Fabrication of Functionally Graded NiTi Alloy with Large Stress Windows and High Cyclic Stability by Direct Current Heat Treatment

The most widely utilized functional property of the NiTi alloy is superelasticity, which originates from stress-induced martensitic transformation. However, the narrow transformation stress window brings difficulties to the application of NiTi components and functional degradation limit service reliability. To broaden the transformation stress windows and enhance the cyclic stability of superelastic NiTi alloys, we employed direct current heat treatment (DCHT) to tailor the superelasticity of NiTi wires. Initially, NiTi wires with a 1 mm diameter underwent localized annealing through DCHT at 12 A/600 s along their length. The step-gradient stress plateau resulted in stress windows of 300 MPa and 480 MPa for the forward and reverse transformations, respectively. Subsequently, a local DCHT ageing treatment at 7 A/2 h was applied to the section previously treated with 12 A/600 s to enhance its corresponding cyclic stability. In comparison to the non-aged sample, the aged sample exhibits a reduction in residual strain by 37.8%. This innovative DCHT method provides the opportunity to induce step superelastic behaviour and enhances the stability of superelasticity in NiTi alloys for various applications.