Band 59, Heft 8-9

To further improve the range of battery-powered electric vehicles, new concepts in power electronics are required. The use of so-called “wide-bandgap” semiconductor materials, such as SiC, could meet the need for even more powerful power electronics, but this also increases the demands on the thermal management of the components. Complex copper cooler structures adapted to the temperature field could be a suitable way to meet these increasing requirements for heat dissipation. However, conventional manufacturing techniques such as forging, casting and milling are reaching their limits for the production of complex Cu structures, and new processes such as additive manufacturing are coming into focus. Additive manufacturing processes such as selective laser melting (SLM) place high demands on the laser system for highly-conductive copper (Cu) [1]. This means that less established sinter-based processes such as binder jetting (BJ) or fused filament fabrication (FFF) are also suitable for the production of complex Cu structures. In sinter-based methods, the sintering process and the microstructure, which is strongly influenced by it, is crucial for achieving good physical properties of the component [2]. In this work, methods are presented that allow for a quantitative and qualitative evaluation of the sintering process of copper at the microstructure level in order to derive optimized process parameters that enable higher sintering densities and thus greater conductivities. The influence of residual porosity and impurities on conductivity was investigated and allows for a specific prediction of the expected conductivity of sintered Cu structures.

After a thorough structural examination, a “Campo del Cielo” iron meteorite found in Argentina was forged at temperatures of about 1100 °C. This meteorite is classified as an IA group coarse octahedrite. Besides Fe, it also contains about 7 wt.% Ni and 1.5 wt.% Co, as well as small amounts of Mg, Si, Al, S, and P. Heat treatments above the eutectoid temperature induce a α-γ transformation of the present (Fe,Ni) solid solution. Owing to the retransformation that takes place upon cooling, the typical microstructure of the meteorite vanishes and a mainly ferritic polycrystalline microstructure forms (kamacite α-(Fe,Ni)). Only minor changes could be observed for the initially present taenite γ-(Fe,Ni). Schreibersite, also referred to as rhabdite (Fe,Ni) 3 P, melts and dissolves into the surrounding matrix. At high temperatures, locally present graphite likewise dissolves and cementite or martensite are formed upon cooling. Changes of other phases such as troilite (FeS) or lawrencite (Fe,Ni)Cl 2 could not be detected. Given an initial weight of the meteorite fragment of approx. 1.5 kg, the intention was to manufacture a sword. However, since the meteorite broke apart during forging, only a knife could be manufactured.

Color metallography of aluminum alloys can provide new structural knowledge or extend and refine structural knowledge compared to a black and white image. The basic principle of color metallography is to create a suitable film by etching. On samples coated with a suitable film, light interference is caused by the splitting of the incident light into components reflected at the air-layer interface and the layer-metal interface. The thickness of the film essentially determines at which colors the interference occurs. For films of suitable thickness, interference occurs in the blue, green and yellow regions. The film thickness depends on the chemical composition of the sample material, the chemical composition of the etchant and the etching time. If the etching conditions are kept constant (etchant type, etching time) and the chemical composition of the individual micro-areas changes significantly, it will be possible to distinguish the microareas by color contrast in the bright field. The results of color etching of aluminum alloys in different types of etchants will be presented. Using an automated apparatus, the same etching conditions will be maintained, including time and repeatability of movements in the etching, cleaning and drying process.

Gas and ion diffusivity within the membrane electrode assembly (MEA) are of central importance for the functioning of a proton exchange membrane (PEM) fuel cell. These properties are, among other things, directly impacted by the microstructure of the layering and the morphology of the individual layers. Different types of preparation were used to prepare the inner layer composite of the MEA/PEM of a fuel cell. The preparation quality provided for high resolution electron microscopy and the microstructure quantification were compared. The conventional mechanical preparation is compared to planar ion polishing, and cryo ion polishing at -80 °C. Advantages and disadvantages of the methods are discussed by way of examples related to the microstructure such as porosity, elemental distribution, and the measurement of the layer thickness. It is shown that cryo ion polishing facilitates a largely artifact-free preparation of the highly thermo-sensitive, partially highly porous layered PTFE composites, and the porous catalyst layers. Thus, not only easily ascertainable layer thicknesses but also qualitative features and important quantitative microstructural parameters, such as nanometer-sized pore structures and the porosity of the various composite layers can be revealed.

