Bending-fatigue-resistant hierarchical NiTi shape memory alloy

Bending-fatigue-resistant hierarchical NiTi shape memory alloy

 

Kai Yan^{1, 2}, Kangjie Chu¹, Maoli Wang¹, Peng Hua³, Pengbo Wei^{1, 2}, Hanlin Gu⁴, Qiming Zhuang¹, Weifeng He⁵, Qingping Sun^{2, *}, Robert O. Ritchie^{6, *}, Fuzeng Ren^{1, *}

¹Southern University of Science and Technology, Shenzhen, China

²The Hong Kong University of Science and Technology, Hong Kong, China

³Harbin Institute of Technology, Shenzhen, China

⁴Peking University, Beijing, China

⁵Xi¡¯an Jiaotong University, Xi¡¯an, China

⁶University of California, Berkeley, CA, USA

 

Abstract

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¡ä¨CR¨CB2 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%¡ªrepresenting 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.