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 > direction. While previous studies only report a D1(a)-type compound at a stoichiometry of Ni4W with this < 100 >-ordering tendency, we predict that within the solubility range of tungsten in nickel, structures with tungsten < 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 >-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 (C-ij) over bar (x(W)) and partial derivative(C-ij) over bar/partial derivative x(W). (C) 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.