Electronic structures and shape-memory behavior of Ti50Ni50−xCux (x=0, 6.25, 12.5, 18.75 and 25.0 at%) by density functional theory

The electronic structures and mechanical properties of a series of CsCl-type Ti50Ni50−xCux (x=0, 6.25, 12.5, 18.75 and 25.0 at%,) shape-memory alloys have been investigated using the density functional theory (DFT) with a plane-wave pseudo-potential. It is found that the calculated formation energies and equilibrium lattice constants increase with Cu content increasing. The calculated bulk modulus (B) and Young modulus of these alloys are in good agreement with the available values. By comparing the shear constants C′ and anisotropy factors A of these alloys, the tendency of their martensitic transformation (MT) behaviors is deduced. The softening of both C′ and A for TiNi (Ti50Ni50−xCux (x=0)) would lead an MT of B2→B19′ while a MT of B2→B19 would occur when Cu contents ⩾18.75 at%. A two-step MT, B2→B19→B19′, however, would take place with Cu content increasing.