Stability and Physical Properties of the L12-γ′ Phase in the CoNiAlTi-System

Abstract

There is a current interest in Co-based superalloys hardened by a L12-γ′ phase because Co has a higher melting point than Ni and is more resistant against sulfidation attack. However, the Co-Al-W system many of those γ′ hardened Co-based superalloys are based on, has a number of drawbacks. The γ′ phase Co3(Al,W) is not stable at high temperature, the density of the alloys is very high and the oxidation resistance is insufficient. Due to this, there is an ongoing interest to develop γ′-hardened Co-based superalloys based on other systems. Here, first principles calculations are presented to investigate the properties of the γ′ L12-(Co0.5,Ni0.5)3(Al0.5,Ti0.5) phase and related L12 structures. (Co0.5,Ni0.5)3(Al0.5,Ti0.5) exhibits a lower energy of formation than Co3Ti and Co3(Al0.5,W0.5). Nevertheless, Ni3(Al0.5,Ti0.5) has an even lower energy of formation which is further lowered if Ti is enriched on the second sublattice. This finding is supported by analyzing the electronic densities of states. Ni3(Al0.5,Ti0.5) and especially Ni3(Al0.25,Ti0.75) exhibit Fermi levels close to the gap between binding and antibinding states, which is an indicator for stability. In addition to the stability of the γ′-phase in dependence on Ni and Ti content, also the elastic properties were calculated. Ni3(Al0.25,Ti0.75) is less elastic anisotropic and has higher Young’s and shear modulus compared to Ni3(Al0.5,Ti0.5) and (Co0.5,Ni0.5)3(Al0.5,Ti0.5).