The effect of aging on the B2-R transformation behaviors in Ti-51at%Ni alloy

Aging is an effective way to control the transformation behaviors of Ti–Ni shape memory alloys. In the present study, the transformation behaviors of Ti-51at%Ni polycrystals after aging at various temperatures (250 °C–500 °C) for different time were investigated by DSC measurement. It is found that B2-R transformation temperature (RS) of the aged samples is uniquely determined by the aging temperature for aging above 400 °C, being almost independent on aging time. Higher aging temperature causes a lowering of RS temperature. At lower aging temperatures (e.g., 250 °C, 300 °C), RS gradually increases with aging time and finally saturates. The saturated RS value is also a unique function of the aging temperature. These facts can be explained by the phase equilibrium between Ti–Ni matrix (B2) and Ti3Ni4 precipitates. The Ti3Ni4 precipitates formed at different aging temperatures alter the Ni concentration in B2 matrix and consequently cause an aging temperature dependence of the R-phase transformation of B2 matrix. By considering the kinetics of precipitation reaction, the “time-independence” of RS for high temperature aging and the “time-dependence” of RS for low temperature aging can also be reasonably understood. Besides, the R–B19′ transformation temperature (MS) gradually increases with aging time as well, which can also be understood by considering the growth of precipitates during aging.