5 Werkstofftechnik
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To apply high-entropy alloys (HEA) of the CrMnFeCoNi family in challenging atmospheres, their degradation behavior under harsh environments needs to be investigated. Oxidation studies to HEAs have not been extensively investigated and most of them are concentrated on environments like synthetic air, laboratory air, CO/CO2, O2 and H2O atmospheres. Main corrosion products which were identified after aging times of up to 100 h are Mn2O3 (≤800°C) and Mn3O4 (≥800°C).
Another corrosive medium in high temperature applications is SO2, which preferentially forms sulfides on commercial steels for example. These can be occurred both in the oxide layer and at the oxide/metal interface. For instance, on Fe-Cr based alloys sulfides (Cr5S6) were detected along grain boundaries and their number increases with exposure time and Cr-content in the alloy. These sulfides show an increased hardness, compared to the bulk alloy, and cause an embrittlement of the grain boundaries. This is a serious material degradation phenomenon, now addressed for the case of HEAs.
In the present study metal sulfides were identified after corrosion of the HEA CrMnFeCoNi alloy in an Ar-0.5vol.%SO2 atmosphere at 800°C for 24 h, 48 h, 96 h and 192 h exposure time. After all three duration times, a thin non-protective Cr2O3 layer has formed at the oxide/alloy interface. At the gas side a thick Mn3O4 layer with local voids containing sulfur could be detected by SEM-EDS analysis. Furthermore, S precipitates could be detected in the bulk material near the surface. These sulfides were characterized in detail by scanning and transmission electron microscopy. Based on these results, a model for grain boundary sulfidation of high-entropy alloy CrMnFeCoNi is discussed.
Alkali and alkaline earth chlorides are discussed as heat storage media and are characterized by their low price and high availability. Disadvantages are a high corrosion rate and formation of Cr6+ ions in the melt, as observed in various binary chlorine salt melts. In our work the system NaCl-KCl-MgCl2 is considered. The storage capacity in this salt system is between 2 and 3 MWh per 10 t salt, depending on composition, melting temperature and working temperature. At the same time the system offers a eutectic line, which allows a high variance of the composition and possibly different corrosion rates can be observed. Corrosion tests in melts were carried out and the corrosion layers investigated. The tests with chloride melts on 12% Cr steel show an inner corrosion zone of up to 40 µm depth after 96 hours. The corrosion mechanisms and potential solutions are discussed.
This contribution provides current findings regarding materials susceptibility for CCUS applications. Basing on results gathered in 2 German long-term projects (COORAL and CLUSTER) suitable materials are introduced as well as dominating impurities of the CO2 stream and corrosion mechanisms. Investigations cover the whole CCUS process chain and provide the following recommendations for certain parts. 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.
Fiber-reinforced-polymers (FRPs) are in current research focus in the lightweight construction industry, because of their extraordinary characteristics (stiffness and strength-to-density relation). The structure of polymer matrix and the interaction with reinforcement are crucial for optimization of the mechanical and thermal properties of FRPs. Due to the macromolecular chain structure, the mechanical properties of a polymer strongly vary with temperature: Below the glass transition, the chain segments of a polymer are “frozen”. Regarding fracture, the total changed energy during fracture if only dissipates for the generation of the new surfaces. However, in the region of the glass transition, the polymer chain segments start to get “unfrozen”, and the energy is not only required for generating new surfaces, but also for irreversibly deformation. This irreversible deformation is affected by the global temperature and the local temperature near the crack tip, which is affected by the local strain rate and crack propagation velocity.
Hence, in this research project, the irreversible deformation of neat and reinforced polymers will be controlled by changing the global temperature as well as the local temperature. With using different fracture experiments, the amount of energy required for creating new surfaces and for the irreversible deformation will be separated. The fracture tests include the conventional tensile test, the macroscopic peel test and the single fiber peel – off test.
