Hysteresis in NiTi alloys

Large changes in hysteresis are reported as a function of applied stress under thermal cycling experiments in NiTi single crystals. In the low Ni alloy the thermal hysteresis expanded with increasing stress while in the high Ni alloy the thermal hysteresis contracted with increasing stress. The results in both cases meet the limit obtained from Differential Scanning Calorimetry at zero stress. The changes are attributed to the relaxation of elastic stored energy which is primarily due to dislocations emanating at martensite/austenite interfaces. Modifications in thermodynamics formulation are proposed to account for the change in hysteresis via change in the elastic stored energy. Memory effects due to dislocation arrangements imposed under high stress thermal cycles on subsequent thermal hysteresis under low stresses were found to be significant, while variations in thermal hysteresis from cycle to cycle under constant stress are noted to be rather small.