Thermal Cycling Effects in an Aged Ni-rich Ti–Ni Shape Memory Alloy

Abstract

Thermal cycling effects in an aged Ni-rich Ti–Ni shape memory alloy have been examined by means of differential scanning calorimetry (DSC) and transmission electron microscopy (TEM), changing aging temperature and time, temperature range for thermal cycling, and the number of thermal cycles up to 10⁴ times. Two kinds of thermal cycling termed “complete cycling” and “incomplete cycling” were adopted: The complete cycling caused the B2 matrix\ ightleftarrowsrhombohedral intermediate (R phase)\ ightleftarrowsmonoclinic martensite (M) transformations in the appropriately aged alloy, and the incomplete cycling did only the first B2\ ightleftarrowsR transformations. The DSC measurement revealed that the complete cycling significantly affected not only the R→M transformation temperature, M^*, but also the B2→R one, T_R^*, of the aged alloy, M^* being lowered to a great extent whereas T_R^* being raised up considerably. Meanwhile, the R→B2 reverse transformation temperature, T_R^r*, was raised like wise with T_R^*, and the M→R reverse one, A^*, was lowered a little. On the other hand, the incomplete cycling brought about substantially no change in the transformation temperatures. The TEM observation on the aged alloy subjected to the complete cycling of 10⁴ times showed that the B2 matrix regions between the Ti₃Ni₄ precipitates were heavily roughened by some lattice defects. However, the TEM observation on the alloy subjected to the incomplete cycling showed no change. The observed decrease in M^* and increase in T_R^* are discussed standing on a point of view that stress fields around the precipitates formed during aging may be relaxed by the thermal cycling.