Effect of Heat Treatment on the Martensitic Transformation in TiNi Compound

Abstract

In order to clarify the effect of heat treatment on the martensitic transformation in the TiNi compound, the electrical resistivity vs temperature curves of TiNi containing hyper-stoichiometric concentration of nickel which were heat-treated under various conditions were obtained. TiNi containing more than 51 at%Ni showed a remarkable quenching effect. The Ms temperature of water-quenched specimens decreased with increasing nickel concentration. The Ms temperature of Ti-50 at%Ni was constant regardless of the change in quenching temperature, whereas that of Ti-51 and 52 at%Ni decreased with increasing quenching temperature above 500°C. The Ms temperature of Ti-51 at%Ni, however, became constant by quenching from a temperature above 600°C. In Ti-52 at%Ni the Ms temperature decreased below liquid nitrogen temperature by quenching from a temperature above 750°C. The Ms temperature increased up to −5°C by aging at 500°C after water quenching from 1000°C in both Ti-51 and −52 at%Ni. In Ti-49.5, 50.0 and 50.4 at%Ni the appearance of a peak in resistivity-temperature curve was observed after some complete thermal cycles. The Ms temperature decreased during initial five cycles and then became constant. From the results obtained by electrical resistivity measurement and X-ray diffraction, it is concluded that the TiNi compound has a CsCl-type ordered structure and order-disorder transformation or peritectoid transformation does not exist in the temperature range from the Ms temperature to 1000°C. It is also found that the nonstoichiometric range of the compound becomes narrower with decreasing temperature and the maximum nickel concentration at 500°C is 50.8 at%Ni.