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Organisationseinheit der BAM
- 5 Werkstofftechnik (1057) (entfernen)
Paper des Monats
- ja (10)
It is widely accepted that the magnetic state of a ferromagnetic material may be irreversibly altered by mechanical loading due to magnetoelastic effects. A novel standardized nondestructive testing (NDT) technique uses weak magnetic stray fields, which are assumed to arise from inhomogeneous deformation, for structural health monitoring (i.e., for detection and assessment of damage). However, the mechanical and microstructural complexity of damage has hitherto only been insufficiently considered. The aim of this study is to discuss the phenomenon of inhomogeneous “self-magnetization” of a polycrystalline ferromagnetic material under inhomogeneous deformation experimentally and with stronger material-mechanical focus. To this end, notched specimens were elastically and plastically deformed. Surface magnetic states were measured by a three-axis giant magnetoresistant (GMR) sensor and were compared with strain field (digital image correlation) and optical topography measurements. It is demonstrated that the stray fields do not solely form due to magnetoelastic effects. Instead, inhomogeneous plastic deformation causes topography, which is one of the main origins for the magnetic stray field formation. Additionally, if not considered, topography may falsify the magnetic signals due to variable lift-off values. The correlation of magnetic vector components with mechanical tensors, particularly for multiaxial stress/strain states and inhomogeneous elastic-plastic deformations remains an issue.
Corrosion fatigue specimen with different surfaces (technical surfaces after machining and polished surfaces) of high alloyed martensitic stainless steel X46Cr13 (1.4043) and duplex stai nless steel X2CrNiMoN22 3 2 (1.4462) were compared at load amplitudes from 175 MPa to 325 MPa in the geothermal brine of the N orthern German Basin at 98 °C. Surface corrosion layers and pits reveal carbonate corrosion products on the surface such as FeCO 3 and FeOOH as the main precipitation phases with no dependence on the original surface roughness . At high stress amplitudes above 275 MPa technical surfaces (P50% at σa 300 MPa=5 × 10 5 ) resulted in more cycles to failure than polished (P50% at σa 300 MPa=1.5 × 10 5 ). The greater slope coefficient for technical surfaces k = 19.006 compared to polished surfaces k =8.78 demonstrate s earlier failure at given stress amplitude σa .
Coupons of X5CrNiCuNb16-4 that may be used as injection pipe with 16% Chromium and 0.05% Carbon (1.4542, AISI 630) were exposed for 3000 h to CO2-saturated saline aquifer water similar to the conditions in the Northern German Basin at ambient pressure and 60 °C. Surface corrosion layers and pits reveal carbonate corrosion products on the surface such as FeCO3 and FeOOH as the main precipitation phases with no dependence on the original surface roughness. Corrosion rates for polished and technical surfaces were below 0.005 mm/year compared to corrosion rates of 0.035 mm/year after shot peening.
Due to their unique physico-chemical properties, nanoparticles are well established in research and industrial applications. A reliable characterization of their size, shape, and size distribution is not only mandatory to fully understand and exploit their potential and develop reproducible syntheses, but also to manage environmental and health risks related to their exposure and for regulatory requirements. To validate and standardize methods for the accurate and reliable particle size determination nanoscale reference materials (nanoRMs) are necessary. However, there is only a very small number of nanoRMs for particle size offered by key distributors such as the National Institute of Standards and Technology (NIST) and the Joint Research Centre (JRC) and, moreover, few provide certified values. In addition, these materials are currently restricted to polymers, silica, titanium dioxide, gold and silver, which have a spherical shape except for titania nanorods. To expand this list with other relevant nanomaterials of different shapes and elemental composition, that can be used for more than one sizing technique, we are currently building up a platform of novel nanoRMs relying on iron oxide nanoparticles of different shape, size and surface chemistry. Iron oxide was chosen as a core material because of its relevance for the material and life sciences.
BAM is currently building up a platform of novel nanoRMs relying on iron oxide nanoparticles of different shape, size and surface chemistry. Iron oxide was chosen as a core material because of its relevance to the material and life sciences.
As a first candidate of this series, we present cubic iron oxide nanoparticles with a nominal edge length of 8 nm. These particles were synthesized by thermal decomposition of iron oleate in high boiling organic solvents adapting well-known literature procedures. After dilution to a concentration suitable for electron microscopy (TEM and SEM) as well as for small-angle X-ray scattering (SAXS) measurements, the candidate nanoRM was bottled and assessed for homogeneity and stability by both methods following the guidelines of ISO 17034 and ISO Guide 35.
The particle sizes obtained by both STEM-in-SEM and TEM are in excellent agreement with a minimum Feret of 8.3 nm ± 0.7 nm. The aspect ratio (AR) of the iron oxide cubes were extracted from the images as the ratio of minimum Feret to Feret resulting in an AR of 1.18 for TEM to 1.25 for SEM. Alternatively, a rectangular bounding box was fitted originating from the minimum Feret and the longest distance through the particle in perpendicular direction. This led to AR values of 1.05 for TEM and 1.12 for SEM, respectively. The results confirm the almost ideal cubic shape.
The effect of two types of scanning strategies on the grain structure and build-up of Residual Stress (RS) has been investigated in an as-built IN718 alloy produced by Laser Powder Bed Fusion (LPBF). The RS state has been investigated by X-ray diffraction techniques. The microstructural characterization was performed principally by Electron Backscatter Diffraction (EBSD), where the application of a post-measurement refinement technique enables small misorientations (< 2°) to be resolved. Kernel average misorientation (KAM) distributions indicate that preferably oriented columnar grains contain higher levels of misorientation, when compared to elongated grains with lower texture. The KAM distributions combined with X-ray diffraction stress maps infer that the increased misorientation is induced via plastic deformation driven by the thermal stresses, acting to self-relieve stress. The possibility of obtaining lower RS states in the build direction as a consequence of the influence of the microstructure should be considered when envisaging scanning strategies aimed at the mitigation of RS.
The relaxation of macroscopic residual stresses in laser powder bed fused stainless steel 316L
(2021)
The processing of stainless steel 316L using the additive manufacturing process Laser Powder Bed Fusion (LPBF) can widen its field of application due to a strong increase in Yield strength, without making major compromises on the ductility nor its outstanding corrosion and oxidation properties. Furthermore, improved designs that either reduce the weight or optimise the function of a part can be obtained using LPBF. These benefits are however counterbalanced by the proneness of LPBF to inducing high Residual Stresses (RS) during manufacturing. The characterisation and monitoring of these RS are of paramount importance for the wider acceptance of the LPBF process. This study focuses on the relaxation of the initial macroscopic RS present in an LPBF 316L as-built prism that undergoes various routes of manufacturing steps to achieve different specimen geometries and stress relieving treatments. The RS are determined using Angle-Dispersive (AD) and Time-of-Flight (TOF) neutron diffraction. The results reveal high tensile RS close to the surfaces and compressive RS near the centre of the as-built parts. The reduction in size and change of geometry heavily impact the stress ranges of the remaining RS, with lower stress ranges in cylindrical shaped compared to rectangular shaped specimens. Also, the application of different stress relieving heat treatments showed that heat-treating temperatures above 800 °C are necessary to obtain a strong relaxation in LPBF 316L.
The Ni-Cr-Mo-W alloy is characterized by the excellent high-temperature corrosion resistance, good strength and ability to work in aggressive environments. To protect the surface of the substrate 13CrMo4-5 steel against aggressive environments the Inconel 686 as clad layer was used. The corrosion process was performed in oxidizing mixture of gases like O2, COx, SOx. The second part of corrosion Experiment concerned the corrosion test of the coating in reducing atmosphere of the specified gases with ashes, which contained e.g. Na, Cl, Ca, Si, C, Fe, Al.
The application and benefits of Semantic Web Technologies (SWT) for managing, sharing, and (re-)using of research data are demonstrated in implementations in the field of Materials Science and Engineering (MSE). However, a compilation and classification are needed to fully recognize the scattered published works with its unique added values. Here, the primary use of SWT at the interface with MSE is identified using specifically created categories. This overview highlights promising opportunities for the application of SWT to MSE, such as enhancing the quality of experimental processes, enriching data with contextual information in knowledge graphs, or using ontologies to perform specific queries on semantically structured data. While interdisciplinary work between the two fields is still in its early stages, a great need is identified to facilitate access for nonexperts and develop and provide user-friendly tools and workflows. The full potential of SWT can best be achieved in the long term by the broad acceptance and active participation of the MSE community. In perspective, these technological solutions will advance the field of MSE by making data FAIR. Data-driven approaches will benefit from these data structures and their connections to catalyze knowledge generation in MSE.
The influence of the support structure on residual stress and distortion in SLM Inconel 718 parts
(2018)
The effect of support structure and of removal from the base plate on the residual stress state in selective laser melted IN718 parts was studied by means of synchrotron X-ray diffraction. The residual stresses in subsurface region of two elongated prisms in as-built condition and after removal from the base plate were determined. One sample was directly built on a base plate and another one on a support structure. Also, the distortion on the top surface due to stress release was measured by contact profilometry. High tensile residual stress values were found, with pronounced stress gradient along the hatching direction. In the sample on support, stress redistribution took place after removal from the base plate, as opposed to simple stress relaxation for the sample without support. The sample on support structure showed larger distortion compared to sample without support. We conclude that the use of a support decreases stress values but stress-relieving heat treatments are still needed.
This work presents the microstructure and chemical composition of oxide scales created on Ni – base coating after corrosion experiments in aggressive gases and ashes. The Inconel 686 coating applied on the low carbon steel 13CrMo4-5 was performed by a CO2 laser cladding process. The experiments were carried out in oxidizing gas atmospheres containing O2, COx, SOx. The second Variation of corrosion experiments were performed in a reducing atmosphere with ashes containing elements such as: Na, Cl, Ca, Si, C, Fe, Al etc. After 240 h and 1.000 h corrosion experiments the oxide scales on the substrate and overlay were created in both cases.
The sulfur compounds were found on the top of the coating surface (EPMA) and also higher contents of silica compounds were revealed on specimens covered by ashes during the experiments. The microstructure and chemical composition of the clad and scales were investigated by means of a light microscope and an electron microscope (SEM)equipped with an EDS detector.
To understand the impact of dissolved water on structure and properties, four boron-rich glasses of molar compositions 15-x Na2O x CaO 15 SiO2 70 B2O3 (with x=0, 7.5, 10) and 10 Na2O 15 SiO2 75 B2O3 were prepared and subsequently hydrated (up to 8 wt% H2O). Density measurements show a non-linear trend upon hydration implying large structural changes in particular at water contents<2 wt%. Near-infrared spectroscopy shows hydroxyl groups are the dominant species in all glasses upon the entire range of water content. Molecular H2O is detectable only at total water contents>2 wt%. 11B MAS NMR spectra show that the abundance of BO4 species is mainly controlled by ratio of (Na2O+CaO)/B2O3 while incorporation of water plays a minor role.
Compared to borate glasses, the efficiency of formation of BO4 tetrahedra is favored by crosslinking of the network by SiO4-units. The glass transition temperatures, determined by differential thermal analysis, decreases continuously with water content due to breakage of B-O-B bonds by hydrolysis. However, compared to Silicates and aluminosilicates, the effect of dissolved water is less pronounced which can be explained by weaker B-O-B bonds in comparison to Si-O-Si bonds.
Comprehensive insights into the early stages of corrosion mechanisms provide fundamental knowledge to further understand and model long time material behaviour. The present work studies the early stages of combined oxidation and sulphidation of ferritic model alloys for time scales up to 250 h at 650 °C to observe the influence of chromium during the corrosion under SO2. Model alloys were used to focus on the reaction of the intended elements: Fe, Cr, S, and O. Pure iron simultaneously forms magnetite and iron sulphide in an early stage of corrosion, covered by a pure oxide layer after 100 h. Iron with 13 wt% Cr shows hematite and mixed Fe-Cr-oxides first, before sulphides nucleate in the inner corrosion zone. With increasing ageing time a magnetite layer is observed below the hematite layer. Quantitative phase fractions of all corrosion products observed were determined from cross section images. Characterization of the Fe13Cr corrosion scale by FIB revealed a highly porous structure in the inner corrosion zone where Cr-rich (Fe, Cr)-sulphides are present, and caused the scale to spall easily.
