Phase stability and mechanical property of γ′- (Ni1-xCox)3Al1-yCry alloys

The phase stability and mechanical property of γ ′-(Ni 1-x Co x ) 3 Al 1-y Cr y phase of Ni-Co based superalloys are studied by the first-principles method. The calculated mixing enthalpies at 0 K indicate that the alloys are thermodynamically unstable against phase separation into Ni 3 AlCr and Co 3 AlCr alloys. Additions of 6.25 at % of Cr into (NiCo) 3 Al alloy can decrease the positive mixing enthalpy by 34%, hence Cr has a significant stabilizing effect on γ ′ phase. The configurational entropy can further stabilize the alloy at finite temperature. A strong tetragonal shear softening is observed for (Ni 1-x Co x ) 3 Al 1-y Cr y alloys with large Co concentration. Cr additions could increase C ′ of the alloys, and thus offset the softening effect of Co by a certain extent. The electronic density of states analysis demonstrates that the flexibility of Al p band and synergistic alloying effect are the physics behind the stabilization the γ ′ phase of Ni-Co based superalloy by Cr alloying.