Enhanced stability of the strengthening phase Ni₂(Cr,Mo) in Ni-Cr-Mo alloys by adjacent instability

Abstract The stability of the strengthening phase Ni₂(Cr,Mo) in Ni-Cr-Mo alloys is subtle and its formation can only be obtained via well-controlled heat treatments. Here the energetic and mechanical stabilities of Ni₂(Cr,Mo) have been studied using density-functional theory calculations. The results show that the phase has an enhanced stability at the composition of Ni₂Cr_0.67Mo_0.33 due to the appearance of unstable Ni₂Cr_0.5Mo_0.5, which is at a local energy maximum in the energy profile. Through the calculations of single-crystal elastic constants, we show that both the bulk and shear modulus of Ni₂Cr_0.5Mo_0.5 are abnormally soft. It indicates that the strength and directionality of the chemical bonds in Ni₂Cr_0.5Mo_0.5 are reduced abnormally, giving its relative instability and enhanced stability of Ni₂Cr_0.67Mo_0.33.