Crack propagation in polymers: Separation of surface energy and irreversible deformation energy
(2019)
Fiber-reinforced-polymers (FRPs) are in current research focus in the lightweight construction industry, because of their extraordinary characteristics (stiffness and strength-to-density relation). The structure of polymer matrix and the interaction with reinforcement are crucial for optimization of the mechanical and thermal properties of FRPs. Due to the macromolecular chain structure, the mechanical properties of a polymer strongly vary with temperature: Below the glass transition, the chain segments of a polymer are “frozen”. Regarding fracture, the total changed energy during fracture if only dissipates for the generation of the new surfaces. However, in the region of the glass transition, the polymer chain segments start to get “unfrozen”, and the energy is not only required for generating new surfaces, but also for irreversibly deformation. This irreversible deformation is affected by the global temperature and the local temperature near the crack tip, which is affected by the local strain rate and crack propagation velocity.
Hence, in this research project, the irreversible deformation of neat and reinforced polymers will be controlled by changing the global temperature as well as the local temperature. With using different fracture experiments, the amount of energy required for creating new surfaces and for the irreversible deformation will be separated. This poster is the summary of the first part of the whole project. In the first part, the basic crack propagation theory for neat polymers is established and the special fracture experiment sample is prepared and tested at room temperature. In addition, the fracture experiment at room temperature is validated numerically.
The aim of the task area Materials Data Infrastructure (TA-MDI) of the consortium Materials Science and Engineering (MatWerk) of National Research Data Infrastructure (NFDI) is to shape scientific datasets obtained through the Participant Projects (PPs) from a data management perspective conforming to the FAIR principles, making use of the FAIR Digital Object (FAIR DO) concept, including structured metadata and storage solutions. As an example, they apply PP18 (BAM) as a use case to demonstrate the proposed technical workflow.
Creep and fracture behavior of conventionally and additively manufactured stainless steel 316L
(2020)
A critical task within the frame of establishing process-structure-property-performance relationships in additive manufacturing (AM) of metals is producing reliable and well-documented material behavior’s data and knowledge regarding the structure-property correlation, including the role of defects. After all, it represents the basis for developing more targeted process optimizations and more reliable predictions of performance in the future. Within this context, this contribution aims to close the actual gap of limited historical data and knowledge concerning the creep behavior of the widely used austenitic stainless steel 316L, manufactured by Laser-Powder-Bed-Fusion (L-PBF). To address this objective, specimens from conventional hot-rolled and AM material were tested under application-relevant conditions according to existing standards for conventional material, and microstructurally characterized before and after failure. The test specimens were machined from single blocks from the AM material. The blocks were manufactured using a standard scan and build-up strategy and were subsequently heat-treated. The creep behavior is described and comparatively assessed based on the creep lifetime and selected creep curves and characteristic values. The effect of defects and microstructure on the material’s behavior is analyzed based on destructive and non-destructive evaluations on selected specimens. The AM material shows shorter creep lives, reaches the secondary creep stage much faster and at a lower strain, and features lower creep ductility compared to its conventional counterpart. The creep damage behavior of the AM material is more microstructure than defect controlled and is characterized by the formation and accumulation of single intergranular damage along the whole volume. Critical features identified are the grain morphology and the grain-boundary as well as the dislocation’s density. Micro-computed tomography (µCT) proves to be an alternative to metallography to analyze the creep damage.
The current lack of quantitative knowledge on processing-microstructure–property relationships is one of the major bottlenecks in today’s rapidly expanding field of additive manufacturing. This is centrally rooted in the nature of the processing, leading to complex microstructural features. Experimentally-guided modeling can offer reliable solutions for the safe application of additively manufactured materials. In this work, we combine a set of systematic experiments and modeling to address creep anisotropy and its correlation with microstructural characteristics in laser-based powder bed fusion (PBF-LB/M) additively manufactured Inconel-738LC (IN738LC). Three sample orientations (with the tensile axis parallel, perpendicular, and 45° tilted, relative to the building direction) are crept at 850 °C, accompanied by electron backscatter secondary diffraction (EBSD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) investigations. A crystal plasticity (CP) model for Ni-base superalloys, capable of modeling different types of slip systems, is developed and combined with various polycrystalline representative volume elements (RVEs) built on the experimental measurements. Besides our experiments, we verify our modeling framework on electron beam powder bed fusion (PBF-EB/M) additively manufactured Inconel-738LC. The results of our simulations show that while the crystallographic texture alone cannot explain the observed creep anisotropy, the superlattice extrinsic stacking faults (SESF) and related microtwinning slip systems play major roles as active deformation mechanisms. We confirm this using TEM investigations, revealing evidence of SESFs in crept specimens. We also show that the elongated grain morphology can result in higher creep rates, especially in the specimens with a tilted tensile axis.
