Softening process during reverse martensitic transformation in TiNi shape memory alloy

• Strain accumulation depends on the fraction of the reverse martensitic transformation.
• Softening process takes place on heating of shape memory alloy up to 140 °C.
• Softening process is controlled by the maximum temperature attained on heating.

Strain variation in a Ti50Ni50 shape memory alloy subjected to 30 thermal cycles under a constant stress of 50 MPa through the temperature range of incomplete reverse martensitic transformation was studied. It is found that the rate of plastic strain accumulation depends on a fraction of the reverse transformation temperature range realized on heating of the Ti50Ni50 alloy during thermal cycling. The larger the fraction of the reverse transformation temperature range, the higher the rate of plastic strain accumulation was observed during thermal cycling. If the fraction of the reverse transformation temperature range is 0.75 or less, strain hardening results in a full arrest in the plastic strain accumulation at the stationary stage. Otherwise, the alloy accumulates the plastic strain with a constant rate at the stationary stage. It was shown that the strain hardening incompletely arrests the plastic strain accumulation at the last quarter of the reverse transformation temperature range due to the realization of a relaxation process on heating, which results in alloy softening. This is caused by an increase in dislocation mobility owing to a rise in sample temperature hence, this process is thermally activated. Completion of the softening process is controlled by the maximum temperature attained by heating during thermal cycling.