Effect of annealing on stress-induced transformation behaviors at low temperatures in a Ti-51.8 at.% Ni shape memory alloy

For polycrystalline Ti-51.8Ni (at.%) specimens annealed at 473 and 943K, the microstructures at room temperature were observed by transmission electron microscopy and the stress-induced transformation behaviors at low temperatures were investigated by compressive test. The precipitation of Ti₃Ni₄ was confirmed in the both annealed specimens, where the precipitates at 943K had a lenticular shape in micrometer size, and those at 473K seemed to be clusters in nanometer size. Superelasticity with almost complete shape recovery was detected at temperatures below 200K for all the annealed specimens. In comparison to the as-homogenized specimen, although the temperature dependence on the stress hysteresis only slightly changed in each annealing condition, the equilibrium stresses in the specimen annealed at 473 and 943K apparently increased and decreased, respectively.