Microstructure Characterisation of Ni-75 at.% Al Raney Type Alloy as a Result of Cooling Rate and Chromium Doping

The effect of cooling rate on the phase composition of gas atomized Raney type catalysts was studied using the Ni-75 at.% Al composition. The resulting particles were sieved into 3 standard size fractions and analysed using XRD with Rietveld refinement: as expected the three phases, Al3Ni2, Al3Ni, and Al-Al3Ni eutectic were identified. Differing phase compositions in the 3 size ranges were identified offering a possible explanation for varying catalytic activity with cooling rate, the higher cooling rates experienced by the smaller droplets allow less time for the peritectic conversion of Al3Ni2 to Al3Ni to proceed. This in turn results in a more Al-rich residual liquid, increasing the volume fraction of eutectic. This was further confirmed when analysing the microstructure using SEM backscatter imaging. Al3Ni2 was found to be encased in a shell of Al3Ni characteristic of peritectic reactions. The remainder of the alloy was found to consist of Al-Al3Ni eutectic. The SEM backscatter imaging also indicated that the larger particles displayed and a more globular structure than smaller particles. Similar Raney type Ni-75 at.% Al doped with 1.5 at.% Cr were synthesised using the same method and sieved into the same 3 standard size fractions. It was found that the Cr doped alloys exhibited a more dendritic character than the undoped samples in the corresponding size fraction, although the material still displayed an increasingly dendritic character with increasing cooling rate. The phase composition found by Rietveld refinement also followed a similar trend to the undoped samples with decreasing amounts of Al3Ni formed at the higher cooling rates. However, significant amounts of an additional phase, Al13Cr2, were also observed. Rietveld refinement found that a larger amount of Al13Cr2 was present than could be accounted for by the addition of 1.5 at.% Cr . This can be explained by the substitution of Ni onto the Cr lattice, as confirmed by Rietveld refinement. Al13Cr2 was found to be located mostly at the boundary of the Al3Ni and Al-Al3Ni eutectic phases during elemental mapping and quantitative image analysis of backscattered electron micrographs. This indicates that precipitation of Al13Cr2 is towards the end of the solidification process. The relatively large amounts of the Al-rich Al13Cr2 may explain the enhanced catalytic activity observed following leaching of Cr-doped Raney catalysts