Downsizing Hysteresis in VIM melted Ni-Ti-Cu-X alloys

Ternary Ni-Ti-Cu alloys have been deeply investigated in research works for low hysteresis shape memory applications. Based on crystallographic theory of martensite and co-factor theory, it is possible to design alloy formulations with near zero hysteresis by satisfying the first cofactor condition λ₂ = 1, reducing power losses in actuation and improving fatigue life under cycling. This approach was successfully proven by thin film and low scale synthetization approaches. However, in the real industrial practice, in which large amounts of material need to be melted, laminated and drawn, such alloys face several practical issues. Above all, ternary alloys that satisfy the λ₂ = 1 condition are often composed of highly off-stoichiometric formulations, leading to a significant number of second phases and incoherent precipitates which reduce the workability by embrittlement. In order to solve this issue, a feasible approach could be the addition of a fourth element for engineering an alloy that satisfy the cofactor condition, and a microstructure with a low amount of secondary phases optimized for conventional manufacturing processes. The scope of this work is to highlight and review on the most important key factors in low hysteresis alloy design for the industrial world by providing examples of ternary and quaternary Ni-Ti-Cu alloys, which were studied by DFT calculations, DSC, XRD and microscopy.