An investigation on the crystal structures of Ti50Ni50−xCux shape memory alloys based on density functional theory calculations

•We studied martensite crystal structures of TiNiCu SMAs through DFT calculation.•The lattice parameters and stable crystal structures changed with changing at% Cu.•When >20 at% Cu, orthorhombic structure became more stable than monoclinic.•Electron transfer might be the fundamental reason for the structural instability.•The results suggest a plausible method for predicting crystal structures of SMAs.

The present research has investigated the martensite crystal structures and electronic structures of Ti50Ni50−xCux (x = 0, 5, 12.5, 15, 18.75, 20, 25) shape memory alloys using density functional theory (DFT). The computational results are compared with the reported data and it is found that the equilibrium lattice constants are in good agreement with reported values. It is also found that with Cu addition to NiTi, the lattice parameters (a and c) and the monoclinic angle decrease, whereas the lattice parameter b increases. With increasing Cu content, fewer electrons were transferred from Ti to Ni in comparison with that in binary NiTi alloys, and the NiTi monoclinic structure becomes unstable. When Cu content is increased to around 20 at%, an orthorhombic crystal structure is formed which agrees well with reported experimental observations.