Scientific Development Research

NiTi shape memory alloys have become core materials for biomedical interventional devices due to their excellent superelasticity. However, performance degradation under cyclic service conditions severely limits their long-term reliability. This paper systematically investigates the effect of grain size on stress-induced martensitic transformation and superelastic cyclic stability of Ni50.8Ti49.2 alloy. A series of specimens with gradient grain sizes ranging from nanocrystalline (~30 nm) to coarse-grained (>5 μm) were prepared through heavy cold rolling combined with annealing at different temperatures. The evolution of phase transformation behavior, microstructure, and superelastic stability with grain size was systematically characterized using DSC, EBSD, TEM, and cyclic tensile tests. The results demonstrate a significant size-dependent effect of grain size on superelastic stability. Coarse-grained specimens (>1 μm) exhibit typical stress-induced martensitic transformation but rapid degradation of cyclic performance. In ultrafine-grained specimens (200-500 nm), martensitic transformation coexists with grain boundary coordinated deformation, with optimal cyclic stability achieved at ~300 nm, maintaining over 80% recoverable strain after 50 cycles. In nanocrystalline specimens (<100 nm), martensitic transformation is significantly suppressed, and superelastic characteristics essentially disappear. This study quantitatively determines, for the first time, the critical lower limit of grain size for maintaining good superelasticity to be 80-100 nm, and proposes an optimal "window range" of 200-400 nm. This research provides theoretical guidance for microstructure design of high-performance and long-life NiTi alloy devices.

NiTi alloy; Grain size; Martensitic transformation; Superelasticity; Cyclic stability