Effect of cooling rate and chromium doping on the microstructure of Al-25 at.% Ni Raney type alloy

Al-25 at.% Ni and Al-23.5 at.% Ni-1.5 at.% Cr alloys were synthesised via gas atomisation to study the effect of rapid cooling on the microstructure and phase composition of Raney type catalyst precursor powders. In the undoped powders, the three phases, Al₃Ni₂, Al₃Ni, and Al-Al₃Ni eutectic were identified, while in the Cr-doped powders the additional phase Al₁₃Cr₂ was also identified. Extensive substitution of Ni onto the Cr lattice sites is observed, which generates the observed phase fraction of Al₁₃Cr₂. Elemental mapping and quantitative image analysis of backscattered electron micrographs indicates that the Al₁₃Cr₂ phase precipitates late in solidification, probably direct from the melt, during the final stages of Al₃Ni growth. As such, this explains previous observations that Cr is found on the surface of the activated catalyst without the need to invoke migration of Cr. Leaching of such an Al-rich compound offers a plausible explanation for the enhanced catalytic activity observed in Cr-doped Raney catalysts.