TRANSFORMATION BEHAVIOR OF NITI IN RELATION TO THERMAL CYCLING AND DEFORMATION

The transformation behaviour of an equiatomic NiTi alloy was investigated by measurement of the electrical resistivity and by differential scanning calorimetry. The temperature region of transformation depends on the number of thermal cycles and the intermediate phase appears progressively during cooling with increasing thermal cycles, resulting in a two-stage transformation, while the reverse transformation during heating remains a one-stage transformation. The temperature region of transformation is broadened by plastic deformation, and with subsequent annealing the intermediate phase is enhanced. depending on the annealing temperature. Effects of thermal cycling and annealing after deformation on the transformation of NiTi are attributable to transformation-induced defects as dislocations, and it is concluded that the defects make it possible to stabilize the intermediate phase.