Theoretical Exploration of Alloying Effects on Stabilities and Mechanical Properties of γʹ Phase in Novel Co–Al–Nb-Base Superalloys

The new developed γ/γʹ Co–Al–Nb-base alloys show great potentials as high-temperature materials. However, finding appropriate compositions to improve performance of alloys still poses a great challenge to the development of Co–Al–Nb-base alloys. Motivated by the lack of alloying effects on fundamental properties of critical γʹ phase, we systematically performed a theoretical investigation on the effect of alloying elements TM (TM: Ti, V, Cr, Zr, Mo, Ta, W, Re, and Ru) on phase stabilities and mechanical properties of L1₂-type γʹ (Co, Ni)₃(Al, Nb). By analyzing the stability of γʹ phase with respect to its competitive B2 and D0₁₉ phases, the results shown that Ti, V, and Cr enhance the L1₂ stability and widen the L1₂–D0₁₉ energy barrier, in which V yields the maximum influence. The analysis of electronic structure indicated that the alternation of valence electrons at fermi level would be the atomic origin for doping TM in γʹ phase. The calculated results of mechanical properties shown that V and Cr are expected to be optimal dopant for enhancing the strength and the ductility of γʹ phase. The addition of Ta is also beneficial for enhancing the strength at the slight expense of ductility of γʹ phase. By drawing the mechanical maps, the preferred composition range for the phases with desired properties is roughly demarcated in theory for the multi-addition of V/Cr and V/Ta in γʹ phase. The findings would be useful for optimizing the performance of novel γ/γʹ Co–Al–Nb-base superalloys.