In this study, the influence of aging and mean stress on fatigue of the aluminium-alloy EN AW-2618A is investigated. Therefore axial fatigue tests are carried out on smooth specimens. The experiments show that the fatigue life decreases with increasing mean stress. Furthermore, the tests with the overaged specimens demonstrate that the number of cycles to failure is decreasing with increasing aging time.
Sulfur and water have a fundamental impact on the corrosion rate and potential failure of materials. It is therefore necessary to understand the mechanisms, rates, and potential means of transport, as well as the reactions of these elements with an alloy. This work investigates the effect of water vapor in the initial stages of SO2 corrosion of an ferritic model alloy containing 9 wt% Cr and 0.5 wt% Mn.
The exposure experiments were studied at 650°C in situ under laboratory conditions using energy-dispersive x-ray diffraction analysis. Two separate experiments were run, one with a 99.5% Ar + 0.5% SO2 atmosphere and one with a 69.5% Ar + 0.5% SO2 + 30% H2O atmosphere. With a wet atmosphere, the alloy formed a scale with decreasing oxygen content towards the scale–alloy interface. Sulfides were identified above and below a (Fe, Cr)3O4 layer in the inner corrosion zone. In contrast to this, the overall scale growth was slower in a dry SO2 atmosphere.
The impact of the microstructure of Fe-16Cr-0.2C on high-temperature oxidation – sulphidation in SO2
(2021)
This study elucidates the impact of the microstructure of Fe-16Cr-0.2C on oxide layer formation at 650 ◦C in Ar-0.5 % SO2. A cold-rolled and two heat-treated states of the alloy were exposed for up to 1000 h. The samples were characterised in detail from microstructural and chemical perspectives using scanning electron microscopy (SEM), X-ray diffraction (XRD) and time-of-flight secondary ion mass spectrometry (ToF-SIMS). The microstructural modification of the alloy by heat-treatment was advantageous. It was found that Cr-carbides support chromia formation and reduce sulphidation when their area fraction is low and diameter is small.
The unusual behavior observed in the coefficient of thermal expansion and specific heat capacity of CrFeNi, CoCrNi, and CoCrFeNi medium/high-entropy alloys is commonly referred to as the K-state effect. It is shown to be independent of the Curie temperature, as demonstrated by temperature-dependent magnetic moment measurements. CoCrFeNi alloy is chosen for detailed characterization; potential reasons for the K-state effect such as texture, recrystallization, and second-phase precipitation are ruled out. An examination of the electronic structure indicates the formation of a pseudo-gap in the Density of States, which suggests a specific chemical interaction between Ni and Cr atoms upon alloying. Hybrid Monte Carlo/Molecular Dynamic (MC/MD) simulations indicate the presence of non-negligible chemical short-range order (CSRO). Local lattice distortions are shown to be negligible, although deviations around Cr and Ni elements from those expected in a fully disordered structure are experimentally observed by X-ray absorption spectroscopy. The determined bonding distances are in good agreement with MC/MD calculations. A mechanism is proposed to explain the anomalies and calorimetric experiments and their results are used to validate the mechanism.
As more industrial interests focusing on using salt caverns and repurposed gas or petroleum reservoirs for alternative fuel storage, i.e. CO2/H2, the question raises whether microorganisms may impact the infrastructure, gas purity and storage condition over time.
Environments with high salinity (> 1.5 Meq of NaCl) are resided by halophiles (salt-loving microorganisms). To compensate for the intensive osmotic stress, they have resorted to two main adaptation strategies: 1) production of compatible solutes and 2) accumulation of intracellular KCl. Microbial community analysis of several high salinity environments revealed a number of recurring genera, including Halomonas and Halanaerobium. However, the impact of halophiles on the overall integrity and stability of the storage facilities remain largely unknown.
To evaluate the suitability and stability of saline storage facilities, several model halophilic microorganisms, such as members of Halomonas, will be selected as testing subjects. First, the impact of halophiles on the infrastructure will be determined using an integrative approach by combining a number of techniques, including electrochemistry, TOF-SIMS, SEM/FIB/EDS and FIB-TEM. Second, the abilities of halophiles to alter the fuel composition (i.e. increase/decrease the fractions of H2) will be monitored using gas chromatography by growing them under high pressure.
As a result of climate change and the accompanying mandatory shift to renewable energy resources, microorganisms will continue to play an important role in the energy sector, both to their benefit and detriment. Thus, it is important to achieve a certain level of understanding regarding the activities and mechanisms of halophiles prior to large-scaled excursions.
A round-robin study has been carried out to estimate the impact of the human element in small-angle scattering data analysis. Four corrected datasets were provided to participants ready for analysis. All datasets were measured on samples containing spherical scatterers, with two datasets in dilute dispersions and two from powders. Most of the 46 participants correctly identified the number of populations in the dilute dispersions, with half of the population mean entries within 1.5% and half of the population width entries within 40%. Due to the added complexity of the structure factor, far fewer people submitted answers on the powder datasets. For those that did, half of the entries for the means and widths were within 44 and 86%, respectively. This round-robin experiment highlights several causes for the discrepancies, for which solutions are proposed.
A Round Robin study has been carried out to estimate the impact of the human element in small-angle scattering data analysis. Four corrected datasets were provided to participants ready for analysis. All datasets were measured on samples containing spherical scatterers, with two datasets in dilute dispersions, and two from powders.
Most of the 46 participants correctly identified the number of populations in the dilute dispersions, with half of the population mean entries within 1.5 % and half of the population width entries within 40 %, respectively. Due to the added complexity of the structure factor, much fewer people submitted answers on the powder datasets.
For those that did, half of the entries for the means and widths were within 44 % and 86 % respectively. This Round Robin experiment highlights several causes for the discrepancies, for which solutions are proposed.
The interest to additively manufacture Nickel-based superalloys has substantially grown within the past decade both academically and industrially. More specifically, additive manufacturing processes such as laser powder bed fusion (LPBF) offer the ability to produce dense parts within a single manufacturing step. In fact, the exceptional freedom in design associated with the layer-based nature of the processes is of particular interest for the complex shapes typically required in turbine applications. In certain cases, the overall part performance can be achieved by tailoring the microstructure and the crystallographic texture to the specific application. However, these advantages must be paid at a price: the large local temperature gradients associated with the rapid melting and solidification produce parts that inherently contain large residual stress in the as-manufactured state. In addition, the presence of pores in the final part may further affect the in-service part failure. As among Nickel-based alloys Inconel 718 exhibits excellent weldability, this alloy has been widely studied in open research in the domain of LPBF. However, significant microsegregation of the heavier alloying elements such as Niobium and Molybdenum accompanied by dislocation entanglements may preclude the application of conventional heat treatment schedules. Therefore, different post processing heat treatments are required for laser powder bed fused Inconel 718 as compared to conventional variants of the same alloy.
In this study, we investigated two different heat treatment routes for LPBF Inconel 718. In a first routine, the samples were stress relieved and subsequently subjected to hot isostatic pressing (HIP) followed by a solution heat treatment and a two-step age (referred to as FHT). In a second routine, the samples were subjected to a single-step direct age post stress relieving heat treatment (referred to DA). We investigated the consequences of such heat treatment schedules on the microstructure, texture, and mechanical behavior. We show that by applying a DA heat treatment the typical columnar microstructure possessing a crystallographic texture is retained, while an equiaxed untextured microstructure prevails in case of an FHT heat treatment. We further evaluate how these heat treatments affect the mechanical behaviour on the macroscopic and microscopic scale.
BAM! This issue of Advanced Engineering Materials celebrates 150 years of scientific and technical research at the interface between academia, industry and politics. Rooted in 1871 at the birth of the German Empire and at that time located in simple basements and barracks, the institutional development began around mechanical metallurgy of iron and steel and represents today a diverse portfolio of fore-front research that orients itself along tomorrow's societal challenges and long-term research horizons.
Microplastics are increasingly entering marine, limnic and terrestrial ecosystems worldwide, where they sorb hydrophobic organic contaminants. Here, the sorption behavior of the fuel-related water contaminants benzene, toluene, ethyl benzene and xylene (BTEX) and four tertiary butyl ethers to virgin and via UV radiation aged polypropylene (PP) and polystyrene (PS) pellets was investigated. Changes in material properties due to aging were recorded using appropriate polymer characterization methods, such as differential scanning calorimetry, Fourier transform infrared spectroscopy, gel permeation chromatography, X-ray photoelectron spectroscopy, and microscopy.
Pellets were exposed to water containing BTEX and the ethers at 130-190 mg/L for up to two weeks.
Aqueous sorbate concentrations were determined by headspace gas chromatography. Sorption to the polymers was correlated with the sorbate's Kow and was significant for BTEX and marginal for the ethers.
Due to substantially lower glass transition temperatures, PP showed higher sorption than PS. Aging had no effect on the sorption behavior of PP. PS sorbed less BTEX after aging due to an oxidized surface layer.
Wind turbine rotor blades commonly fail before their projected 20-year lifespan largely due to defects that originate during manufacturing and are propagated by operational fatigue and environmental conditions. The cost-intensive replacement outcomes lead to a high loss of earnings, and are one of the inhibitors of wind turbine production. A potential repair alternative to restoring the mechanical properties of such lightweight fiber reinforced polymer (FRP) structures is to locally patch these areas with scarf joints. This type of repair allows for a smoother load distribution across the joint, and is favored especially on structures where minor aerodynamic contour changes are key. The effects of such repairs on the structural integrity, however, is still largely unknown. Building upon an understanding of the static load failure mechanism of GFRP scarf joints, presented at the ICCS23 Joint Event in 2020, the influence of the fiber orientation mismatch between parent and repair materials of 1:50 scarf joints on the failure mechanism of monolithic glass FRP specimens under cyclic fatigue load were examined in this study. Specimens with various layups were produced with the vacuum-assisted resin infusion (VARI) process using biaxial E-glass non-crimp fabric (NCF). The patch layers were then joined directly to the parent structure with the VARI using biaxial E-glass NCF with half the areal weight of the parent side to allow for better drapability. This mimics the soft-to-hard patch style utilized in wind turbine blade shell field repairs. The specimens were tested under uniaxial fatigue load, during which they were periodically monitored for damage onset. A comparison of the +45/-45° and 0/90° layups allowed for an understanding of the role of a highly mismatching fiber orientation in the transition zone between parent and patch material on the failure mechanism of the scarf joint. In addition to the tensile strength and stiffness property recovery assessment, a grayscale analysis using in-situ camera images determined the damage state leading to failure in each region across the scarf joint, which varied in the parent material versus scarf joint region, providing insight to the critical regions in this composite structure under cyclic loading.
Results of an extended TMF test program on grade P92 steel in the temperature range of 620 °C - 300 °C, comprising in-phase (IP) and out-of-phase (OP) tests, partly performed with symmetric dwells at Tmax/Tmin, are presented. In contrast to previous studies, the low-strain regime is also illuminated, which approaches flexible operation in a power plant with start/stop cycles. At all strain amplitudes, the material performance is characterized by continuous cyclic softening, which is retarded in tests at lower strains but reaches similar magnitudes in the course of testing. In the investigated temperature range, the phase angle does not affect fatigue life in continuous experiments, whereas the IP condition is more detrimental in tests with dwells. Fractographic analyses indicate creep-dominated and fatigue-dominated damage for IP and OP, respectively. Analyses of the (micro)hardness distribution in the tested specimens suggest an enhanced microstructural softening in tests with dwell times for the low- but not for the high-strain regime. To rationalize the obtained fatigue data, the fracture-mechanics-based D_TMF concept, which was developed for TMF life assessment of ductile alloys, was applied. It is found that the D_TMF parameter correlates well with the measured fatigue lives, suggesting that subcritical growth of cracks (with sizes from a few microns to a few millimeters) governs failure in the investigated range of strain amplitudes.
