Effects of aging temperature and aging time on the mechanism of martensitic transformation in nickel-rich NiTi shape memory alloys

The effects of aging temperature and time on the shape memory behavior of the high nickel-rich Ni₅₄Ti₄₆ (at.%) alloys were investigated. The changes in transformation temperatures, microstructure, lattice structure and mechanical behavior with aging were revealed. Multiple-stage phase transformation was observed for aged specimens due to the formation of Ti₃Ni₄, Ti₂Ni_3, and TiNi₃ precipitates. It was revealed that the transformation temperatures can be tailored by aging where M_s was −3 °C after 550 °C–24 h aging and 29 °C after 400 °C–3 h aging. Aged samples showed a shape memory effect with a recoverable strain of <1.6% where 550 °C–3 h aged Ni₅₄Ti₄₆ alloy showed a maximum recoverable strain of 1.55% under 500 MPa. The maximum work output was determined as 7.75 J/cm³ for 550 °C–3 h aged Ni₅₄Ti₄₆ alloys. Partial shape recovery was also observed in aged samples.