Microstructure Characterisation of Rapidly Solidified Al-Ni-Cr and Si-Ge Alloy Powders.

Al-Ni and Si-Ge alloys were subject to microstructure and crystal characterisation techniques following rapid solidification processes.
Firstly, Al-25 at.% Ni was synthesised via close-coupled gas atomization and sieved into standard size fractions to investigate the microstructural effect of cooling rate. Cooling rate in the Al-Ni samples was found to affect not only the microstructure morphology but also the phase fractions. Al-23.5 at.% Ni-1.5 at.% Cr samples were also created using the same method to investigate the changes in the microstructure with the addition of Cr. When Cr was added to the alloy the same changes in microstructure and phase fraction was found with a change in cooling rate but the addition of a fourth phase: Al13Cr2 was revealed. While Cr doping has previously proven to enhance catalytic activity this study was conducted to understand the mechanism(s) that contribute to this increase in activity. The formation of better defined dendrites, and the addition Al-rich Al13Cr2 phase, means that after leaching a better nanoporous structure is achieved, which may explain the enhanced catalytic activity that has previously been observed in Cr-doped Al-Ni catalysts.
Si-30 wt.% Ge samples were created using a 6.5m drop tube and sieved into size fractions to understand the effect of cooling rate and solidification sequence. As the cooling rate increased finer grains are found as expected in rapid solidification. Partitioning however, does not follow the expected trend; a greater amount of partitioning was found in the faster cooled samples which is characteristic with the absence of solute trapping, instead it is hypothesised that the mechanism responsible is back-diffusion. Si-Ge is expected to solidify as a solid solution, but by using EDX analysis in the TEM small regions of preferred stoichiometry were observed. An interesting finding was that the Si-60 at.% Ge region, potentially Ge3Si2, was also chemically disordered like the rest of the alloy.