Study on electrothermal driving performance and cyclic stability of Ti45Ni55 shape memory alloy springs

Abstract

As a significant branch of NiTi shape memory alloys (SMAs), Ni-rich NiTi alloys have garnered considerable research attention. This study employs Ti₄₅Ni₅₅ (at.%) alloy wires to prepare dual-way SMA springs under various annealing temperatures via deformation-aging constraint methods, focusing on optimizing their phase transformation behavior, cyclic stability, and electrothermal driving performance under thermo-mechanical coupling. The results indicate that the phase transformation sequence of the alloy wire evolves with annealing temperature (400–550 °C), progressing from B2 → R to B2 ↔ R ↔ B19′ and ultimately to B2 ↔ B19′ transition, accompanied by generally decreasing phase transformation temperatures. Optimal superelasticity (residual strain < 0.2 %) with distinct stress plateaus is achieved at 400–450 °C, while higher annealing temperatures reduce platform stresses but increase residual strain. The SMA spring annealed at 500 °C demonstrates superior actuation performance under thermomechanical coupling (3 N-1.5A), achieving maximum actuation displacement (73.9 ± 5.1 mm), fastest response (1.12 ± 0.2 s), and excellent fatigue resistance. Thermal hysteresis expansion correlates with increased A_f* and decreased R_f*. The minimal lattice distortion of R phase enables low-energy, high-speed actuation with enhanced cyclic stability. Functional degradation arises from residual stress, dislocation accumulation and concurrent nanocrystalline coarsening, which collectively destabilize the shape memory effect and martensitic transformation. These findings provide critical insights for designing high-performance SMA actuators through microstructure optimization.