| There are many problems to be solved for more wide practical applications of near-equiatomic TiNi shape memory alloys. In this point of view, a study has been made of the effect of isothermal heat treatment in the temperature range of 450℃ to 750℃ on the transition temperatures, especially M_s and A_f, and on the mechanical properties and the effect of thermal cycling on the electric resistivity curve. For this investigation, a Ti-rich TiNi shape memory alloy containing 52.36 at.% Ti was prepared. Thermal cycling caused the electric resistivity peak and the decrease of M_s. but did not affect T_R, the temperature at which electric resistivity started increasing. Both M_s and A_f increased with the temperature of isothermal heat-treatment above 450℃. when the increasing rate of M_s was higher than that of A_f. Elongation and tensile strength were highly increased by long-time (100 hr) isothermal heat-treatment and these were dependent on the holding time rather than the temperature. There also existed the variation in the fracture mode about 550℃. |
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