A New Prototype Two-Phase (TiNi)–(β-W) SMA System with Tailorable Thermal Hysteresis



The Ti–Ni–W two-phase shape memory alloy (SMA) thin film system is presented as a prototype for new SMAs with tailorable thermal transformation hysteresis (ΔT). The concept is to combine the SMA TiNi with almost insoluble W to create the two-phase system (TiNi)–(β-W). This system behaves like a pseudobinary TiNi system. Phase transformation behavior for compositions above the solubility limit of W in TiNi exhibit a B2–R phase transformation with characteristically small ΔT. Moreover, ΔT is dependent on the amount of W and it can be tailored to zero and even negative. This phenomenon is rationalized as being due to the mechanical interaction between the phases B2-TiNi and β-W. The presented results are very promising for the development of high-speed Ti–Ni-based SMA actuators.