Effect of ternary additions on the stability of ordered phases in Ni-Mo alloys-transmission electron microscopy results and first principles calculations

Abstract

The ordering behavior of a Ni-Mo alloy in the presence of ternary additives X (X = Al, Cr, Mn, V) has been studied using transmission electron microscopy (TEM) as well as first-principles calculations using the tight-binding-linear muffin tin orbital (TB-LMTO) method. The sequence of ordering transformations in binary Ni-Mo alloys has been shown earlier to be controlled by a competition between several fcc-based superlattices, viz. Ni₂Mo (Pt₂Mo type), Ni₃Mo (D0₂₂), Ni₄Mo (D1_a), and the so-called short-range ordered (SRO) structure characterized by the presence of {1½0} reflections. These ternary additives have been observed to stabilize the Ni₃Mo (D0₂₂) phase compared to the Ni₂Mo + Ni₄Mo phase mixture, leading to a sequence of transformation different from that obtained in binary alloys. The calculated energies of formation were observed to conform to the experimentally observed stability hierarchy in these binary intermetallics and their ternary analogues. The third element stabilizes the D0₂₂ structure by contributing to the covalent component of bonding in these compounds.