Internal pressure as a key thermodynamic factor to obtain high-temperature superelasticity of shape memory alloys

Stress–strain loops illustrating the superelastic behaviour of shape memory alloys (SMAs) were computed based on the theory of ferroelastic phase transitions. The predictions of the theory demonstrate the possibility of drastic changes in the stress–strain dependences due to the expansion of the SMA upon heating. Specifically, the computations were carried out taking into account the characteristics of Co-Ni-Ga alloys, which exhibit a high-temperature superelasticity. It is shown that the expansion of crystal lattice, which can be caused by the appearance of small particles and crystal defects, or change of chemical order in SMA, can induce (i) an extension of the temperature range of superelastic behaviour of SMA to high temperatures; (ii) an increase of the superelastic strain at elevated temperatures; (iii) an increase of the stress needed to reach the superelastic strain plateau and (iv) a widening of the hysteresis of stress-induced martensitic transformation. Theoretical results are in a qualitative agreement with experimental data obtained for Co-Ni-Ga alloys.