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

• Ni2Cr1−xMox phases have an enhanced stability at the composition of Ni2Cr0.67Mo0.33 due to the appearance of unstable Ni2Cr0.5Mo0.5.
• All the Ni2Cr1−xMox phases are mechanically stable.
• Both the strength and directionality of the chemical bonds in Ni2Cr0.5Mo0.5 are reduced abnormally.

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 Ni2Cr0.67Mo0.33 due to the appearance of unstable Ni2Cr0.5Mo0.5, 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 Ni2Cr0.5Mo0.5 are abnormally soft. It indicates that the strength and directionality of the chemical bonds in Ni2Cr0.5Mo0.5 are reduced abnormally, giving its relative instability and enhanced stability of Ni2Cr0.67Mo0.33.

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