Experimental observations of transient phases during long‐range ordering to Ni₄Mo in a NiMoFeCr alloy

Experimental observations are reported of transient phases which form during long‐range ordering to Ni4Mo (f.c.c. → Dla superlattice) in the quaternary alloy Ni–19.2 at% Mo–1.2 at% Fe–1.06 at% Cr using electron diffraction. In the early stages of ordering during isothermal annealing, diffuse intensity maxima centered at the short‐range order reflections {1 1/2 0}f.c.c. and along 〈100〉f.c.c. directions are observed. Subsequently, a DO22 superlattice is generated from the short‐range order state. The coexistence of the DO22, Pt2Mo‐type, and Dla superlattices is observed in this alloy system which indicates that these three superlattices have similar energy. With continued annealing, both the DO22 and Pt2Mo‐type superlattices disappear, indicating that they are transient phases. These results are not inconsistent with the theoretical treatments of ordered alloys which are based on an Ising model with pairwise atomic interactions. Copyright © 1987 WILEY‐VCH Verlag GmbH & Co. KGaA