This presentation shows some experimental results of the characterization of the creep behavior of LPBF 316L, which has been poorly studied and understood to date. The presentation includes results regarding the mechanical properties, the initial microstructural state and its evolution under loading, and the damage mechanism. This work was done within the BAM focus area materials project AGIL. As a benchmark to assess the material properties of the LPBF 316L, a conventionally manufactured variant was also tested.
In Germany spent nuclear fuel and high level radioactive waste is stored in interim storage containers with double lid systems. Those lids are equipped with metal seals (e.g. Helicoflex®) that ensure the safe enclosure of the inventory. The used metal seals consist of three components as can be seen in the cross-sectional view in Figure 1. The innermost part is a helical spring that is surrounded by an inner jacket made of stainless steel. The outer jacket that is made of a softer material which in case of assembly in the aforementioned storage containers is silver or aluminum (i.e. Al 99.5). During application the seal is compressed and due to the restoring force of the helical spring, the outer jacket is plastically deformed and adapts to the sealing surface. Hence, leakage paths are closed and the sealing function is generated. In Germany the above-mentioned containers are licensed for up to 40 years of interim storage, which in case extended storage becomes necessary before a final repository is available will have to be extended to even longer periods. Therefore, the evaluation of the long-term behavior of the seals is necessary, taking into account storage conditions, decay heat and possible mechanical loads as well.
At Bundesanstalt für Materialforschung und –prüfung (BAM) long-term investigations are being conducted in which seals are assembled in test flanges and aged at temperatures ranging from room temperature to 150°C for accelerated aging. The aged seals are tested semi-annually (after the first 6 months in which the seals are tested more frequently) regarding the sealing performance, the remaining seal force, and the useable resilience upon decompression. Results of these investigations have been published over the past years (e.g. Grelle, Wolff, Probst, Jaunich, & Völzke, 2017; Völzke, Wolff, Probst, Nagelschmidt, & Schulz, 2014). It was found that the seal force and the useable resilience decrease with time and temperature, which is in agreement with the result of other studies (Sassoulas et al., 2006; Wataru et al., 2016) as well. Geometry change of the outer jacket has been identified as the main reason for this seal behavior. At the prevailing operating temperatures and stresses the aluminum is subjected to creep deformation leading to a thinning of the outer jacket. Since the seal groove depth remains unchanged the helical spring expands, which in turn leads to a decrease of the generated spring and seal force.
Although the main reason for the change of seal parameters over time and temperature is known, a detailed characterization of the seal behavior and a reliable prediction of the parameter development for aging times that exceed the experimental time frame have not been possible, yet.
For deeper understanding of the aging processes, an Investigation program, which is covered in this contribution, is conducted at Bundesanstalt für Materialforschung und –prüfung (BAM) that focusses on the behavior of the aluminum jacket and its influence on the long-term sealing performance. The program investigates properties of material samples as well as the behavior of the seal as a component.
Original sheet material of the same aluminum that is used for manufacturing of the seals is investigated in compression creep tests. For this, a DMA (dynamic mechanical analysis) machine is employed (here used for static tests) that allows for a measurement of the specimens deformation under forces of up to 500 N. The advantage of this method is that the original material can be tested in the same shape as used for the seals which is 0.5 mm thick sheet material. For investigation of tensile creep standard specimens are used, that were machined from surrogate material of the same composition and annealing condition.
