Precipitation of Disordered Ni-X Solid Solution Phases in Off-Stoichiometric Ordered Ni₃X Alloys

Summary

Nucleation of the solid solution γ phase from ordered γ’ phase (Ni₃Al) is quite difficult. Undercooling the 22.6 at.% Al alloy by more than 50 °C to a temperature (700 °C) at which diffusion is reasonably rapid fails to produce observable γ precipitates, even after aging for 120 h. The phase boundary can be best found through the use of dissolution experiments, which take advantage of the absence of a barrier to the dissooution of γ phase.

The γ precipitates are equiaxed when small, then non-equiaxed as they grow and eventually become plate-like shaped. The γ precipitates transform into plates far more readily than γ’ precipitates, which is attributed to the absence of anti-phase boundaries in the γ phase. Despite the non-equiaxed shapes, <r>³ depends approximately linearly to aging time t. The experiments show that the kinetics of coarsening of γ precipitates is much slower than that in the reverse system. This is consistent with behavior expected from diffusion in the two phases.

The Ni₃Ge phase behaves similarly to Ni₃Al in that it can be aged to produce disordered precipitates of the Ni-Ge solid solution from Ni₃Ge matrix. We do not yet know if this is also true for Ni₃Ga, but preliminary experiments suggest that it is not.