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.

A fully implicit numerical method, based upon a combination of adaptively refined hierarchical meshes and geometric multigrid, is presented for the simulation of binary alloy solidification in three space dimensions. The computational techniques are presented for a particular mathematical model, based upon the phase-field approach, however their applicability is of greater generality than for the specific phase-field model used here. In particular, an implicit second order time discretization is combined with the use of second order spatial differences to yield a large nonlinear system of algebraic equations as each time step. It is demonstrated that these equations may be solved reliably and efficiently through the use of a nonlinear multigrid scheme for locally refined grids. In effect this paper presents an extension of earlier research in two space dimensions (J. Comput. Phys., 225 (2007), pp. 1271-1287) to fully three-dimensional problems. This extension is validated against earlier two-dimensional results and against some of the limited results available in three dimensions, obtained using an explicit scheme. The efficiency of the implicit approach and the multigrid solver are then demonstrated and some sample computational results for the simulation of the growth of dendrite structures are presented.

Models of semi-solid processing currently utilise constitutive equations to describe slurry rheology. However, using the techniques of molecular dynamics it may be possible to go beyond this approach to obtain direct relationships between applied shear rates, the resultant microstructural changes and rheology.

The results of a simple molecular dynamics simulation of semi-solid flow within a Couette rheometer are presented and compared with experimental data for the system Sn -15% Pb. We find that the model correctly predicts the apparent viscosity under steady state conditions, up to solid fractions of ≈ 40%.