Furthermore, aluminum seals that are cut into smaller segments are assembled in flanges and placed in heating chambers at temperatures ranging from 23°C to 150°C. After different periods of time from 3 days to 300 days the segments are taken out of the flanges and are investigated, thus giving information on different states of aging. Measurements of the development of the seal contact width and the aluminum jacket thickness are done with an optical microscope. Further investigations on the segments will include metallography and hardness measurements.
From the detailed material and component behavior including the results of the long-term seal force and useable resilience investigations a better understanding of the overall seal behavior can be gained. The aim is to contribute to the development of material models and analytical approaches for the prediction of the sealing behavior in dependence of time and temperature.
In Germany spent nuclear fuel (SNF) and high level radioactive waste (HLW) are stored in interim storage containers with double lid systems. Those lids are equipped with metal seals (e.g. Helicoflex®) that ensure the safe enclosure of the inventory. Being licensed for up to 40 years of interim storage the evaluation of the long-term behavior of the seals is necessary, taking into account storage conditions, decay heat and possible mechanical loads.
Porosity in single-crystal nickel-base superalloys is removed by hot isostatic pressing (HIP) at temperatures above gamma’-solvus where the material is very soft and ductile. For example, single-crystal nickel-base superalloy CMSX-4 is HIPed at temperature 1288 °C, which is slightly higher than the gamma’-solvus temperature of this alloy equal to about 1280 °C. It is assumed that pore shrinking during HIP is mostly due to dislocation creep. Such a modelling of HIP of CMSX-4 was started in our group on the base of results of creep tests of [001] single-crystals at 1288 °C [1]. However, it was found later [2] that the alloy CMSX-4 shows very strong creep anisotropy at 1288 °C. Therefore, for calibration of the creep law, creep tests of different orientations under different stress levels are required at the HIP temperature. This was the main task of present work. Single-crystals of CMSX-4 of axial orientations [001], [011], [123] and [111] were cast by VIAM Moscow and tested by BAM Berlin under creep conditions at 1288 °C and stress levels between 4 MPa and 16 MPa. At all stress levels, the creep rate increases by an order of magnitude when changing the orientation from [001] to [111] with [011] and [123] orientations in between. Such a character of creep anisotropy corresponds to the orientation dependence of the Schmid factor for octahedral glide.
The crystal viscoplasticity model developed in [1] was improved to better represent the time induced softening observed during creep. The creep tests for different stresses and orientations as well as pore closure were simulated. The results of pore closure simulation are compared with measurements of porosity decrease during Hiping.
The creep behavior of single-crystals of the nickel-base superalloy CMSX-4 was investigated at 1288°C, which is the temperature of the hot isostatic pressing (HIP) treatment applied to this superalloy in the industry. It was found that at this super-solvus temperature, where no Gamma’-strengthening occurs, the superalloy is very soft and rapidly deforms under stresses between 4 and 16 MPa. The creep resistance was found to be very anisotropic, e.g. the creep rate of [001] crystals was about 11 times higher than that of a [111] crystal. The specimens of different orientations also showed a very different necking behavior. The reduction of the cross-section area psi of [001] crystals reached nearly 100%, while for a [111] crystal psi=62%. The EBSD analysis of deformed specimens showed that despite such a large local strain the [001] crystals didn’t not recrystallize, while a less deformed [111] crystal totally recrystallized within the necking zone. From the shape of deformed specimens and TEM investigations it was concluded that the main strain contribution resulted from <011> {111} octahedral slip.
The development of refractory CCAs has been explored for potential use in high temperature applications. An example of this is the AlMo0.5NbTa0.5TiZr alloy, which resembles the well-known γ/γ’ microstructure in Ni-Base superalloys with cuboidal particles embedded in a continuous matrix. The aim of this work is to evaluate the alloy’s mechanical behavior under tension in the temperature range 800-1000°C, by applying creep tests under vacuum (excluding oxidation effects). Some little temperature influence on minimum creep rate @ 1000 and 1100 °C was found and at a first glance, and Norton plots shows that deformation is probably both diffusion and dislocation controlled. However, further work is needed to stablish deformation and degradation micro mechanisms in the studied creep regime.
