HYSTERESIS OF PHASE TRANSFORMATION TEMPERATURE IN NiTi SHAPE MEMORY ALLOY

The relationship between structure and hysteresis of phase transformation temperature in NiTi shape memory alloy has been investigated by means of TEM observation,positron an- nihilation and electrical resistivity measurement.The sequence of hysteresis for the alloy aged under different regimes was found to be:plate martensite>R-phase>tie-like martensite. The reversible displaeement of phase boundaries of these transformations is blocked by the co- herent stress field around Ti_(11)Ni_(14)phase particles.A linear relationship between S paramet- er of positron annihilation and maximum values of temperature hysteresis showed that the mismatch dislocation and elastic stress field established by Ti_(11)Ni_(14)phase precipitation are the main factor to determine the temperature hysteresis of phase transformation in NiTi shape memory alloy.