The prediction of structural parameters and optoelectronic properties of compound semiconductors is very important. However, calculations often neglect chemical variability and structural defects. In chalcopyrite type semiconductors one of the major defects are copper vacancies (V Cu). The four cation neighbors of the anion determine its position in the chalcopyrite type structure expressed by the Wyckoff position 8d (x, 1/4, 1/8). Intrinsic point defects like V
Cu and anti-sites may cause variations of the Anion position in the middle of the cation tetrahedron, especially in the Anion position Parameter x. For stoichiometric chalcopyrite type compounds a formalism according to the principle of conservation of tetrahedral bonds (CTB) can be applied to calculate the anion position parameter, but it fails in the case of off-stoichiometric chalcopyrites. This case study of chalcopyrite type CuGaS 2 and Mn-substituted GuGaS 2
shows that the experimentally determined anion position Parameter x
deviate from values calculated by CTB approach. The systematic investigation of off-stoichiometric CuGaS 2 and Mn-substituted GuGaS
2 demonstrates the effect of copper vacancies on the average radii of the cation sites (Wyckoff positions 4a and 4b) as well as on the anion position Parameter x. By applying an elaborated CTB Approach implementing copper vacancies an agreement between experimental and calculated anion position Parameter x can be obtained.
Suitable material solutions are of key importance in designing and producing components for engineering systems – either for functional or structural applications. Materials data are generated, transferred, and introduced at each step along the complete life cycle of a component. A reliable materials data space is therefore crucial in the digital transformation of an industrial branch.
A great challenge in establishing a materials data space lies in the complexity and diversity of materials science and engineering. It must be able to handle data from different knowledge areas over several magnitudes of length scale.
The Platform MaterialDigital (PMD) is expected to network a large number of repositories of materials data, allowing the direct contact of different stakeholders as materials producers, testing labs, designers and end users. Following the FAIR principles, it will promote the semantic interoperability across the frontiers of materials classes. In the frame of a large joint initiative, PMD
works intensively together with currently near 20 research consortia in promoting this exchange (www.material-digital.de).
In this presentation we will describe the status of our Platform MaterialDigital. We will also present in more detail the activities of GlasDigital, one of the joint projects mentioned above dealing with the digitalization of glass design and manufacturing.
(https://www.bam.de/Content/EN/Projects/GlasDigital/glasdigital.html)
Detailed microstructural characterization of the AlMo0.5NbTa0.5TiZr refractory high entropy superalloy in the as-cast state is reported for first time and compared with the state annealed at 1400 oC for 24 h. The former shows a dendritic structure, with a mixture of A2/B2 phases < 20 nm in both the dendritic and interdendritic regions. A mostly amorphous phase, rich in Al and Zr, is found within the interdendritic region. The annealed state reproduced the combination of A2/B2/Al-Zr-rich phases reported previously. Calculations from two relevant ThermoCalc databases were compared with the experimental results. Equilibrium calculations were compared with results for the annealed alloy, whereas solidification paths calculated using Scheil-Gulliver model were used for comparison with the as-cast alloy. A previously hypothesized spinodal decomposition during cooling as the mechanism responsible for the patterned A2/B2 microstructure is confirmed via the CALPHAD calculations, pointing to its use as an efficient design tool for such alloys. Finally, the comparison between the experimental and computational findings allowed better understanding the solidification path and equilibrium stability of this alloy, giving a base to make better decisions on the field of new refractory superalloy design.
Unlike conventional alloys, which typically consist of one main element, high-entropy alloys (HEAs) contain five or more principal elements, which broaden chemical complexity and with it a realm of synergistic mechanisms. The AlMo0.5NbTa0.5TiZr HEA initiated a subclass of Al-containing refractory (r)HEAs that has recently drawn attention [2]. The alloy has a superalloy-resembling B2/bcc nanostructure, which inspired its name refractory high entropy superalloy (RSA). With high-temperature (HT) compressive strengths beyond conventional Ni-based superalloys, this nanostructure could be used for improved HT structural applications. However, in the application-relevant HT regime the Al-Zr-rich B2 phase decomposes to form a hexagonal Al-Zr-based intermetallic (Al4-xZr5; x: 0..1) [3,4]. This work explores the fascinating yet fatal micromechanisms associated to this phase transformation, in the context of creep, annealing and oxidation experiments performed between 800 and 1200 °C.
The material was produced by arc-melting and heat treatment in argon, which lead to grain boundaries decorated with up to 7%. Interrupted constant-load creep tests were performed under vacuum (at 10-4 Pa), at 900–1100 °C with external tensile stresses of 30–120 MPa. Oxidation experiments were separately conducted for 24 hours at 800 and 1000 °C in both dry (21% O2 + 79% N2) and humid (8% O2 + 74% N2 + 18% H2O) air. After the experiments, the samples were characterized by X-ray diffraction, scanning electron microscopy and transmission electron microscopy to reveal degradation mechanisms. Crystallographic texture, orientation relationships and stabilization of an oxygen-containing iso structure (Al4-xZr5(Ox-y); y: 0..x) of the Al-Zr-rich intermetallic are found and discussed.
High-performance fibre-reinforced polymer composites are important construction materials based not only on the specific properties of the reinforcing fibres and the flexible polymer Matrix but also on the compatible properties of the composite interphase. First, oxygen-free (a-CSi:H) and oxygen-binding (a-CSiO:H) plasma nanocoatings of different mechanical and tribological properties were deposited on planar silicon dioxide substrates that closely mimic E-glass. The nanoscratch test was used to characterize the nanocoating adhesion expressed in terms of critical normal load and work of adhesion. Next, the same nanocoatings were deposited on E-glass fibres, which were used as reinforcements in the polyester composite to affect its interphase properties. The shear properties of the polymer composite were characterized by macro- and micromechanical tests, namely a short beam shear test to determine the short-beam strength and a single fibre push-out test to determine the interfacial shear strength. The results of the polymer composites showed a strong correlation between the short-beam strength and the interfacial shear strength, proving that both tests are sensitive to changes in fibre-matrix adhesion due to different surface modifications of glass fibres (GF).
Finally, a strong correlation between the shear properties of the GF/polyester composite and the adhesion of the plasma nanocoating expressed through the work of adhesion was demonstrated.
Thus, increasing the work of adhesion of plasma nanocoatings from 0.8 to 1.5 mJ·m−2 increased the short-beam strength from 23.1 to 45.2 MPa. The results confirmed that the work of adhesion is a more suitable parameter in characterising the level of nanocoating adhesion in comparison with the critical normal load.
Texturing of calcium cobaltite for thermoelectric applications by pressure assisted sintering
(2021)
Thermoelectric materials can convert waste heat directly into electrical power by using the Seebeck effect. Calcium cobaltite (CCO) is considered as a promising thermoelectric p-type oxide for energy harvesting applications at temperatures above 500 °C. The properties and morphology of single-crystal CCO are strongly anisotropic because of its crystal structure of alternating layers of CoO2 and Ca2CoO3. By aligning the plate-like grains, the anisotropic properties of the grains can be assigned to the poly-crystalline parts.
In this study, the combination of tape casting and pressure-assisted sintering is used to texture and densify large scale components (50 cm²). Thereby, the influence of powder preparation and applied pressure during sintering on texturing and thermoelectric properties is investigated.
The analysis of XRD pole figures revealed that tape casting already leads to highly textured CCO. By pressure variation during sintering, the microstructure of CCO can be tailored either toward maximum power factor as required for energy harvesting or toward maximum figure of merit as required for energy recovery. Low pressure lead to a porous microstructure and maximum figure of merit and higher pressure to full densification and maximum power factor. The electrical and thermal conductivity of CCO seem depending on both texture and sinter density.
Dense ZnO films with a strong c‐axis texture have been deposited on transparent conductive oxide glass, glass, and Si wafers, respectively, with a two‐step pressureless wet chemical method using zinc acetate dihydrate as Zn‐precursor. The crystallographic structure of the films has been studied with XRD and scanning electron microscopy. Optical measurements reveal a high transparency of the ZnO films with a thickness of up to 10 μm. This new cost‐effective route for ZnO film deposition does not require expensive sophisticated equipment and is easily upscaled.
Although layer-based additive manufacturing methods such as laser powder bed fusion (PBF-LB) offer an immense geometrical freedom in design, they are typically subject to a build-up of internal stress (i.e. thermal stress) during manufacturing. As a consequence, significant residual stress (RS) is retained in the final part as a footprint of these internal stresses. Furthermore, localized melting and solidification inherently induce columnar-type grain growth accompanied by crystallographic texture. Although diffraction-based methods are commonly used to determine the RS distribution in PBF-LB parts, such features pose metrological challenges in their application. In theory, preferred grain orientation invalidates the hypothesis of isotropic material behavior underlying the common methods to determine RS. In this work, more refined methods are employed to determine RS in PBF-LB/M/IN718 prisms, based on crystallographic texture data. In fact, the employment of direction-dependent elastic constants (i.e. stress factors) for the calculation of RS results in insignificant differences from conventional approaches based on the hypothesis of isotropic mechanical properties. It can be concluded that this result is directly linked to the fact that the {311} lattice planes typically used for RS analysis in nickel-based alloys have high multiplicity and less strong texture intensities compared with other lattice planes. It is also found that the length of the laser scan vectors determines the surface RS distribution in prisms prior to their removal from the baseplate. On removal from the baseplate the surface RS considerably relaxes and/or redistributes; a combination of the geometry and the scanning strategy dictates the sub-surface RS distribution.
Additive manufacturing methods such as laser powder bed fusion (LPBF) allow geometrically complex parts to be manufactured within a single step. However, as an aftereffect of the localized heat input, the rapid cooling rates are the origin of the large residual stress (RS) retained in as-manufactured parts. With a view on the microstructure, the rapid directional cooling leads to a cellular solidification mode which is accompanied by columnar grown grains possessing crystallographic texture. The solidification conditions can be controlled by the processing parameters and the scanning strategy. Thus, the process allows one to tailor the microstructure and the texture to the specific needs. Yet, such microstructures are not only the origin of the mechanical anisotropy but also pose metrological challenges for the diffraction-based RS determination. In that context the micromechanical elastic anisotropy plays an important role: it translates the measured microscopic strain to macroscopic stress. Therefore, it is of uttermost importance to understand the influence of the hierarchical microstructures and the texture on the elastic anisotropy of LPBF manufactured materials.
This study reveals the influence of the build orientation and the texture on the micro-mechanical anisotropy of as-built Inconel 718. Through variations of the build orientation and the scanning strategy, we manufactured specimens possessing [001]/[011]-, [001]-, and [011]/[111]-type textures. The resulting microstructures lead to differences in the macroscopic mechanical properties. Even further, tensile in-situ loading experiments during neutron diffraction measurements along the different texture components revealed differences in the microstrain response of multiple crystal lattice planes. In particular, the load partitioning and the residual strain accumulation among the [011]/[111] textured specimen displayed distinct differences measured up to a macroscopic strain of 10 %. However, the behavior of the specimens possessing [001]/[011]-and [001]-type texture was only minorly affected. The consequences on the metrology of RS analysis by diffraction-based methods are discussed.
The locally varying tetragonality in martensite grains of a high-carbon steel (1.2 mass percent C) was resolved by electron backscatter diffraction (EBSD) with a spatial resolution in the order of 100 nm. Compared to spatially integrating X-ray diffraction, which yielded an average tetragonality fo c/a=1.05, the EBSD measurements in the scanning electron microscope allowed to image a local variation of the lattice papameter ration c/a in the range of 1.02 ≤ c/a ≤ 1.07. The local variation of tetragonality is confirmed by two different EBSD data analysis approaches based on the fitting of simulated to experimental EBSD patterns. The resulting EBSD-based tetragonality maps are pointing to a complex interaction of carbon concentration and local lattice distortions during the formation process of martensitic structures.
With this presentation, the push-out technique is explained. The focus of the experimental work is on the characterization of the fiber-matrix interface of short fiber reinforced composites. The reinforcing component was glass fibers and the matrix polymer was PA6.6 and PPA.
It is demonstrated for the first time that the push-out technique ca be applied on injection molded short fiber PMC and is sensitive to the mechanical interface properties. Further studies are planned on the influence of multiple processing, the temperature and humidity.
