Bulk Versus Surface Regulation of Cyclic Superelasticity in LPBF-Fabricated NiTi Alloy

Cyclic superelasticity in laser powder bed fusion (LPBF)-fabricated NiTi alloys is strongly influenced by the scale of structural regulation. While conventional post-processing strategies are typically interpreted from a microstructural perspective, the distinct roles of bulk and surface regulation in governing cyclic functional response remain unclear. In this study, heat treatment and laser shock peening (LSP) are employed as representative bulk and surface regulation routes, respectively, to systematically investigate their effects on phase transformation and cyclic superelasticity. The results reveal that heat treatment and LSP operate through fundamentally different regulation modes. Heat treatment acts as a bulk regulation route, reconstructing the overall microstructure, promoting precipitation (NiTi₂ and Ni₄Ti₃), and modifying transformation pathways, which enhances recovery ratio but reduces recoverable strain. In contrast, LSP acts as a surface/subsurface regulation route, inducing gradient grain refinement and near-surface hardening while maintaining a B2-dominated matrix. As a result, the LSP-treated sample exhibits superior cyclic stability, with a stable recoverable strain of 9.93% and a superelastic strain of 5.10% after 10 cycles. These findings demonstrate that cyclic superelasticity is governed not only by phase constitution but also critically by the scale of structural regulation. This work provides a practical framework for selecting post-processing strategies to optimize functional performance in LPBF NiTi alloys.