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Surface roughness, RZ, normal transmittance, ΤN, total transmittance, ΤT, and photovoltaic (PV) module efficiency, ηS, were measured for commercial solar glass plates and PV test modules identically sandblasted with different loads of quartz sand (200–400 μm), impact inclination angles, and sand particle speed. Measured data are presented versus the specific energy uptake during sand blasting, E (J/m2). Cracks, adhering particles, and scratch-like textures probably caused by plastic flow phenomena could be observed after sand blasting. Their characteristic size was much smaller than that of sand particles. After blasting and subsequent cleaning, the glass surface was still covered with adhering glass particles. These particles, cracks, and scratch-like textures could not be removed by cleaning. For sand blasting with α = 30° inclination angle and E = 30 000 J/m2, normal transmittance, total transmittance, and relative module efficiency decreased by 29%, 2% and ∽2%, respectively. This finding indicates that diffusive transmission of light substantially contributes to PV module efficiency and that the module efficiency decrease caused by sand erosion can be better estimated from total than by normal transmittance measurements.
The presentation shows how the three-dimensional quantification of dislocations and their characteristic features, e.g. Burgers vector, line direction, dislocation density, is carried out at the transmission electron microscope in scanning mode (STEM) at Division 5.1 at BAM. Exemplarily, the methods are shown for Ni-base superalloy single crystals, for which a short introduction is given using further TEM techniques. Additional examples on low angle grain boundaries, nucleation of oxides at dislocations and interaction of dislocations and carbides are shown.
The content of the presentation was addapted, aiming at scientists who work within the DFG Priority Programme 1713 "Strong coupling of thermo-chemical and thermo-mechanical states in applied materials".
The LEK94 is a Ni-base superalloy single crystal (SX) of the second generation, a materials class which is important for its high temperature creep resistance (>1000°C) in first stage blades of low-to-medium pressure gas turbines. Monocrystalline Ni-base superalloy SXs have a two-phase microstructure consisting of small cubes (' phase with the ordered L12 crystal structure, cube edge length: 500 nm), separated by thin channels (γ phase with fcc solid solution structure, channel width: 20 nm). The microstructural evolution during high temperature and low stress tensile creep has been thoroughly investigated previously, mainly for [001] loading, both in terms of dislocation activity (filling of γ channels, formation of dislocation networks, cutting of the γ’ phase) as well as phase coarsening (rafting, topological inversion). Other loading geometries have received less attention.
The present work studies high temperature and low stress creep deformation of the superalloy LEK 94 at temperatures around 1000°C, where rafting occurs. Differences between loading under different uniaxial, biaxial and triaxial stress states are discussed. Stereo-microscopy and g∙b analysis in the scanning transmission electron microscopy mode (STEM) are combined for microstructural analysis. The focus is set on the role of dislocation interactions with the aging microstructure. Both development in STEM characterization methods, as well as the roles of phase coarsening, γ channel filling, microstructural heterogeneity and γ’ phase cutting are discussed.
The demand for improved castability and low angle grain boundary (LAGB) resistance has led to the addition of low contents of e.g., B, Hf, Zr or C, into large industrial gas turbine components made of Ni-base superalloy single crystals (SXs). Due to the long-term application of Ni-base superalloy SX components in the temperature regime > 1000 °C, the formation of carbides is highly probable, which could jeopardize mechanical properties, such as high cycle fatigue. In the present contribution, the effect of internal and external stresses on the nucleation and growth characteristics of M23C6 carbides is investigated. Creep experiments are performed on the Ni-base superalloy SX LEK 94, which shows a low C concentration (= 0.1 at. %), at 1020 °C under parallel and circularly notched tensile specimens at a nominal stress of 160 MPa in the crystallographic direction [001]. The carbides are then characterized via scanning (S) and transmission (T) electron microscopy (EM). Nucleation is enhanced in the dendritic cores, often as coalesced colonies, extending over micrometers within M-rich (M: Cr, Re, W, Mo) γ channels. Lath shapes with facets on {100} (parallel to growth direction) and {111} are common. These facets exist since early stages (Fig.1a) and later develop misfit dislocations (Fig.1b), preserving the orientation relationship {100}γ || {100}M23C6. Fig. 1c shows a region from the creep gage, where carbides interact with superdislocations in the γ’ phase. Possible mechanisms are discussed.
The field of materials science is defined as “the study of the properties of solid materials and how those properties are determined by a material’s composition and structure.”. Many –if not most– of the materials that are produced nowadays owe their properties to structures engineered down to the nanoscopic level. This need has been partly realized thanks to the understanding of materials’ building blocks via characterization techniques that reach this level of resolution. Transmission electron microscopy, since its first implementation in the early 1930s (in Berlin), has been implemented to achieve imaging –and spectral– analysis at lateral resolutions down to the atomic level.
In this contribution, a series of practical examples will be presented, where applied materials are characterized by a range of transmission electron microscopy techniques to understand structural and functional properties of a wide range of materials. Among these materials examples will be presented on structural conventionally and additively manufactured metallic alloys, high entropy alloys, dissimilar aluminum-to-steel welds, magnetic nanoparticles, ceramic coatings, high temperature oxidation products. Addressed will be either the effect of processing route or that of the exposure to experimental conditions similar to those found in the respective intended applications.
