Bending-fatigue-resistant hierarchical NiTi shape memory alloy

<title>Abstract</title> The bending fatigue resistance of superelastic shape memory alloys (SMAs) is a key determinant for their reliable function in cyclic applications such as biomedical implants, adaptive actuators, and elastocaloric devices. However, conventional NiTi alloys exhibit limited fatigue life due to premature crack initiation and propagation under cyclic tensile loading. Here, we report a surface engineering strategy that overcomes this limitation by inducing a hierarchical surface architecture via pre-strain warm laser shock peening (pw-LSP). This architecture integrates a high-strength titanium nitrides-enriched top layer, an ultrafine-grained layer with an inverse grain size gradient and a B19′–R–B2 phase gradient layer, and a substantial compressive residual stress exceeding 1 GPa. These features act synergistically to suppress crack nucleation and arrest propagation through a crack-tip shielding mechanism. As a result, the treated NiTi demonstrates a record-setting bending fatigue life exceeding 5 million cycles at a maximum surface tensile strain of 1.94%—representing a more than 3,000-fold enhancement over untreated nanocrystalline NiTi. This work presents a robust and scalable approach for designing fatigue-resistant SMAs with broad implications for high-cycle, high-reliability applications.