Phase stability and mechanical property of γ′- (Ni_1-xCo_x)3Al_1-yCr_y alloys

The phase stability and mechanical property of γ′-(Ni_1-xCo_x)₃Al_1-yCr_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 Ni₃AlCr and Co₃AlCr alloys. Additions of 6.25 at % of Cr into (NiCo)₃Al 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-xCo_x)₃Al_1-yCr_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. (paper)