Phase behavior and mechanical properties of Ni–W studied by first-principles calculations and ab initio based thermodynamics

The substitutional ordering phenomena of face-centered cubic Ni-rich Ni–W alloys are investigated by means of cluster-expansion Hamiltonians based on density functional theory calculations. We observe a strong tendency for ordering in Ni–W characterized by the formation of tungsten chains along the 〈100〉〈100〉 direction. While previous studies only report a D1aD1a-type compound at a stoichiometry of Ni4W with this 〈100〉〈100〉-ordering tendency, we predict that within the solubility range of tungsten in nickel, structures with tungsten 〈100〉〈100〉-chains are the predominant form of short-range order even at elevated temperatures. In particular, we find a Pt8Ti-structured compound (which also shows this 〈100〉〈100〉-ordering) to be stable in Ni–W at low temperatures and low tungsten contents. Even at high temperatures, Ni–W solid solutions are found to exhibit strong remnant order. The implications of these findings are discussed from a metallurgical perspective with an emphasis on the stiffening effect of tungsten alloying on Ni-rich matrices. Using these insights on the short-range order, we then determine a parametrization of the composition-dependent stiffening cij‾(xW) and ∂cij‾/∂xW.