Data analysis methods play an important role in both the experimental and simulation-based digital description of materials but have so far been poorly structured. The platform Material Digital (PMD) should contribute to a standardized description of data processing methods in materials research. The goal is the quality assurance of the processes and the output data, the acquisition and definition of their accuracy as well as the interoperability between applications.
Therefore, application ontologies are created to explicitly describe processes and test methods. In this presentation, the first efforts in creating a tensile test application ontology in accordance with the ISO standard 6892-1:2019-11 are shown. Especially, the path of ontology development to be pursued based on standards was focused.
Data analysis methods play an important role in both the experimental and simulation-based digital description of materials but have so far been poorly structured. The platform Material Digital (PMD) is supposed to contribute to a standardized description of data processing methods in materials research. The goal is the quality assurance of the processes and the output data, the acquisition and definition of their accuracy as well as the interoperability between applications.
Therefore, application ontologies are created to explicitly describe processes and test methods. In this presentation, the first efforts within the joint project PMD in creating a tensile test application ontology in accordance with the ISO standard 6892-1:2019-11 are shown. Especially, the path of ontology development to be pursued based on standards was focused. Furthermore, the presentation includes a live demonstration of queries possibly performed to query data that was uploaded in the PMD triple store.
Tensile Test Ontology (TTO)
(2023)
This is the stable version 2.0.1 of the PMD ontology module of the tensile test (Tensile Test Ontology - TTO) as developed on the basis of the 2019 standard ISO 6892-1: Metallic materials - Tensile Testing - Part 1: Method of test at room temperature.
The TTO was developed in the frame of the PMD project. The TTO provides conceptualizations valid for the description of tensile test and corresponding data in accordance with the respective standard. By using TTO for storing tensile test data, all data will be well structured and based on a common vocabulary agreed on by an expert group (generation of FAIR data) which will lead to enhanced data interoperability. This comprises several data categories such as primary data, secondary data and metadata. Data will be human and machine readable. The usage of TTO facilitates data retrieval and downstream usage. Due to a close connection to the mid-level PMD core ontology (PMDco), the interoperability of tensile test data is enhanced and data querying in combination with other aspects and data within the broad field of material science and engineering (MSE) is facilitated.
The TTO class structure forms a comprehensible and semantic layer for unified storage of data generated in a tensile test including the possibility to record data from analysis, re-evaluation and re-use. Furthermore, extensive metadata allows to assess data quality and reproduce experiments. Following the open world assumption, object properties are deliberately low restrictive and sparse.
Extensometer sind seit Jahrzehnten im Einsatz und bilden die Basis für eine präzise Dehnungsmessung in werkstoffmechanischen Versuchen. Jedoch ist die Temperaturstabilität von Extensometern häufig noch nicht ausreichend beachtet worden, um Fehler verursacht durch Temperaturschwankungen zu minimieren. Dieses wird derzeit nicht nur in Deutschland, sondern auch international zunehmend diskutiert. Datenblätter von Extensometern sind teils unvollständig besonders hinsichtlich von Temperatureinflüssen. Extensometer für den Einsatz bei höheren Temperaturen sollen z.B. laut Normen für dehnungsgeregelte Ermüdungsversuche mit Luft oder Wasser aktiv gekühlt werden. Jedoch schreiben die Normen keine Temperatur-Toleranzen für Extensometer vor. Dehnungsfehler verursacht durch Temperaturschwankungen am Extensometer bleiben meist verborgen, da die Temperatur des Extensometers standardmäßig gar nicht gemessen wird. Die Ursache von Einflüssen zur Temperaturstabilität von Extensometern wird erläutert. Ein einfaches Rechenbeispiel zeigt die Empfindlichkeit von Extensometern gegenüber Temperaturschwankungen und die Auswirkungen auf den Dehnungsfehler. Lösungsmöglichkeiten zur Optimierung werden aufgezeigt. Der Anwender von Extensometern soll sensibilisiert werden, um der Temperaturstabilität von Extensometern sowohl hinsichtlich Herstellerspezifikationen als auch bei der Anwendung von Extensometern im Prüflabor mehr Aufmerksamkeit zu schenken, um letztlich die Qualität von Versuchsergebnissen zu verbessern. Empfehlungen an Extensometerhersteller werden formuliert. Dieses Thema wird zukünftig auch in Prüfnormen mehr Aufmerksamkeit geschenkt werden. Es werden Maßnahmen zur Verbesserung der Temperaturstabilität von Extensometern Einzug in Prüfnomen finden, um letztlich verlässlichere Versuchsergebnisse zu erhalten. Es ist beabsichtigt, eine Verbesserung der Temperaturstabilität von Extensometern z.B. bei der nächsten Revision der Prüfnormen ISO 12106 und ISO 12111 für dehnungsgeregelte LCF- und TMF-Versuche zu berücksichtigen.
Extensometer werden seit langer Zeit erfolgreich für die Ermittlung von werkstoffmecha-nischen Kennwerten eingesetzt. Allerdings wurden in den letzten Jahren immer wieder durch Temperaturschwankungen bedingte Drift an Extensometern festgestellt, die be-sonders in Langzeitversuchen zu Problemen führen können. Es wurden Ursachen für die thermische Drift von Extensometern aufgezeigt. Anhand von einem Beispiel wurde gezeigt, wie sich Temperaturschwankungen an einem Extensometer auf die Ergebnisse von werkstoffmechanischen Ermüdungsversuchen auswirken können. An einem weiteren Beispiel wurde beschrieben, dass der durch Temperaturschwankungen verursachte Dehnungsfehler eines Extensometers stark von der Höhe der Temperaturschwankung und von der Dehnungsschwingbreite abhängt und, je nach Anwendungsfall größer als 1% werden kann. Es wurde verdeutlicht, dass bisherige nach Norm vorgeschriebene Maß-nahmen oft nicht ausreichen, um Einflüsse von Temperaturschwankungen am Extenso-meter zuverlässig zu unterbinden. Vorschläge wurden aufgezeigt, um die Temperatursta-bilität von Extensometern zu verbessern. Das gesamte Temperaturmanagement von Raumtemperatur, Kühlwassertemperatur, Temperaturmessung am Extensometer etc. sollte von Grund auf neu betrachtet werden, um Verbesserungen bei der Temperatur-konstanz von Extensometern zu erlangen. Ferner wurde vorgeschlagen, Extensometer in temperaturkompensierter Ausführung zu entwickeln und zu verkaufen. Dieses sollte Ret-rofits für existierende Extensometer miteinschließen. Konkrete Maßnahmen zur Imple-mentierung in Materialprüfungsnormen müssen weiter diskutiert werden.
Temperature-induced, stress-induced martensitic phase transitions and martensite reorientation process in Ni rich (50.9 at.%) NiTi pseudoelastic NiTi wires with ultra-fine grained (UFG) microstructure were studied by electrical resistance measurements. Measurements of the electrical resistance as a function of temperature at different constant mechanical loads accompanied by complementary experiments with variable loads at constant temperature were performed. Results show that the transformation mechanisms in UFG microstructures exhibit a higher level of complexity when compared with those characterizing the behavior of other microstructures (e.g., recrystallized or larger grains size). It was found that a threshold stress level below 150 MPa delimits the transition from a homogeneous (low stress) to localized but reversible Lüders type transformation (high stress) when the transformations are induced under constant applied stress and that reorientation processes require stresses of 100 MPa in the present UFG wires. Even though the strain evolutions do not always show two distinct yielding events during cooling or heating, electrical resistance measurements proved that a two-step transformation involving R-phase and B19' martensite was always present in the extended range of temperatures and stresses investigated here.
Misfits of γ- γ’ based Al10Co25Cr8Fe15Ni36Ti6 and its Mo- and Hf-variations are studied up to a temperature of 980 °C and compared with Ni- and Co-based superalloys. The trace elements decrease (Hf) or increase (Mo) the edge radii of the γ’ cuboids without changing their sizes. Atom probe measurements revealed that the Hf alloy prefers the γ’ phase while Mo prefers the γ matrix, leading to a lattice parameters enhancement of both phases, as could be revealed by synchrotron X-ray diffraction. The misfit is influenced in opposite ways: Hf increases the positive misfit, while Mo reduces it at all investigated temperatures.
The main difference between high entropy alloys and conventional alloys is the solid solution strengthening effect, which shifts from a single element to a multi-element matrix. Little is known about the effectiveness of this effect at high temperatures. Face-centered cubic, equiatomic, and single crystalline high entropy alloy CrMnFeCoNi was pre-alloyed by arc-melting and cast as a single Crystal using the Bridgman process. Mechanical characterization by creep testing were performed at temperatures of 700, 980, 1100, and 1200°C at different loads under vacuum and compared to single-crystalline pure nickel. The results allow a direct assessment of the influence of the chemical composition without any disturbance by grain boundary sliding or diffusion. The results indicate different behaviors of single crystalline pure nickel and CrMnFeCoNi. At 700°C CrMnFeCoNi is more creep-resistant than Ni, but at 980°C both alloys show a nearly similar creep strength. Above 980°C the creep behavior is identical and the solid solution strengthening effect of the CrMnFeCoNi alloy disappears.
Ceramic springs combine attractive properties for applications in machinery, metrology, and sensor technology. They are electrically insulating, non-magnetic, provide a linear stress-strain behavior, and are stable at high temperatures and in corrosive environments. Generally, the precise dimensioning of a ceramic spring with respect to the spring constant is challenging. Different models are described, but many of these calculations do not match the actual spring properties. We demonstrate a reliable approach for the dimensioning and manufacturing of helical compression springs with a rectangular winding cross-section. Based on the German standard DIN 2090, which is referring to metallic springs, the spring constant can be calculated based on shear modulus, diameter, height, widths, and number of windings. Different ceramic springs were produced by milling of sintered hollow cylinders of zirconia, alumina and silicon nitride. The experimental spring constants are in very good agreement with the calculated values. Spring constants of zirconia springs were varied over three orders of magnitude between 0.02 N/mm and 5 N/mm by purposeful adaption of the spring geometry. The combination of dimensioning based on DIN 2090 and precise hard machining offers a reliable technology for the fabrication of tailored ceramic springs for special applications.
The present work provides a comparative study on the interface and adhesion properties of surface modified single glass fibers embedded in an acrylate matrix. To facilitate a covalent bonding at the fibermatrix interface, the fibers are functionalized with selected organosilanes that comprise either passive (unsaturated C¼C bonds of methacrylate moieties) or photoactive functionalities (photocleavable bis(acyl)phosphane oxide groups). Immobilization of the functional silanes is carried out by a classic silanization reaction involving a condensation reaction across the surface hydroxyl groups of the inorganic glass fibers. The change of the physico-chemical properties of the fibers due to desizing and subsequent surface modification is monitored by X-ray photoelectron spectroscopy and zeta potential measurements. In addition, scanning electron microscopy is used to follow the changes in surface morphology. After the modification step, the desized and modified single fibers are embedded in a photocurable acrylate resin formulation. By performing single fiber pull-out tests, maximum pull-out force, friction strength and apparent interfacial shear strength are determined as a function of the coupled silanes. The results reveal that the attached organosilanes lead to a significant increase in adhesion strength, whilst the performance of the photo-cleavable organosilane is superior to the passive methacryl-functional derivative.
In order to be able to manipulate ceramic powder compacts and ceramic suspensions (slurries) within their volume with light, a minimum transparency of the materials is required. Compared to polymers and metals, ceramic materials are characterized by the fact that they have a wide electronic band gap and therefore a wide optical window of transparency. The optical window generally ranges from less than 0.3 µm to 5 µm wavelength. In order to focus light into the volume of a ceramic powder compact, its light scattering properties must therefore be tailored. In this study, we present the physical background and material development strategies for the application of two-photon polymerization (2PP) and selective volumetric sintering for the additive manufacturing of structures in the volume of ceramic slips and green compacts.
Manipulating ceramic powder compacts and ceramic suspensions (slurries) within their volume with light requires a minimum transparency of the materials. Compared to polymers and metals, ceramic materials are unique as they offer a wide electronic band gap and thus a wide optical window of transparency. The optical window typically ranges from below 0.3 µm up to 5µm wavelength. Hence, to penetrate with light into the volume of a ceramic powder compound, its light scattering properties need to be investigated and tailored. In the present study we introduce the physical background and material development strategies to apply two-photon-polymerization (2PP), and other volumetric methods for the additive manufacture of filigree structures within the volume of ceramic slurries.
