Transmission electron and optical microscopic observations were made on the Ni-24.4 at% Mo alloy (nearly stoichiometric Ni₃Mo) to clarify the decomposition process during isothermal annealing after quenching from the α-region (face-centered cubic structure).
It is found that the alloy quenched from the α-region shows a short-range order similar to that found in the Ni₄Mo alloy, and when annealed isothermally below the peritectoid temperature, γ(Ni₃Mo) does not precipitate directly from the short-range-ordered matrix, but a meta-stable phase Ni₂Mo (Pt₂Mo type superlattice) does. These precipitates of Ni₂Mo are lenticular in shape and grow to plates. The habit plane of the plate-shaped Ni₂Mo precipitates is of the {110}_α type. With increase in annealing time, β(Ni₄Mo) precipitates in the matrix, and Ni₂Mo and β coexist. On the other hand, γ nucleates predominantly at grain boundaries of α and grows. When annealed for a longer time, γ predominates by consuming the pre-existing Ni₂Mo and β.
The orientation relationships of each of precipitates (Ni₂Mo, γ and β) with the matrix were determined.