<Original Papers>Effect of the Heat-treatment on the Microstructure of Martensite in TiNi Compounds

In order to clarify the effect of heat treatment on the shape memory effect of TiNi, the microstructure of TiNi martensite has been studied by the observation of transmission electron microscopy and the measurement of electrical resistivity under various heat treatments. The martensite in starting materials, transformation twins with (111)_M twinning plane and stacking faults with (001)_M basal plane were observed as the internal defects. The orientation relationship between the matrix and martensite was decided. The Ms temperature of Ti-50 at % Ni was constant regardless of changes in the heat treatment temperature, whereas the peak in ρ-T curves became clearly with the increase of heat treatment temperature. Effect of the heat treatment on the microstructure of martensite is summarized as follows. The transformation twins of the samples that were heated at higher temperature (700℃) grew wider. The stacking faults were remarkably observed in samples that were heated at an intermediate temperature (500℃). As compared with starting materials, the microstructure of the samples that were heated at lower temperature (300℃) was only slightly different. The width of the twinning plate increased with increase of the heat treatment temperature. The possible relation between the shape memory effect and microstructure of martensite are discussed. It is supposed that the variation of microstructure in martensite that was heated at higher temperature is caused by the change of transformation process from B2 (parent) → M (martensite) to B2→ R (intermediate phase)→ M