Recherche verschiedener jeweils nationaler/internationaler Regelwerke
und Prozeduren für die Systematische Schadensanalyse
• Literaturrecherche zu Methoden der systematischen interdisziplinären Schadensanalyse
• Vergleich der Regelwerke VDI 3822, ASTM, ASM sowie der Methoden
• Anwendung der Regelwerke/Prozeduren und Methoden am Beispiel
Kirkendall porosity that forms during interdiffusion in a diffusion couple of nickel-base superalloy CMSX-10 with pure nickel is investigated. The diffusion experiments are conducted at a temperature of 1250 °C, where the strengthening ƴ'-phase ist partially dissolved. The porosity is studied by X-ray sub-μ tomography with a spatial resolution of about 0.35³ μm³ at the European Synchrotron Radiation Facility (ESRF) in Grenoble, France. It is found that depending on the distance from the interface the Kirkendall pores take different shapes: octahedral, rounded pyramidal, drop shaped, dendritic, pear shaped, and joint shapes. Such a variety of pore morphologies indicates a complex multistage process of porosity nucleation and growth under vacancy supersaturation of different degrees. The experimental findings are interpreted on the basis of the results of diffusion modeling. It is shown that the kinetics of porosity growth is essentially influenced by the dissolution of the ƴ'-phase.
LTCCs (Low-temperature co-fired ceramics) consist of three-dimensionally distributed, hermetically bonded ceramic and metallic components with structure sizes within [10; 100] µm. A non-destructive imaging technique is needed that provides 3D, sharp, high-contrast resolution of these structures, as well as porosity and defect analysis, which is made difficult by the very different X-ray absorption coefficients of the individual components of the microstructure. A HDR method is being developed that allows a combination of different tomograms, each with X-ray energies adapted to individual materials.
Spherical mesoporous bioactive glasses in the silicon dioxide (SiO2)-phosphorus pentoxide (P2O5)–calcium oxide (CaO) system with a high specific surface area of up to 300m2/g and a medium pore radius of 4 nm were synthesized by using a simple one-pot surfactant-assisted sol–gel synthesis method followed by calcination at 500–700°C. The authors were able to control the particle properties by varying synthesis parameters to achieve microscale powders with spherical morphology and a particle size of around 5–10 mm by employing one structure-directing agent. Due to a high Calcium oxide content of 33·6mol% and a phosphorus pentoxide content of 4·0mol%, the powder showed very good bioactivity up to 7 d of immersion in simulated Body fluid. The resulting microspheres are promising materials for a variety of life science applications, as further processing – for example, granulation – is unnecessary. Microspheres can be applied as materials for powder-based additive manufacturing or in stable suspensions for drug release, in bone cements or fillers.
Up to now, oriented surface crystallization phenomena are discussed controversially, and related studies are restricted to few glasses. For silicate glasses we found a good correlation between the calculated surface energy of crystal faces and oriented surface nucleation. Surface energies were estimated assuming that crystal surfaces resemble minimum energy crack paths along the given crystal plane. This concept was successfully applied by Rouxel in calculating fracture surface energies of glasses. Several oriented nucleation phenomena can be herby explained assuming that high energy crystal surfaces tend to be wetted by the melt. This would minimize the total interfacial energy of the nucleus. Furthermore, we will discuss the evolution of the microstructure and its effect on the preferred crystal orientation.
Up to now, the mechanisms of surface nucleation and surface-induced texture formation are far from being understood. Corresponding phenomena are discussed hypothetically or even controversial, and related studies are restricted to very few glasses. In this talk the state of the art on mechanisms of surface nucleation are summarized. On one hand, mechanical damaged surfaces show high nucleation activity, at which the nucleation occurs at convex tips and edges preferentially. On the other hand, solid foreign particles are dominant nucleation sites at low damaged surfaces. They enable nucleation at temperatures even far above Tg. The nucleation activity of the particles is substantially controlled by their thermal and chemical durability. But no systematic studies on initially oriented crystal growth or nucleation from defined active nucleation sites have been pursued, so far. Therefore, the main objective of a just started project is to advance the basic understanding of the mechanisms of surface-induced microstructure formation in glass ceramics. We shall answer the question whether preferred orientation of surface crystals is the result of oriented nucleation or caused by other orientation selection mechanisms acting during early crystal growth. In both cases, crystal orientation may be caused by the orientation of the glass surface itself or the anisotropy and orientation of active surface nucleation defects. As a first attempt we focused on possible reorientation of separately growing surface crystals during early crystal growth. First results show clear evidence that separately growing crystals can reorient themselves as they are going to impinge each other.
A summery of previous research regarding surface crystallization is given as well as results regarding thecurrent DFG project are presented.
Overview about the surface initiated microstructure formation in glass surfaces. Samples which are exposed to a temperature treatment, can develop a crystalline microstructure above Tg at the surface. These separated crystals can be preferably oriented towards the surface of the sample. First experiments about the origin of these orientation phenomenon as well as the potentially causing mechanisms are presented and discussed within the presentation.
Fresnoite glass with excess SiO2 exhibits oriented surface crystallization, in contrast to the stoichiometric glass composition. Recent EBSD studies documented that the crystals in BTS (2BaO-TiO2-xSiO2, x=0-3) can occur in a distinct [101]-orientation perpendicular to the surface and claimed that this orientation is not a result of growth selection. During these previous studies, however, the effect of surface preparation and surrounding atmosphere during the crystallization experiments were not considered. As these parameters may influence crystal orientation, we studied the surface crystallization of a BTS glass (2BaO-TiO2–2.75SiO2) under controlled conditions with the help of light, electron and polarisation microscopy as well as EBSD. Heat treatments for one hour at 840°C of fractured BTS glass surfaces in air resulted in a large number of not-separable surface crystals. This large number of crystals can be caused by dust particles, which act as nucleation agents. As crystal growth velocity could further be influenced by humidity, our experiments are performed in a filtered and dried air atmosphere. The crystal morphology and orientation will be analysed in dependence of the sample preparation and a differing surrounding atmosphere.
Controlled oriented crystallization of glass surfaces is desired for high precision applications, since the uppermost crystal layer significantly influences the properties of the material. In contrast to previous studies, the data presented here deal with separated crystals growing at defect-free surfaces in four atmospheres with different degrees of humidity (ambient/dry air, argon and vacuum). A glass with the composition 2 BaO–TiO2–2.75 SiO2 was heat-treated at 825 °C until fresnoite (Ba2TiSi2O8) grew to a significant size. The crystal growth rate is found to increase with increasing humidity. The morphology of the crystals changes from highly distorted dendrites in the driest atmosphere (vacuum) to circular/spear-head-shaped crystals in the wettest atmosphere (ambient air), which we attribute to a decrease in viscosity of the glass surface due to water uptake. The least distorted crystals appear in the form of depressions of up to 6 µm. This has an influence on the observed crystal orientation, as measured by electron backscatter diffraction (EBSD). The pulled-in crystals change the orientation during growth relative to the flat glass surface due to an enrichment in SiO2 at the crystal fronts. This confirms that the orientation of crystals is not fixed following nucleation.
The crystallization behavior of sodium ion conductive Na2MnP2O7 glass was examined to clarify the crystallization mechanism. The formation of thermodynamically metastable phase, layered Na2MnP2O7, at the surface of the glass occurred. Heat treatment at 430 °C for 3 h lead to surface crystals of Na2MnP2O7 oriented with the (101) direction perpendicular to the sample surface. As the heat treatment temperature increased, the glass-ceramic samples deformed, and the presence numerous micro bubbles due to dissolved water was detected.
Homogeneous introduction of organic additives is a key of ceramic powder processing. Addition of organics to ceramic slurries holds advantages compared to dry processing like organic content reduction and a more homogeneous additive distribution on the particle surface.
Investigations of the alumina slurries were primarily based on zeta potential measurements and sedimentation analysis by optical centrifugation. Both methods were combined to determine a suitable additive type, amount and composition, whereas the spray drying suitability has been ensured by viscosity measurements. Granules, yielded by spray drying of such ideally dispersed alumina slurries, are mostly hollow and possess a hard shell. Those granules cannot easily be processed and can only hardly be destroyed in the following shaping step, leading to sinter bodies with many defects and poor strength and density.
The precise slurry destabilization, carried out after ideally dispersing the ceramic powder, shows a strong influence on the drying behavior of the granules and hence on the granule properties. A promising degree of destabilization and partial flocculation was quantified by optical centrifugation and resulted in improved granule properties. Spray drying the destabilized alumina slurries yielded homogeneous “non-hollow” granules without the above mentioned hard shell. Sample bodies produced of these granules exhibited a reduction of defect size and number, leading to better results for sinter body density and strength.
The positive effect of the slurry destabilization has been further improved, by exchanging the atomizing unit from a two-fluid one to an ultrasound atomizer with only minor slurry adjustments necessary. The controlled destabilization and ultrasound atomization of the ceramic slurry show excellent transferability for zirconia and even ZTA (zirconia toughened alumina) composite materials.
Carbon Capture Utilization and Storage (CCUS) is a promising technology to reach the target for reduction of CO2 emissions, e.g. from fossil-fuel operated power plants or cement mills. Crucial points for a sustainable and future-proof CCUS procedure are reliability and cost efficiency of the whole process chain, including separation of CO2 from the source, compression of CO2, its subsequent transportation to the injection site and injection into geological formations, e.g. aquifers.
Most components that are in contact with CO2-stream consist of steel. Depending on the operating conditions (e.g. temperature, pressure, and CO2-stream composition) specific suitable steels should be used. The compressed CO2-stream is likely to contain process specific impurities; small amounts of SO2 and NO2 in combination with oxygen and water are most harmful.
One approach, as currently preferred by pipeline operators, is to clean the CO2-stream to such levels, acceptable for carbon steel, commonly used as pipeline material. Another consideration would be, to use more corrosion resistant alloys for CO2-streams with higher amounts of impurities.
Due to the absence of certified benchmarks for upper limits, systematic experiments with impurities in the CO2-stream were carried out reflecting mainly transport and injection conditions.
Within the COORAL project (German acronym for “CO2 purity for capture and storage”) levels of impurities in the CO2-stream, being acceptable when using specific steels, were evaluated. Material exposure to dense or multiphase carbon dioxide (CO2) containing specific amounts of water vapor, oxygen (O2) sulfur dioxide (SO2), nitrogen dioxide (NO2), carbon monoxide (CO) can be a challenge to steels. In some situations, condensation of impurities and reaction products from the CO2 stream can occur.
CO2 saturated brine is supposed to rise in the well when the injection process is interrupted. The material selection shall ensure that neither CO2 nor brine or a combination of both will leak out of the inner tubing.
This COORAL-work was extended by a follow-up project, called CLUSTER. Here the additional influence of impurities was investigated when merging CO2 streams from different sources, combined within a “so-called” cluster.
Results are summarized within the following table regarding suitability for different parts of the process chain.
Commercially available carbon steels are suitable for compression and pipelines as long as moisture content and impurities are limited. (water 50 to 100 ppmv, SO2 and NO2 ca. 100 ppmv).
Corrosion rates increase with increasing water content.
(0.2 – 20 mm/a).
Condensation of acids and therefore droplet formation is always possible, even at low water contents.
A low SO2 content within the CO2-stream might be more important than a low water content.
Cr13-steels showed a general susceptibility to shallow pitting and pitting. So, they seem to be not suitable for CCUS applications.
Low alloyed steels showed better corrosion behavior. (predictable uniform corrosion)
For direct contact with saline aquifer fluids only high alloyed steels shall be used.
