Effect of Co additions on B2 phase stability of Ni-poor NiTi-based alloys

Based on the generalized atomic site preference model and the quasi-chemical bond approximation, the atomic distribution state free energy of the third addition in a binary alloy with the B2 lattice is derived analytically. The configurational energy coefficients are independent of the alloy composition and can be determined directly by the first principle calculation. The calculated B2 ↔ bcc transformation temperature of the Ni-poor NiTi–Co alloys changes at 200 K at.%−1 by the increasing of Co concentration. The experimental results about the effect of the third Co additions on the martensitic transformation procedure of the Ni-poor NiTi-based alloys are well explained.

► We describe the atomic site preference state in crystalline compounds clearly.
► Configurational energy is expressed in terms of atomic site preference degrees.
► First principle calculation method of energy coefficients is developed.
► Calculated B2 phase stability of ternary NiTi alloys decreases by Co addition.