Strip material made of 80CrV2 was bainitized for a further development of the master blade steels for thin-cutting circular and band saw blades. Severe distortion after tempering of the material led to the search for a metallographic preparation method to identify the microstructural constituents retained austenite and martensite. Mechanical preparation followed by chemical etching was unsuccessful because the surface was too rough to identify the targeted phases under SEM. Performing an electrolytic preparation produced a less rough surface with sharp phase boundaries, which allowed identification at high magnifications under the SEM. Microhardness testing was used to distinguish between the microstructural constituents of retained austenite and martensite.

Representative macroscopic images of a sample are an integral part of many material testing methods. Usually, a digital camera attached to a macro stand is used. Recurring problems include reflections from the surface of the sample or the fact that regular 2D representations of fracture surfaces, for example, do not always permit a correct interpretation at a later point. In this study, a novel objective lens from K|Lens GmbH, capable of recording the depth information of the surface with a digital camera in a single shot and enabling three-dimensional reconstruction will be used to evaluate potential applications. Furthermore, application limits in macroscopic imaging and the analysis of typical metallographic samples, with the focus being on three-dimensional imaging and quantification, will be studied.

Glass substrates are playing an increasingly important role in the production of microchips. This paper will demonstrate the potential of scanning acoustic microscopy (SAM) used for the characterization of laser-welded structures in thin glass slides. Structures within the volume of a glass sample can be imaged quickly and with high accuracy using this method. In particular, it should be noted that not only the depth information, but also the height profile of the structures can be obtained from ultrasonic cross-sectional images. Measurements only take a few minutes. While certain structures can be visualized in 3D using ultrasonic Time-of-Flight (ToF) measurements, current research is focusing on the 3D representation of more general structures based on the Synthetic Aperture Focusing Technique (SAFT).

In this work, the fields of application of mobile metallography in technical practice are presented on the basis of numerous examples. This nearly non-destructive testing method (NDT) is used primarily for quality assurance of components in operational applications. It includes the fabrication of microstructure replicas, mobile hardness testing and analysis methods for the determining of the chemical composition of materials. The microstructure replicas are taken for different purposes. On the one hand, it can be used to document the initial state of the microstructure, to evaluate the microstructure condition for the control of the manufacturing quality, and conclude on the required properties. On the other hand, this method can be used to evaluate defects (formation of cracks, performance of welds, correct heat treatment, creep damage, etc.). On the basis of different test cases, the results for low-alloyed and highalloyed ferritic and austenitic steels, for nickel alloys and for the possibility of subsequent determination of precipitation populations in the transmission electron microscope (TEM) are presented.

A gold-steel composite material for decorative applications was manufactured by forging. This procedure can be referred to as modified damascene technique. Sheets made of steel K110 (C-rich cold work steel) and gold sheets were assembled and forged together at approx. 800 °C. The bar was subsequently twisted to obtain the typical torsion pattern. The end portions of this bar were subjected to a metallographic examination. The steel K110 shows broken carbides which were formed during the forging process. The carbides can be etched using Murakami’s reagent, whereas a 3 % Nital solution can be used to etch the steel matrix. Gold etching was challenging because many etchants suitable to etch gold preferentially attack the steel. In reference to gold extraction by cyanide leaching, a KCN solution was used as etchant. It could be shown that atmospheric oxygen acting as an oxidant is already sufficient to etch gold. However, using a KCN-H 2 O 2 mixture provides a more efficient etching effect. Gold grains in the gold layers can reach a size of up to 1 mm. A diffusion zone with a thickness of approx. 10 μm can be observed at the steelgold interfaces.

Failures of stainless and heat-resistant austenitic CrNi steels frequently occur in practice due to incorrect material selection, improper processing or unforeseeable factors under operating conditions. Selected examples will be used to illustrate various of these factors influencing the development of failure.