Effect of chromium addition on the ordering behaviour of Ni-Mo alloy: experimental results vs. electronic structure calculations

The ordering behaviour of a Ni-Mo alloy in the presence of a ternary additive, viz. Cr, has been studied. The sequence of ordering transformations in binary Ni-Mo alloys has been shown earlier to be controlled by a competition between several f.c.c.-based superlattices, viz. Ni2Mo (Pt2Mo type), Ni3Mo (DO22), Ni4Mo (D1a) and the so-called short range ordered (SRO) structure characterized by the presence of diffraction intensity maxima (11/20)at the positions in reciprocal space. The effect of ternary addition of chromium in the selection of the superlattice structure has been examined in this paper in an alloy of composition Ni-24 at.% Mo-6 at.% Cr. The presence of Cr has been experimentally found to favour the formation of Ni2(Mo,Cr) (Pt2Mo-type) phase in preference to the Ni3Mo (DO22) and Ni4Mo (D1a) superlattices. This leads to a sequence of transformation different from that obtained in binary alloys. The effect of Cr on the ground state phase stability is determined, in this alloy, using the first-principles local-density based full-potential augmented plane wave (FP-LAPW) method. From the differences in the electronic structures, densities of states and total energies of binary Ni2Cr, Ni2Mo (Pt2Mo type), Ni3Mo (DO22) and Ni4Mo (D1a) phases and the corresponding ternary superlattice structures, an attempt has been made to predict the hierarchy of relative phase stabilities. Theoretical predictions based on these electronic structure calculations have been found to be consistent with the experimental microstructural observations of the evolutionary stages of ordering in the ternary Ni-24 at.% Mo-6 at.% Cr alloy