Papers by Author: V. Klemm

Abstract: The combination of a TRIP steel with the MgO stabilized ZrO2 ceramics (MgO•ZrO2) is regarded as a promising way to increase the energy absorption in engineering materials. An additional contribution to the energy absorption in the counterparts, i.e. in the TRIP steel and in MgO•ZrO2, is expected to arise at the interfaces between the individual materials. However, the mutual crystallographic orientation of the TRIP steel and MgO•ZrO2 at their interface plays a crucial role both for the adhesion of the counterparts and for the energy absorption process. In this work, the interfaces between the TRIP steel and MgO•ZrO2 were studied on simplified systems, which were prepared in form of the TRIP steel thin films that were deposited using the magnetron sputtering on various substrates, e.g. Si wafer, MgO•ZrO2 and the Al2O3/ZrO2 composites. The substrates were both single-crystalline (Si wafer) and polycrystalline (MgO•ZrO2, Al2O3/ZrO2). The basic characteristics of the thin films (morphology, thickness, chemical composition) were obtained from the scanning electron micrographs and from the energy dispersive analysis of the characteristic X-rays (EDX). X-ray diffraction (XRD) and transmission electron microscopy with high-resolution (HRTEM) that was complemented by the Fast Fourier Transform (FFT) of the HRTEM micrographs were employed as the crucial experimental methods for the microstructure analysis of these thin films. XRD was used for the phase analysis and for the global texture analysis. The global texture analysis was performed via the pole figure measurements. FFT/HRTEM was used for the characterisation of the local orientation relationships between the TRIP steel and the respective substrate and for the visualisation of the interfaces between individual crystallites.

Abstract: Formation of microstructure defects at the phase boundaries in TRIP steels was investigated with the aid of microstructure analysis on a TRIP steel crystal, which was grown by the Bridgman technique. The microstructure studies comprised scanning electron microscopy (SEM), X-ray diffraction (XRD), electron backscatter diffraction (EBSD), transmission electron microscopy (TEM) and transmission electron microscopy with high resolution (HRTEM). Initial XRD measurements revealed that the crystals under study consist of austenite and ferrite with extremely strong preferred orientations. Subsequent XRD pole figure measurements and EBSD scans have shown that the orientation relationship between austenite and ferrite can be described by the Nishiyama-Wassermann model. For a detailed description of the microstructure of the Bridgman crystal, the orientation distribution of crystallites within the individual phases was investigated using the XRD reciprocal space mapping and the rocking curve measurements. These experiments have shown that the density of microstructure defects is much lower in ferrite than in austenite. The direct information about the defect structures at the phase boundaries between austenite and ferrite was obtained from the TEM micrographs, which revealed complicated micro-twin structures at the boundaries between the neighbouring phases. HRTEM discovered very narrow stripes of ferrite embedded in austenite that were regarded as a source of the microstructure defects in austenite.