Very High Cycle Fatigue of Wrought NiTi Shape Memory Alloys

Abstract Very-high-cycle fatigue (VHCF) data for superelastic NiTi beyond 10 7 cycles remain limited, particularly for wrought, precipitate-tuned conditions tested under ultrasonic loading. Here, the VHCF response of a Ni-rich wrought NiTi (Ni50.8Ti49.2) was quantified by coupling calorimetry and monotonic mechanics with 20 kHz axial ultrasonic fatigue in fully reversed loading (R = − 1) using pulse-pause control and forced-air cooling to mitigate self-heating. The alloy was solutionized at 1000 °C for 2 h and aged at 600 °C for 1 h to promote Ni4Ti3 precipitation. DSC showed suppressed transformation temperatures relative to the non-heat-treated state, with a two-step B2 → R → B19′ sequence on cooling and a sharp reverse transformation on heating (A f = 9–10 °C). Room-temperature tensile tests confirmed a fully austenitic starting state (E = 61.9 GPa) and a stress-induced martensitic transformation (SIMT) onset near 223 MPa with a 250–300 MPa transformation plateau. S–N data spanning 10 5 to 10 9 cycles were fitted by a Basquin relation (σ f ’ = 653.7 MPa, b = − 0.089), indicating a gradual fatigue-strength reduction with increasing life and increased scatter in the VHCF regime without a clear endurance limit. Fractography and EDS revealed a stress-dependent transition in crack initiation: higher-amplitude failures were dominated by surface slip/damage, whereas VHCF failures initiated subsurface at Ti − rich non-metallic inclusions (TiC and Ti2NiOx). These results provide a benchmark VHCF dataset for wrought NiTi under R = − 1 ultrasonic loading and clarify how precipitation-tuned transformation behavior and inclusion populations govern VHCF crack initiation.