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 (R_S) 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 R_S temperature. At lower aging temperatures (e.g., 250 °C, 300 °C), R_S gradually increases with aging time and finally saturates. The saturated R_S 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 Ti ₃Ni₄ 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 R_S for high temperature aging and the "time-dependence" of R_S for low temperature aging can also be reasonably understood. Besides, the R-B19′ transformation temperature (M_S) gradually increases with aging time as well, which can also be understood by considering the growth of precipitates during aging.