Effect of varisized Ni4Ti3 precipitate on the phase transformation behavior and functional stability of Ti-50.8 at.% Ni alloys

The functional stability of Ti-50.8 at.% Ni alloys was improved by conducting various aging treatments (250–550 °C for 0.5–12 h) after stretching-bending deformation (SBD). The aforementioned was performed to explore the effects of varisized Ni4Ti3 precipitate on the phase transformation behavior and functional stability of NiTi alloys. As the aging temperature increased from 250 to 550 °C, a two-stage phase transformation A → R and R → M replaced the one-stage transformation A → M. At a lower aging temperature (250 °C, 0.5 h) after SBD, no Ni4Ti3 precipitates were detected. Aging at 350 °C for 0.5 h after SBD yielded a uniform distribution of nanoscale Ni4Ti3 precipitates (size: <15 nm), thereby improving the superelasticity. A higher aging temperature of 450 °C for 0.5 h after SBD resulted in an uneven distribution of Ni4Ti3 precipitates with an average size of ~50 nm. Further, aging at 550 °C for 0.5 h after SBD resulted in oversized Ni4Ti3 precipitates with an average size of ~80 nm (up to 150 nm), thus reducing the superelasticity.