The nature of reversible change in M_S temperatures of Ti-Ni alloys with alternating aging

Ni-rich Ti-Ni alloys were aged alternately between 713 K and 823 K. The transformation temperatures and the microstructures of samples after the alternating aging treatments were systematically investigated by DSC and TEM. The transformation temperatures (R_s, M_s, A_f) showed nearly reversible changes, corresponding to the two aging temperatures; aging at 713 K (low temperature) leads to higher transformation temperatures and aging at 823 K (high temperature) leads to lower transformation temperatures. Meanwhile, some irreversible features were also found in this experiment, such as the dependence of martensitic transformation temperatures on the aging time and the number of alternating aging cycles. The nature of this interesting behavior (the reversible change and irreversible features) for Ni-rich Ti-Ni alloys found a simple explanation by considering the temperature dependence of the solubility of Ni in TiNi matrix, which is in equilibrium with the metastable Ti₃Ni₄ precipitates, as well as the effect of Ti₃Ni₄ precipitate size.