Accumulation of Residual Strain in TiNi Alloy During Thermal Cycling

Accumulation of residual strain on thermal cycling of the TiNi alloy under constant stress of 50 and 200 MPa through the temperature range of complete and incomplete forward martensitic transformation was investigated. Wire samples were subjected to thermal cycles through 25, 50, 75, 100% of temperature range of forward martensitic transformation. The values of transformation plasticity effect, shape memory effect and irreversible strain were measured. Martensitic transformation kinetics was studied by differential scanning calorimetry and resistivity measurement methods. It was found that thermal cycling under a constant stress of 50 MPa through the temperature range of incomplete forward martensitic transformation (Φ≤50%) allowed to significantly decrease accumulation of residual strain in comparison to complete cycles, however the values of deformation effects were rather low. So increase of temperature range fraction leads to accumulation of residual strain, observation of “training effect” (an increase in values of transformation plasticity and shape memory effects with increase of thermal cycles number) and shifting of transformation temperatures. It was found that thermal cycling TiNi alloy under a constant stress of 200 MPa influenced in other way on its properties. It was observed that rate of residual strain accumulation was lower during thermal cycling through the temperature range of incomplete forward martensitic transformation than in a full cycle. Thus one may conclude that in the initial stage of martensitic transformation martensite accommodates mostly elastically and on the final stage due to large internal stresses martensite accommodates substantially plastically.