It is well known that NiTi shape memory alloys need to be strengthened to display stable stress-strain-temperature responses in cyclic thermomechanical loads. Strengthening by final cold work/heat treatment at 773 K for 10 minutes commonly used in industry gives rise to superelastic NiTi wires and sheets with nanocrystalline microstructure that display recoverable stress-strain-temperature responses with acceptable functional fatigue but limited structural fatigue (N f = ∼5000) and ductility (∼12%). The limited ductility often results in wire fractures due to accidental overloading, which is a serious problem for engineering applications. Plastic deformability of NiTi wires at high stresses thus becomes of concern besides functional thermomechanical properties. Moreover, strengthening via cold work/heat treatment cannot be applied to bulk NiTi components, thin films and/or additively manufactured NiTi components. Therefore, various alternative ways of strengthening NiTi based alloys have been recently explored in the SMA field. In this work, we show that cyclic superelastic performance of coarse grain superelastic NiTi wires can be significantly improved and yield stress for plastic deformation can be increased while assuring high strength (∼1 GPa) and large ductility (∼80%) by low temperature aging at 523 K for times optimized based on the results of Monte Carlo simulation of Ni diffusion. It is claimed that the local chemical inhomogeneity introduced by aging into the alloy microstructure increases critical stress for 100(001) dislocation slip in martensite which stabilizes cyclic superelasticity, widens superelastic window by increasing yield stress for plastic deformation and improves plastic deformability of NiTi at high stresses. This strategy opens a new pathway for designing coarse grained NiTi alloys with improved functional fatigue performance, high strength, and excellent plastic deformability.
Lu et al. (Mon,) studied this question.