Additive manufacturing (AM) of metallic alloys has gained momentum in the past decade for industrial applications. The microstructures of AM metallic alloys are complex and hierarchical from the macroscopic to the nanometer scale. When using laser-based powder bed fusion (L-PBF) process, two main microstructural features emerge at the nanoscale: the melt pool boundaries (MPB) and the solidification cellular substructure.
Here, details of the MPB are revealed to clearly show the three-dimensional nature of MPBs with changes of cell growth of direction and their relation to their surrounding cellular substructure, as investigated by transmission electron microscopy (TEM) for L-PBF 316L austenitic stainless steel (cf. Figure 1). A hitherto unknown modulated substructure with a period of 21 nm is further discovered within cells as the result of a partial Ga+-focused ion beam-induced ferritic transformation of the austenite. Cell cores and cell boundaries differ notably regarding the modulated substructure.
Ni-base superalloy single crystals have been used in turbine blades for hot sections of gas turbines for over four decades. In order to increase the efficiency of the turbines, a continuous increase in the inlet temperature of combustion gases into the turbine has driven the design of turbine blades to complicated shapes and the presence of a complex pattern of cooling channels. These three-dimensional shapes, together with the inhomogeneous distribution of stresses along the blade, induce an also complicated triaxial stress state, which does not compare to uniaxial tests that are performed to characterize high temperature properties such as creep. A round notch on a test piece represents a simple configuration that generates a quasi-isostatic stress state across the notch. In the present contribution, the effect of a sharp round notch on the microstructural micromechanisms within the notched region cylindrical bars, loaded along [001] at 1020 °C and 160 MPa net stress, is studied. To this end, a series of interrupted creep tests is conducted on plain and notched bars and the microstructure is compared. Results are discussed in terms of degree microstructural coarsening, and dislocation activity. The effect of notch generation via grinding is also discussed in these terms. The presence of carbides evolving in from residual carbon is also shown and discussed. .Funding by the German Research Association (DFG) [grant number AG 191/1] is acknowledge
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.
Das langsame Risswachstum (slow crack growth, SCG) sowie der umgebungsbedingte Spannungsriss (environmental stress cracking, ESC) sind relevante Schädigungsmechanismen für teilkristalline Werkstoffe auf Basis von Polyethylen hoher Dichte (PE-HD).
Der Vortrag gibt einerseits einen Überblick über die grundlegenden Struktur-Eigenschafts-Beziehungen in diesem Kontext, andererseits werden auch verschiedene praxisorientierte Prüfverfahren vorgestellt. Letztere wurden in gemeinsamen Forschungsprojekten von BAM und LBF mit dem Schwerpunkt Gefahrgutbehälter bzw. Pflanzenschutzmittel vergleichend untersucht und durch weitergehende Analytik ergänzt.
The industrial use of additive manufacturing for the production of metallic parts with high geometrical complexity and lot sizes close to one is rapidly increasing as a result of mass individualisation and applied safety relevant constructions. However, due to the high complexity of the production process, it is not yet fully understood and controlled, especially for changing (lot size one) part geometries.
Due to the thermal nature of the Laser-powder bed fusion (L-PBF) process – where parts are built up layer-wise by melting metal powder via laser - the properties of the produced part are strongly governed by its thermal history. Thus, a promising route for process monitoring is the use of thermography. However, the reconstruction of temperature information from thermographic data relies on the knowledge of the surface emissivity at each position on the part. Since the emissivity is strongly changing during the process due to phase changes, great temperature gradients, possible oxidation, and other potential influencing factors, the extraction of real temperature data from thermographic images is challenging. While the temperature development in and around the melt pool, where melting and solidification occur is most important for the development of the part properties. Also, the emissivity changes are most severe in this area, rendering the temperature deduction most challenging.
A possible route to overcome the entanglement of temperature and emissivity in the thermal radiation is the use of hyperspectral imaging in combination with temperature emissivity separation (TES) algorithms. As a first step towards the combined temperature and emissivity determination in the L-PBF process, here, we use a hyperspectral line camera system operating in the short-wave infrared region (0.9 µm to 1.7 µm) to measure the spectral radiance emitted. In this setup, the melt pool of the L-PBF process migrates through the camera’s 1D field of view, so that the radiation intensities are recorded simultaneously for multiple different wavelength ranges in a spatially resolved manner. At sufficiently high acquisition frame rate, an effective melt pool image can be reconstructed. Using the grey body approximation (emissivity is independent of the wavelength), a first, simple TES is performed, and the resulting emissivity and temperature values are compared to literature values. Subsequent work will include reference measurements of the spectral emissivity in different states allowing its analytical parametrisation as well as the adaption and optimisation of the TES algorithms. An illustration of the proposed method is shown in Fig.1.
The investigated method will allow to gain a deeper understanding of the L-PBF process, e.g., by quantitative validation of simulation results. Additionally, the results will provide a data basis for the development of less complex and cheaper sensor technologies for L-PBF in-process monitoring (or for related process), e.g., by using machine learning.