Subsurface characterization of high-strength high-interstitial austenitic steels after impact wear
(2018)
The microstructure of the subsurface after impact wear of three high-strength high interstitial austenitic steels has been analysed using transmission electron microscopy (TEM) in alloys with C and N as interstitial elements. In all cases, a nanocrystalline region followed by a transition zone and a cold-worked area are present. Additionally, microhardness and nano-scratching tests were conducted to study the wear-related properties of the impact subsurface and the base material. The results of the microstructural analysis reveal that the following mechanisms are involved during impact wear: abrasion (ploughing), microcrack formation associated with contact fatigue, entrapment and adhesion of SiO2 particles. The analysis of the wear-related properties indicates that the subsurface acts as a self-protective layer that prevents the deterioration of the substrate.
The presence of water in the surrounding atmosphere of a propagating crack has a major influence on the subcritical crack growth. While these external phenomena are well understood, there is still a lack of knowledge on the influence of structurally bound water on crack propagation. Thus, our recent study aims on the analysis of crack propagation in water bearing soda-lime silicate glasses with up to 8 wt.% water. The samples were synthesized in an internally heated pressure vessel at 0.5 GPa. Since this preparation route limits the sample sizes, standard test geometries allowing for the determination of stress intensity factors, such as double cantilever beam, are not feasible. Thus, radial cracks in the hydrous glasses were initiated by Vickers indentation and crack growth was simultaneously captured with a camera system. An automated image analysis algorithm was used for the analysis of the crack length of each single video frame. To minimize influences by atmospheric water, all experiments were conducted in a glovebox purged with dry N2. About 150 cracks per glass composition were analyzed to provide statistical significance of the Vickers-induced SCCG. The results show that structurally bound water has a major influence on SCCG by means of crack lengths, growth rates and time of crack initiation.
Glass strength and fatigue is limited by surface cracks. As subcritical crack growth (SCCG) is governed by ambient humidity, stress corrosion at the crack tip is widely accepted to be the underlying mechanism. However, as water is known to have decisive effect on glass properties and can rapidly enter the crack tip near glass region, SCCG could be affected by such water related phenomena. We tried to mimic these effects studying water dissolution and speciation, mechanical properties, and SCCG in water-bearing glasses. For this purpose, glasses up to 8 wt% water have been prepared by means of high-pressure melting of glass powder - water mixtures.
As part of this effort, SCCG in dry and hydrous commercial micros¬cope slide glass (CW = 6 wt%) was studied in double cantilever beam (DCB) geometry and sub-Tg relaxation was measured by Dynamic Mechanical Analysis (DMA).
For SCCG in ambient air (24% r.h.), SCCG was promoted by the presence of 6wt% bulk water with respect to the dry glass. On the other hand, stress intensity values, KI, required to cause slow crack growth (v < 10-6 ms-1) resemble literature findings for float glass of similar composition in liquid water, which might represent the maximum possible promoting effect of ambient water on SCCG.
For SCCG in vacuum (10-3 mbar), dissolved bulk water causes even more pronounced effects. Most strikingly, it strongly decreases the slope of the log v(KI)-curve, which is a measure of dissipated energy during fracture. A strong increase of sub-Tg relaxation with increasing water content was confirmed by DMA. As a consequence, slow crack growth occurs at KI values as measured in the dry glass whereas fast crack growth occurs at much larger KI than that of the dry glass. Kinks and shoulders shown by the inert log v(KI)-curve indicate that bulk water does not simply affect bulk mechanical properties.
Ambient water influences sub-critical crack growth (SCCG) from microscopic surface flaws, leading to stress corrosion at the crack tip. The complex influence of humidity accelerating slow crack propagation (region I) is well studied only for dry commercial NCS glass (< 1000 ppm water). To shed light on this influence, the effect of water is mimicked by studying SCCG water-bearing glasses. For this purpose, water-bearing silicate glasses of 8 wt% total water were synthesized at 0.5 GPa and compared to dry glasses. SCCG was measured in double cantilever beam geometry. For dry glasses, 3 trends in crack velocity vs. stress intensity, KI, curve were found. The slope in region I increases in the order NCS < NBS < BaCS < NZnS < NAS glass. The velocity range of region II, reflecting the transition between corrosion affected and inert crack growth (region III), varies within one order of magnitude among these glasses. The KI region of inert crack growth strongly scatters between 0.4 and 0.9 MPam0.5. For hydrous glasses, it is found that water strongly decreases Tg, form a new sub-Tg internal friction peak caused by molecular water, and makes the glasses more prone to SCCG. The observed trends will be discussed in terms of the effects of Youngs Modulus on the strain energy release rate and energy dissipation related to mechanical glass relaxation phenomena.
Environmental conditions are known to influence sub-critical crack growth (SCCG) that are released from microscopic flaws at the glass surface, leading to stress corrosion phenomena at the crack tip. The processes at the crack-tip are complex and water has been identified as a key component governing SCCG at low crack velocities (region I). In particular, the influence of humidity accelerating crack propagation is well studied for industrial soda-lime silicate glasses, which are practically free (< 1000 ppm) of dissolved water. To shed light on the corrosion process, the situation at the crack-tip is reversed in the present study as dissolved water in larger fractions is present in the glass and crack propagation is triggered in dry environment. For this purpose, water-bearing silicate glasses of up to 8 wt% total water were synthesized in an internally heated pressure vessel at 0.5 GPa and compared to dry glasses of standard glass manufacturing. SCCG was measured using the double cantilever beam technique and by Vickers indentation. For dry glasses, three trends in the crack growth velocity versus stress intensity curve were found. The slope in region I limited by environmental corrosion increases in the order sodium aluminosilicate < sodium borosilicate ≲ sodium lead silicate. The velocity range of region II reflecting the transition between corrosion affected and inert crack growth (region III), varies within one order of magnitude among the glasses. The KI region of inert crack growth strongly scatters between 0.4 and 0.9 MPam1/2. For hydrous glasses, it is found that those of low Tg are more prone to SCCG. As water strongly decreases Tg, it promotes SCCG. First results indicate that molecular water has a dominating influence on SCCG.
Premature failure of glass under load is caused by sub-critical crack growth (SCCG) originate from microscopic flaws at the surface. While SCCG is related to the humidity of the ambient atmosphere, leading to stress corrosion phenomena at the crack tip, the detailed mechanism and the effect of different network formers are still not fully understood. For more clarity, various soda silicate glasses with a second network former were investigated by double cantilever beam technique: Na2O*Al2O3*SiO2 (NAS), Na2O*B2O3*SiO2 (NBS), Na2O*PbO*SiO2 (NPbS).
Three effects on the crack growth velocity, v, versus stress intensity, KI, curves were found out. The slope in region I, which is limited by corrosion, increases in the order NAS < NBS ≲ NPbS. The velocity range of region II reflecting the transition between corrosion effected and inert crack growth (region III), varies within one order of magnitude between the glasses. The KI region of inert crack growth strongly scatters between 0.4 and 0.9 MPam1/2. For comparison, crack growth at different humidity in commercial soda lime silicate glass (NCS) was measured.
Environmental conditions are known to influence sub-critical crack growth (SCCG) that starts from microscopic flaws at the glass surface, leading to stress corrosion phenomena at the crack tip. The processes at the crack tip are complex and water has been identified as a key component governing SCCG at low crack velocities (region I). In particular, the influence of humidity accelerating crack propagation is well studied for dry industrial soda-lime silicate glasses (< 1000 ppm water). To shed light on this influence, the effect of water is mimicked by studying SCCG in water-bearing glasses. For this purpose, water-bearing silicate glasses of up to 8 wt% total water were synthesized in an internally heated pressure vessel at 0.5 GPa and compared to dry glasses. SCCG was measured using the double cantilever beam technique. For dry glasses, three trends in the crack growth velocity versus stress intensity, KI, curve were found. The slope in region I, limited by environmental corrosion, increases in the order soda-lime silicate < sodium borosilicate < barium calcium silicate < sodium zinc silicate < sodium aluminosilicate glass. The velocity range of region II, reflecting the transition between corrosion affected and inert crack growth (region III), varies within one order of magnitude among these glasses. The KI region of inert crack growth strongly scatters between 0.4 and 0.9 MPam1/2. For hydrous glasses, it is found that water strongly decreases Tg, form a new sub-Tg relaxation peak caused by molecular water, and makes the glasses more prone to SCCG. The observed trends will be discussed in terms of the effects of Youngs Modulus on strain energy release rate and energy dissipation related to glass relaxation phenomena.
Study of Polyaniline/Silicon Dioxide based Coating on Carbon Steel in Artificial Geothermal Brine
(2021)
Geothermal brines are corrosive in nature because of their salt contents and high temperatures. Therefore, they pose a major challenge to geothermal power-plants, which are mostly built of low alloyed steels, e.g., carbon steel. Carbon steel is susceptible to uniform and localized corrosion when exposed to geothermal brines having acidic-saline properties. To overcome this limitation, geothermal power plants should be built by either high alloyed materials or by integrating protection systems on carbon steel, such as coatings and inhibitors. We studied a coating system containing polyaniline/silicon dioxide basing on locally available resources that provides protection against corrosion of carbon steel and enhance the thermal resistance in geothermal environments. Here, exposure and electrochemical tests of coated carbon steels were performed in an artificial geothermal brine. The solution had a pH of 4, with the composition of 1,500 mg/L of chlorides, which is based on the chemical analysis of geothermal brine found in Sibayak, Indonesia. All exposure tests were conducted using autoclaves at 150 °C with a total pressure of 1 MPa, which was performed for up to six months to evaluate the durability of the coating system. Post-experimental analyses were performed by assessing the surface of specimens using optical and electron microscopes. On the other hand, electrochemical tests were performed for seven days at 25 °C and 150 °C to investigate the kinetics of electrochemical reactions by measuring open circuit potential and electrochemical impedance spectra. Experimental results showed the corrosion resistance of PANI/SiO2 composite coatings, where polyaniline and SiO2 play their roles as stabilizers.
Study of Polyaniline/Silicon Dioxide based Coating on Carbon Steel in Artificial Geothermal Brine
(2021)
By using the available resources in Indonesia, such as silicon dioxide and marine coating base, the PANI/SiO2 modified alkyd coating was able to protect carbon steel in a deaerated artificial geothermal water. The screening of coatings shows that the modification by adding individual pigment was not sufficient to protect carbon steel even during a short-term exposure, indicated by the discoloration after only seven days of exposure. Electrochemical tests indicated that there was no significant change in the Ecorr between the coated and uncoated carbon steel at room temperature. At 150 °C, the coated carbon steel has a lower potential than that of carbon steel, indicating that the coating is protecting carbon steel cathodically or slowing down the corrosion reaction. Finally, a long-term exposure test confirmed that the PANI/SiO2 modified coating successfully protects the carbon steel in the Sibayak artificial geothermal water up to 150 °C for 6 months.
Al2O3 has been widely used as a coating in industrial applications due to its excellent chemical and thermal resistance. Considering high temperatures and aggressive mediums exist in geothermal systems, Al2O3 can be a potential coating candidate to protect steels in geothermal applications. In this study, γ-Al2O3 was used as a coating on martensitic steels by applying AlOOH sol followed by a heat treatment at 600 °C. To evaluate the coating application process, one-, two-, and three-layer coatings were tested in the artificial North German Basin (NGB), containing 166 g/L Cl−, at 150 °C and 1 MPa for 168 h. To reveal the stability of the Al2O3 coating in NGB solution, three-layer coatings were used in exposure tests for 24, 168, 672, and 1296 h, followed by surface and cross-section characterization. SEM images show that the Al2O3 coating was stable up to 1296 h of exposure, where the outer layer mostly transformed into boehmite AlOOH with needle-like crystals dominating the surface. Closer analysis of cross-sections showed that the interface between each layer was affected in long-term exposure tests, which caused local delamination after 168 h of exposure. In separate experiments, electrochemical impedance spectroscopy (EIS) was performed at 150 °C to evaluate the changes of coatings within the first 24 h. Results showed that the most significant decrease in the impedance is within 6 h, which can be associated with the electrolyte penetration through the coating, followed by the formation of AlOOH. Here, results of both short-term EIS measurements (up to 24 h) and long-term exposure tests (up to 1296 h) are discussed.
