High Recovery Stress Performance of NiTi Shape Memory Alloy with a Wide Temperature Window

The prestressing applications of shape memory alloys (SMAs) employs the recovery stress of a constrained SMA element that originates from its attempt to revert a pre-deformation to original shape upon heating. As the most readily available SMAs, NiTi alloys have shown poor performance of recovery stress compared to NiTiNb and FeMnSiCrNi due to their relatively small transformation hysteresis. This work investigated the effects of heat treatment and prestraining on the martensitic phase transformation characteristics and mechanical properties. It is found that suitably selected heat treatment and prestraining treatment can effectively tune the transformation hysteresis to greater than 100 °C while retaining reasonably high strength, which greatly elevate the recovery stress of NiTi–SMAs. A large recovery stresses of approximately 300 MPa with a wide temperature window of 180 °C of stable performance can be achieved via a simple one-step heat treatment and prestraining treatment using commercial superelastic NiTi wire products.