Multi-stage reversible transformation types and their evolving processes of Ti-Ni shape memory alloys

The multi-stage reversible transformation types and their evolving processes of 350-800 degrees C annealed and 300-500 degrees C aged Ti-(50.2-50.8)Ni (atomic fraction, %) shape memory alloys were investigated by the partial thermal cycling method in differential scanning calorimetry (DSC). The results show that two kinds of the reversible transformations, R and martensite, occured in these alloys, and each transformation can complete in one-stage, and also in multi-stage. The transformation of aged alloy is more complicated than that of annealed alloy. The lower the aging temperature, the more complicated the transformation. If the transformation type is expressed with the ratios of peak numbers of cooling and heating transformations on DSC curves, the 1/1, 2/1, 2/2, 3/2 and 3/3 type transformations occured upon cooling and heating in the annealed and aged alloys, and the heat treatment processes for each transformation type to take place were given out. The multi-stage reversible transformation is a gradually changing process. When partial thermal cycling was carried out within the temperature range of cooling transformation peak, the transformation temperature of reverse transformation peak decreases with decreasing the cooling stop temperature or heating start temperature.