Lock-in- and flash thermography are standard methods in active thermography. They are widely used in industrial inspection tasks e.g. for the detection of delaminations, cracks or pores. The requirements for the light sources of these two methods are substantially different. While lock-in thermography requires sources that can be easily and above all fast modulated, the use of flash thermography requires sources that release a very high optical energy in the very short time.
By introducing high-power vertical cavity surface emitting lasers (VCSELs) arrays to the field of thermography a source is now available that covers these two areas. VCSEL arrays combine the fast temporal behavior of a diode laser with the high optical irradiance and the wide illumination range of flash lamps or LEDs and can thus potentially replace all conventional light sources of thermography.
However, the main advantage of this laser technology lies in the independent control of individual array areas. It is therefore possible to heat not only in terms of time, but also in terms of space. This new degree of freedom allows the development of new NDT methods. We demonstrate this approach using a test problem that can only be solved to a limited extent in active thermography, namely the detection of very thin, hidden defects in metallic materials that are aligned vertically to the surface. For this purpose, we generate destructively interfering thermal wave fields, which make it possible to detect defects within the range of the thermal wave field high sensitivity. This is done without pre-treatment of the surface and without using a reference area to depths beyond the usual thermographic rule of thumb.
Borosilicate glasses (16Na2O–10B2O3–74SiO2, NBS) with water contents up to 22 mol% H2O were prepared to study the effect of water on structural relaxation using DTA, viscometry and internal friction measurements. The results show that the glass transition temperature Tg of DTA and the isokom temperature T12, of viscometry are in excellent agreement, confirming the equivalence of enthalpy and viscous relaxation for NBS glass. Combining Tg data with water speciation data demonstrates that OH groups are mainly responsible for the decrease of Tg with increasing hydration, while molecular water plays only a minor role. Internal friction spectra at 7.125 Hz confirm the decisive influence of water on mechanical relaxation. The temperature range of α-relaxation (glass transition) strongly decreases while two β-relaxation peaks (sub-Tg) progressively appear with increasing water content. A high temperature β-relaxation peak, attributed to the presence of OH groups, shifts from 670 to 450 K as total water content increases from 0.01 to 5 wt%. A low temperature β-relaxation peak, attributed to molecular water, appears at 380 K and 330 K in glasses containing 3 and 5 wt% H2O, respectively. These findings suggest that relaxation mechanism of different hydrous species at low temperature may contribute to fatigue of stressed glasses.
Dissolved water has major impact on the physical and chemical properties of phosphate glasses. In the present study we have investigated the structural response to water incorporation for glasses in the system Li2O–MgO–Al2O3–P2O5.
Glasses containing 0–8 wt% H2O were synthesised at 500 MPa confining pressure in internally heated gas pressure vessels at 1323 K (LMP, Al-poor glass) and 1423 K (LMAP, Al-enriched glass). Water contents of glasses were determined by pyrolysis and subsequent Karl-Fischer titration (KFT) and/or by infrared spectroscopy. Density varies nonlinearly with water content implying large structural changes when adding up to 2 wt% H2O to the dry glass. Glass Transition temperatures measured by differential thermal analysis (DTA) continuously decrease with water content. The trend can be explained by depolymerisation of the phosphate network. Near-infrared spectroscopy shows that even in Al poor glasses only a minority of dissolved water is present as H2O molecules, but the largest amount is present as OH Groups formed by hydrolysis of P–O–P bonds. The network is stabilised by aluminium which is predominantly six-coordinated in these glasses as shown by 27Al MAS NMR spectroscopy. With increase of Al in the glasses, breaking up of the Phosphate network through hydrolysis is depressed, i.e. much lower OH Contents are formed at same total water content.
Network depolymerisation upon addition of H2O is evident also from 31P MAS NMR spectroscopy. While Phosphate tetraheda are crosslinked by two to three bridging oxygen in dry glasses, diphosphate Groups are dominant in glasses containing 8 wt% H2O.
Subjecting metallic glasses repeatedly to liquid nitrogen temperature has become a popular method to homogeneously rejuvenate the material. Here we reveal the atomic-scale structural dynamics using in- situ x-ray photon correlation spectroscopy (XPCS) during and after cryogenic cycling of a Zr-based metallic glass in two structural states (plate and ribbon). Heterogeneous structural dynamics is observed at 300 K that changes to monotonic aging at 78 K. It is found that cryogenic cycling homogenizes the relaxation time distribution. This effect is much more pronounced in the ribbon, which is the only structural state that rejuvenates upon cycling. We furthermore reveal how fast atomic-scale dynamics is correlated with longtime structural relaxation times irrespective of the structural state, and that the ribbon exhibits unexpected additional fast atomic-scale relaxation in comparison to the plate material. A structural picture emerges that points towards heterogeneities in the fictive temperature as a requirement for cryogenic energy storage.
The main deference between high entropy alloys and conventional alloys is the solid solution strengthening effect, which moves from a single element to a multi-element matrix. Little is known about the effectiveness of this effect at high temperatures. Investigation of temperature dependent solid solution strengthening in single phase multicomponent alloys with medium and high entropy using creep testing was performed. Creep tests carried out on single phase SX CrCoNi , CrMnFeCoNi and pure Ni from 700 to 1200 °C excluding oxidation, grain boundaries and multiphase effects. It was found that the influence of solid solution strengthening of CrCoNi and CrMnFeCoNi increases by decreasing temperature (1200 to 700 °C), and dislocation forests occur in CrCoNi and CrMnFeCoNi in comparison to pure Ni.
Dry pressing of ceramic materials requires homogeneously soft granules with good flowability to allow rapid die filling and to avoid packing defects. Spray-drying granulation appears to be the best method for obtaining granules with high flowability in industrial scale. But, strength reducing internal microstructural defects caused by spray-dried granules with hollow and hard shells are often observed using nano and/or multi-component starting powders.
Using the example of a ZTA composite, the potential of slurry optimization, ultrasound atomization and infrared drying for better granule properties and compaction behavior were investigated. Starting granules produced in a conventional spray dryer (Niro, Denmark) with a two fluid nozzle showed typical defects like large central pores and dimples.
The early step of slurry preparation already possesses an essential optimization possibility in the form of stability adjustments. Granule compaction was clearly improved upon a specific reduction in slurry stability. The second optimization opportunity to improve the granule quality was the atomization step. Implementation of an ultrasound atomizing unit into the conventional spray dryer positively affected granule size distribution and therefor flowability and as well granule yield. But, a combination of both process optimizations delivered the best sinter bodies with highest density and strength due to further reduction in maximum size and fraction of pores.
As last step of a spray drying process, the drying is the focus of further investigations. A current setup implying a spray dryer prototype utilizes stacked infrared heater in a countercurrent setup delivering a further increase in granule yield and enduring spraying process stability.
This work presents strong evidence for structural damage accumulation as a function of shear strain admitted by shear bands in a Zr-based bulk metallic glass. Analyzing the shear-band structure of shear- band segments that experienced shear strains covering four orders of magnitude with high-angle annular dark field transmission electron microscopy (HAADF-STEM) reveals strongly scattered data with on overall trend of increasing local volume dilatation with increasing shear strain. Locally, however, a variety of trends is observed, which underlines the strong heterogeneity of structural damage in shear bands in metallic glasses.
Statistical analysis of Vickers induced subcritical crack growth in soda-lime silicate glasses
(2018)
Studies on Vickers induced subcritical crack growth are controversially discussed since the stresses that drive the crack growth are distributed three dimensionally within the material and cannot be retraced by available methods. Hence, empirical approaches are used to calculate mechanical material parameters such as the stress intensity factor KI. However, the results of these approaches show large deviations from those measured by standardized techniques such as double cantilever beam (DCB) or double cleavage drilled compression (DCDC). Yet, small specimen sizes and low specimen quantities can prevent the execution of DCB and DCDC measurements. Here we present an approach that is based on a statistical analysis of Vickers induced radial cracks. For this purpose more than 150 single radial cracks were analyzed. The cracks were generated in a commercial soda-lime silicate glass. The experiments were performed in a glovebox purged with dry nitrogen gas to minimize the influence of atmospheric water on crack growth. The temporally resolved evolution of the radial cracks was monitored in-situ using an inverted microscope equipped with a camera system directly below the Vickers indenter. An automated image analysis software was used to determine the crack length over time. The data show that the crack propagation and thereby the crack velocities are not uniformly but statistically distributed. These findings allow, using the statistical mean value of the distributions in combination with DCB data, a precise formulation of KI for each measured crack length.
Due to the stochastic nature of crack nucleation by Vickers indentation, a statistical analysis of propagation rates of 185 radial cracks was performed. Crack growth was observed directly using a video camera with high Image acquisition rate. It is found that propagation rates are controlled by the environmental reactions at the crack-tip shortly after their initiation (< 1 s). Calibration of the stress intensity KI showed that the residual stress factor χ and the exponent n of the equation KI==χPc−n (with P==load and c==crack length) are broadly distributed among the 185 analyzed cracks, ranging from 10−16 to 104 and from 0.1 to 5, respectively. For the most frequent crack, the equation KI==0.052Pc−1.47 holds. The results show that correlations of indentation-induced crack length to stress intensity necessitate the use of statistical significant data that are calibrated by the environmental reactions at the crack-tip.
Among the very few techniques to localize hydrogen (H) at the microscale in steels, Time-of-flight secondary ion mass spectrometry (ToF-SIMS) was proven to be a reliable tool. The necessity to detect hydrogen stems from its deleterious effects in metals, that are often used as structural components and to obtain better understanding of the underlying metallurgical mechanisms of hydrogen embrittlement (HE) which are still unclear.
Austenitic stainless steels are nowadays commonly used in a wide variety of application, from hydrogen transport and storage facilities to petrochemical and offshore applications where they are exposed to aggressive environments and therefore prone to HE. One of the greater risks in the austenitic class is the embrittlement of the material due to the instability of the γ austenite and its transformation into a brittle α martensitic phase. This transformation takes place due to the local stresses that are induced by the uptake of hydrogen during service. Nonetheless, it was shown that this transformation can occur as an artefact during SIMS analysis itself where Cs-sputtering is necessary not only to remove surface contaminations but mainly to enhance H/D secondary ion yield.
In the following contribution we show the influence of different sputtering conditions on AISI 304L austenitic stainless steel in order to distinguish the artefact from the hydrogen induced transformation. The material was charged electrochemically in a deuterium based electrolyte. Deuterium (D) must be in these experiments as a replacement for hydrogen which cannot be used because adsorbed hydrogen superimposes hydrogen originating from charging the sample in the SIMS images. ToF-SIMS analyses were conducted by ToF SIMS IV (IONTOF GmbH, Münster, Germany). The experiments were carried out on deuterium charged and non-charged samples. The structural characterization was carried out by SEM and EBSD examinations before and after charging, both with a Leo Gemeni 1530VP field-emission scanning electron microscope and a Zeiss Supra 40 instrument (Carl Zeiss Microscopy GmbH, Oberkochen, Germany). The results showed that the use of 1keV Cs+ beam induces stacking faults while higher sputter beam energies results in γ→α transformation.
One of the most ubiquitous and important defects in solids is oxygen. Knowledge about the solubility and diffusivity of oxygen in materials is crucial to understand a number of important technological processes, such as oxidation, corrosion, and heterogeneous catalysis. Density-functional theory calculations of the thermodynamics and kinetics of oxygen in cobalt show that oxygen diffusing into the two close-packed phases, namely α (hcp) and β (fcc), strongly interacts with vacancies.We observe the formation of oxygen split-vacancy centers (V-Oi-V) in both phases, and we show that this defect complex exhibits a similar migration energy barrier to the vacancy and oxygen interstitials. In contrast to the vacancy and oxygen interstitials, the oxygen split-vacancy centers exhibit an anisotropic strain field that couples to applied stress, making it possible to observe them through an internal friction experiment on quenched cobalt.
