Papers by Author: E. Bischoff

Abstract: Natural formation of the unusual bcc modification for Co was demonstrated to occur upon precipitation of Co in supersaturated Au₉₀Co₁₀ alloy, applying high resolution transmission electron microscopy. The bcc Co phase precipitates as thin (0.5 – 1 nm) plates along {100} habit planes, exhibiting the orientation relationship: {100}_Au,fcc//{100}_Co,bcc and <001>_Au,fcc//<011>_Co,bcc. Prolonged annealing induces a Bain type transformation from bcc Co into fcc Co. The precipitation kinetics of Co in supersaturated solid solutions of Au₉₀Co₁₀ was investigated by DSC upon isothermal annealing at temperatures in the range 567 K to 612 K and upon isochronal annealing at heating rates in the range 5 to 40 K·min^‑1. Results of the fitting of a modular model of transformation kinetics to, simultaneously, all isothermal DSC runs and to, simultaneously, all isochronal DSC runs, could be discussed in terms of the different extents of the effects of quenched‑in defects on isothermally and isochronally conducted transformations.

Abstract: Employing NH3/H2 gas mixtures, Fe-4.65at% Al alloy specimens were nitrided to assess how the presence of Al, originally dissolved in the ferrite matrix, influences the development of γ-Fe4N1-x phase in the surface adjacent region. The nitrided specimens were characterized by light microscopy, X-ray diffraction, Electron Backscatter Diffraction and Electron Probe Micro Analysis. Surprisingly, formation of ε-Fe2N1-x was observed, although, for the applied nitriding parameters (nitriding potential and temperature), only the formation of γ-Fe4N1-x would be expected in case of nitriding pure ferrite. An unusual plate-type morphology of γ-Fe4N1-x was observed, contrasting with the usual continuous layer-type growth observed upon nitriding iron, Fe-Cr and Fe-V alloys. These unexpected phenomena may be explained as consequences of the need to realize a very high nitrogen supersaturation in the ferrite matrix in order to initiate the precipitation of AlN.