The elastic constants are the most basic mechanical properties of a material and are needed for any structural analysis of a component. For example, they have a major influence on the eigenfrequencies of vibrating parts. Single crystals of Ni-base superalloys are strongly anisotropic, which means that the observed properties are orientation dependent. Tensor algebra is then required to mathematically formulate the elastic properties and their relations to the crystal orientation. Hence, this chapter first summarizes some basic definitions and calculation rules for Rotation matrices, including the definition of the Euler angles, which are most commonly used to define the relative orientations of the crystal and the component. Parts of this chapter closely follow the lines of the excellent exposition of the topic by Olschewski.
The large number of TEM investigations and the regular microstructure
of single-crystal nickel-base superalloys has boosted the development of a number of physically motivated constitutive laws. In contrast to the more phenomenological models discussed in the next chapter, these models use dislocation densities as internal variables. Obvious advantages are that the computed densities can be compared to TEM observations and the Deformation mechanisms can be easier translated into mathematical equations.
High-temperature corrosion is a widespread problem in various industries.
As soon as a hot and reactive gas (CO2, O2, H2O, SO2, NOx, etc.) is in contact with a solid, physico-chemical processes at the surface and interfaces lead to material degradation. The processes are dynamic and controlled by thermodynamic and kinetic boundary conditions. Whether a reaction product is protective or not depends on various factors, such as chemical composition of the solid and the reactive media, surface treatment as well as diffusion and transport paths of cations and anions. Resulting chemical and structural inhomogeneities with the corrosion layers are characterized by off stoichiometry within cationic and anionic sub lattices. The competitive processes can be studied by various techniques of applied crystallography.
This chapter gives an overview on the challenges of chemical-structural Analysis of reaction products by crystallographic methods such as X-ray diffraction and X-ray near-edge structure spectroscopy and scanning electron microscopy electron backscatter diffraction (SEM-EBSD) for corrosion science.
This presentation is a summary of the work from the past 20 years’ development of PMC-testing at the BAM-FB 5.3 with respect to safety-relevant design of advanced light weight structures in aircraft, wind turbine and automotive applications. The talk begins with wood as an example from nature, and emphasizes that load case, fiber architectural design and the production process and quality have to go hand in hand to generate an advanced light weight structure. Since PMC-relevant basic findings of mankind span across hundreds of years, high-performance composite applications today are based more on long term experiences than on breakthrough inventions of modern days.
In the second part of the talk, future plans and projects of FB-5.3 are presented, specifically addressing H2-safety, circular economy, recycling by design and digitalization of PMC-technologies.
Cyclic fatigue behavior of glass fiber reinforced epoxy resin at ambient and elevated temperatures
(2018)
The fatigue behavior of ±45° glass fiber reinforced epoxy resin under cyclic mechanical and constant thermal loading is investigated in this study. Tests at three different temperature levels in the range 296 K to 343 K have been performed in order to create S-N curves for each temperature level. The specimen damage is measured in-situ using optical grayscale analysis. The characteristic damage state (CDS) is evaluated for each specimen. It is shown that the point of CDS is suitable as a failure criterion to compare the resulting S-N curves. With micromechanical formulations, the temperature-dependent matrix effort is calculated for each stress-temperature level. In terms of matrix effort, the longest fatigue life is reached at high temperatures, while, in terms of stress, the lowest fatigue life is reached at the highest temperatures.
Cyclic fatigue behavior of glass fiber reinforced epoxy resin at ambient and elevated temperatures
(2018)
The fatigue behavior of ±45° glass fiber reinforced epoxy resin under cyclic mechanical and constant thermal loading is investigated in this study. Tests at three different temperature levels in the range 296 K to 343 K have been performed in order to create S-N curves for each temperature level. The specimen damage is measured in-situ using optical grayscale analysis. The characteristic damage state (CDS) is evaluated for each specimen. It is shown that the point of CDS is suitable as a failure criterion to compare the resulting S-N curves. With micromechanical formulations, the temperature-dependent matrix effort is calculated for each stress-temperature level. In terms of matrix effort, the longest fatigue life is reached at high temperatures, while, in terms of stress, the lowest fatigue life is reached at the highest temperatures.