For more than half a century, spinodal decomposition has been a key phenomenon in considering the formation of secondary phases in alloys. The most prominent aspect of the spinodal phenomenon is the lack of an energy barrier on its transformation pathway, offering an alternative to the nucleation and growth mechanism. The classical description of spinodal decomposition often neglects the influence of defects, such as grain boundaries, on the transformation because the innate ability for like-atoms to cluster is assumed to lead the process. Nevertheless, in nanocrystalline alloys, with a high population of grain boundaries with diverse characters, the structurally heterogeneous landscape can greatly influence the chemical decomposition behavior. Combining atom-probe tomography, precession electron diffraction and density-based phase-field simulations, we address how grain boundaries contribute to the temporal evolution of chemical decomposition within the miscibility gap of a Pt-Au nanocrystalline system. We found that grain boundaries can actually have their own miscibility gaps profoundly altering the spinodal decomposition in nanocrystalline alloys. A complex realm of multiple interfacial states, ranging from competitive grain boundary segregation to barrier-free low-dimensional interfacial decomposition, occurs with a dependency upon the grain boundary character.
The fracture surface of slow and continuous crack propagation during environmental stress cracking of a semicrystalline polyethylene exhibits isotropic roughness exponents at the local scale but resolved across the macroscopic fracture surface a clear position dependence is found. The spatially resolved roughness exponent admits values in the range between 0.1 and 0.4, demonstrating nontrivial exponents in the small length-scale regime. Instead, they vary across the fracture surface according to the stress-state distribution, which suggests that the exponents are intimately linked to the locally dominating dissipation processes during craze cracking.
Ziel von fraktographischen Untersuchungen ist die Auswertung von makroskopischen und mikroskopischen Bruchflächenmerkmalen und daraus resultierend die Bestimmung des Bruchmechanismus eines Bauteils aus einem Schadensfall. Die Grundlage für die Bewertung von Bruchmerkmalen stammt dabei aus zuvor durchgeführten eigenen Vergleichsversuchen und aus der Literatur. Eine fraktographische Analyse kann sehr aufwändig sein und setzt in jedem Fall erhebliches Erfahrungswissen voraus. Im IGF-Vorhaben „iFrakto“ wird Software entwickelt, die auf Basis von digitalisiertem Expertenwissen, Machine Learning und unter Zuhilfenahme von Topographie-Daten anhand von REM-Aufnahmen Bruchmerkmale und Bruchmechanismen quantitativ bestimmt, s. Abbildung. Solche Software soll mittelfristig direkt am REM oder bei der nachfolgenden Auswertung wissensbasierte Vorschläge für die Bewertung von Bruchflächen liefern. Als Grundlage hierfür wurden Ringversuche unter Fraktograph/inn/en durchgeführt, um die Wissensbasis zu schaffen, die praxisrelevanten Anforderungen an solche Software abzufragen und erste Praxistests durchzuführen. Die bisherigen Ergebnisse werden vorgestellt und die Relevanz der Auswertestrategie und unterschiedlicher Sensordaten bewertet.
Small-angle Scattering Data Analysis Round Robin: anonymized results, figures and Jupyter notebook
(2023)
The intent of this round robin was to find out how comparable results from different researchers are, who analyse exactly the same processed, corrected dataset.
This zip file contains the anonymized results and the jupyter notebook used to do the data processing, analysis and visualisation. Additionally, TEM images of the samples are included
Efficient studies of scarce or expensive materials require material saving processes. Therefore, a high yield concept for small batch preparation of ready-to-press powder is exemplarily presented for yttria stabilized nano-zirconia (d50 < 50 nm).
The concept involves small batch preparation in an ultrasound resonator, dispersant selection based on zeta potential measurements, evaluation of slurry stability using an analytical centrifuge, and preparation of ready-to-press powder by freeze drying.
Freeze drying offers key advantages. Process efficiency and high yield above 95 % are independent of sample size. The dried product does not require further mechanical treatment like milling or grinding. Side effects like migration of additives are avoided.
An optimized freeze drying process tolerates slurries with moderate stability. Thus, efforts for slurry development can be reduced. Generally, identifying a suitable dispersing agent requires only 3-5 zeta potential measurements. Slurry stability is rechecked using an analytical centrifuge, which also accounts for steric stabilization. An ultrasound resonator is used to disperse the powder without contamination, which becomes critical for small batches.
The described route is exemplarily presented for the development of an additive recipe for nano-sized zirconia powder, targeting for good pressing behavior and high green density. Therefore, a variety of binding and lubricating agents were tested.
Following the presented route, 80 g zirconia powder were sufficient to conduct a study including slurry development and five sample sets with varying composition, each set comprising five discs (d = 20 mm and h = 2 mm).
The granules commonly yielded by spray drying procedures exhibit a hard shell and an irregular, dimpled shape, which is often described as donut-like morphology. Sintered parts produced from such granules suffer from microstructural defects and reduced mechanical properties resulting from these disadvantageous granule properties.
Using the example of alumina, zirconia and zirconia-toughened alumina (ZTA) batches, this paper shows that the morphology of the granules can be tuned by adjusting slurry stability. High zeta potential is essential to optimally disperse the particles. But to achieve spherical and soft granules the electrostatic repulsion forces between the particles should be reduced before spray granulation. Electrostatic repulsion forces were changed with the addition of nitric acid. Measurements of zeta potential and viscosity, as well as sedimentation investigations with an optical centrifuge were used for precise slurry assessment as a major precondition for optimal and reproducible adjustment of slurries before spray drying. Sedimentation analysis using an optical centrifuge was performed to investigate different influences like that of additive composition, solids content or pH-value on the sedimentation behavior. Adequately flocculated slurries lead to homogeneous, soft granules that can be easily deformed and pressed. The fraction of donut-shaped particles and the rigidity of granules were reduced. Consequently, the sintered parts produced from these granulates show improvements regarding porosity, pore size distribution, sintered density and biaxial strength.
The use of high niobium alloyed cast iron alloys is a relatively new approach in which the niobium addition intends to improve the properties of the material by the precipitation of hard niobium carbides during solidification. Steels can be replaced by ductile cast iron in some rolling applications, such as gears and cams, in order to reduce material costs. The aim of this work is to evaluate ductile iron alloyed with 1 weight percent (wt.%) niobium for the as cast specimens and with 1.8 wt.% and 2.4 wt.% niobium for the austempered specimens under lubricated slip-rolling tests using mixed/boundary conditions in an Amsler-type machine. Austempered ductile iron (ADI) alloyed with 1 wt.% chromium, or Carbidic ADI, was tested for comparison. For the as cast conditions, the niobium addition resulted in an increase of wear resistance owing to the low contact pressure of these tests. However, for the austempered specimens, the best performance was found for unalloyed ADI. The main factor acting in the initiation and propagation of cracks in ductile iron is the presence of the graphite nodules. The coarse carbides also contributed to the initiation of cracks and spalling of the material.
Sintern
(2018)
Unter Sintern versteht man allgemein die Überführung eines aus Pulver geformten Rohlings in ein Formteil mit angestrebter Mikrostruktur bzw. gewünschten Gebrauchseigenschaften durch thermische Prozesse. In diesem Kapitel werden die Grundlagen zu Triebkräften und Kinetik sowie die prinzipiellen Mechanismen für Stofftransport und Verdichtung vorgestellt. Die verschiedenen Sintermechanismen Festphasensintern, Flüssigphasensintern und Reaktionssintern werden erläutert und mit einem Überblick über Drucksinterverfahren ergänzt. Abschließend wird ein Überblick über technologische Einflussfaktoren auf die Sinterung gegeben.
Biomaterials for bone replacement and grafting should possess sufficient strength, be bioresorbable and demonstrate osteoconductive and osteoinductive properties. However, resorption of modern materials for bone grafting (hydroxyapatite (HA) and tricalcium phosphate (TCP)) is reported, in some cases, to be not enough, this is why the search for more soluble compounds compared to HA and TCP looks very perspective. A possible way to increase ceramics solubility leads to partial substitution of Ca2+ -ions in Ca3(PO4)2 by alkali cations, like Na+ or/and K+. Improvement of solubility stems from decreasing lattice energy of a substituted phase, as well as the increase in hydration energy of the ions releasing from the phase to ambient solution. From this viewpoint, bioceramics based on compositions from Ca3(PO4)2 - CaKPO4 - CaNaPO4 ternary system seems to be prospective for bone replacement and grafting in the sense of resorption properties. At the same time, one should bear in mind that solubility level (resorbability) is governed not only by reduction of lattice energy but also by microstructure features. Grain sizes and porosity contribute much to dissolution rate making the study of sintering of the ceramics mentioned above highly important.
To control Ca3(PO4)2 - CaKPO4 - CaNaPO4 based ceramic microstructure it is necessary to know possible phase transformations in the system and the way to manage microstructure by sintering schedule or sintering process.
In this work, an isothermal section for phase diagram of Ca3(PO4)2 - CaKPO4 - CaNaPO4 ternary system is studied with several techniques. According to the XRD of quenched samples, this phase triangle has four single-phase areas at 1200˚C (Figure 1). It was shown that single-phase CaK0.6Na0.4PO4 cannot be sintered to full-dense ceramics by conventional sintering regardless time-temperature schedule. Two-step sintering technique, beneficial in the case of HA-ceramics, was unsuccessful in all cases of calcium-alkali phosphate compositions. However, field-assisted sintering techniques like, e.g. Spark Plasma Sintering (SPS), can overcome this problem due to significant impact on grain boundary diffusion. In connection with this fact, grains grow much slower retaining sintering process in a pore control regime. In this work CaK0.6Na0.4PO4 low-porous ceramics was also fabricated by FAST-methods of sintering. Moreover, other alternative sintering techniques, such as reaction sintering, may be useful in accelerating ions diffusion but stopping excessive grain growth.Strength properties of ceramics were evaluated by B3B-testing, micro- and nanoindentation techniques. Fracture toughness also becomes higher with potassium content increase, guiding porosity level.
Resorption properties of sintered ceramics were studied in different solutions with pH=5 and 7.4.
Acknowledgements. The research of sintering processes in calcium phosphate materials were funded by RFBR according to the research project № 18-33-00974.
Since decades electric contacts based on silver metallization pastes are key components of photovoltaics and advanced microelectronics. For the metallization of commercial Si solar cells, high conductive silver glass pastes are cost effectively applicated by screen printing. Nevertheless, silver pastes are still one of the most crucial and expensive none Si materials in solar cells. Ever shorter time to market as well as increasing demands on reduced Ag consumption and line width require the targeted development of silver-glass-pastes with increased sinter ability and electrical conductivity. As a main difficulty, however, the liquid phase sintering of silver glass pastes is poorly understood so far.
In the present study, the influence of different network modifier in alkali-zinc-borate paste glasses on liquid phase sintering of silver-glass-pastes was investigated. Low melting X2O-ZnO-B2O3 glasses with X = Na, Li and Rb (abbr. LZB, NZB, and RZB) were utilized to prepare silver-glass-composites containing 30 %Vol glass. Shrinkage behavior of the silver-glass-composites compared with that of pure silver and pure glass powder compacts was studied with heating microscopy. The powder compacts were uniaxially pressed and heated at 5 K/min to the glass softening temperature. Glass transformation temperature and viscosity of the glasses were respectively measured with dilatometry and rotational viscometry. The thermal behavior of the pure glasses was analyzed with thermal analysis. Additionally, the contact angle of glass on pure silver foil was determined by means of heating microscopy between room temperature and 830 °C.
Thermal analysis of the alkali-zinc-borate-glasses under study has shown transformation temperatures between 450 °C (RZB), 460 °C (LZB) and 465 °C (NZB). For all glasses crystallization was found to start approximately at about 550 °C. However, different peak areas hint on a different degree of crystallization. Conformingly, the sintering behavior, measured in terms of area shrinkage, significantly differed for the silver-pastes under study. For silver-pastes with NZB or LZB-glass, sintering starts at 464 °C for NZB Ag pastes and at 451 °C for LZB Ag pastes and ends at 597 °C for NZB Ag paste and at 594 °C for LZB Ag paste. The sintering of the RZB Ag paste proceeds between 426 °C and 703 °C. The final densification was retarded possibly due to crystallization or swelling. The low sinter onset at 426 °C seems to correlate with the good wetting behavior of the RZB glass. Thus, the lowest apparent contact angle between the just densified powder compact sintered at a silver substrate was found for this glass. Moreover, microstructure analyses of the various composites indicate differences in silver dissolution and reprecipitation.