Volume 55, Issue 4

This study examines ceramic varistors made from zinc oxide whose electrical resistance drops by many orders of magnitude within a few nanoseconds when a critical voltage threshold is exceeded. These unusual properties are caused by charges which are build into grain boundaries or accumulate near the grain boundaries. Zinc oxide crystals are polar and piezoelectric. Therefore, the charges in the grain boundary area and thus the electrical properties of the varistor depend on the orientation of the crystals towards the grain boundaries and on the mechanical tensions in the individual crystals. In this study, the microstructures of ZnO varistors, particularly the size and distribution of the grains and their orientation towards the grain boundaries will be examined. Among other things, lock-in thermography with a target preparation of strongly active grain boundaries and EBSD analyses in combination with SEM examinations, ion-etching and wet-chemical etching for the determination of crystal orientations are used. For selected grain boundaries, the electrical resistance in dependence of the applied voltage is determined via Micro-4 pole measurements.

In the development of steels with increasing strength and toughness, ultra-high strength steel grades with a martensitic microstructure are gaining more importance. From the martensitic transformation product, however, strength-specific quantities derived from the former austenite grain cannot be accessed conveniently. A profound analysis of the prior austenite microstructure, its size distribution and aspect ratio is essential in order to allow conclusions on the mechanical properties. This paper presents an etchant to reveal the prior austenite grains of thermomechanical processed and press hardened martensitic steels. Furthermore, alternative detecting methods such as electron backscatter diffraction (EBSD) and high-temperature laser scanning confocal microscopy were evaluated. It was found that a picric-acid etchant in combination with a prior tempering treatment of the steel enables the visualization of prior austenite grains and their elongation in a micrometer-scale.

The thermomechanical behavior of cast Al-Si piston alloys is highly dependent on the condition of the highly interconnected hybrid 3D network which is composed of primary and eutectic silicon and intermetallic phases which are found embedded in the α-Al matrix. The very similar X-ray absorption of silicon and the α-Al matrix is problematic for the local identification of these phases and thus for the accurate segmentation and characterization of all rigid phases in the network when laboratory X-ray tomographs are used. A combination of conventional X-ray computed tomography, synchrotron computed tomography, and chemical deep etching performed on the same alloy and at the same position allows for an automatic segmentation and a more accurate characterization of phases across large representative volumes and thus provides the information necessary for a quantification of the whole 3D microstructure of the alloys.

Thermal cracking experiments were performed on anthropogenic resources (post consumer plastics with PVC impurities) in an autoclave under N 2 atmosphere and at 500 °C. After about 48 h, a crack near the flange resulted in pressure loss. Deposits of FeCl 2 · 2H 2 O, NiCl 2 · 2H 2 O and Ni 2 O 2 OH could be evidenced on the cracks using XRF and XRD. In the crack itself, mainly Cr 2 O 3 could be found at the crack initiation site, while chlorides of Fe, Cr, and Ni could be observed at the tip of the crack using EDS. Metallographic examinations using V2A and Beraha etchants revealed that the crack propagated transgranularly. The basic structure exhibits not only dendritic segregations (cast structure) but also segregation bands (deformed microstructure). In addition to that, Ti(C,N) precipitations could be found. TiN (yellow) mainly occurred in the core, while TiC (white) was primarily found at the edge of the sample.