Recoverable strain induced by martensitic transformation during holding of NiTi-based shape memory alloys under constant stress and temperature

The aim of the present paper was to study the strain variation in the NiTi-based shape memory alloys during isothermal holding under stress. The recoverable strain of Ni51Ti49 and Ti40.7Hf9.5Ni44.8Cu5 alloys was studied on holding under stress after cooling (Regime 1), after active loading (Regime 2), or during holding without stress in the sample that demonstrates the two-way shape memory effect (Regime 3). The strain variation was found during holding under a stress in all regimes. This strain was recovered on subsequent heating or unloading, thus, the strain variation on holding was caused by the isothermal martensitic transformation. This isothermal strain depended on the chemical composition of the alloy, the regime for the isothermal holding, the stress, and the holding temperature. The maximum isothermal strain was 3.4% in Ti40.7Hf9.5Ni44.8Cu5 (Regime 1) and 6.1% in Ni51Ti49 (Regime 2). Holding in Regime 3 was accompanied by a small strain (less than 0.3%). The influence of the stored elastic energy on isothermal martensitic transformation on holding was discussed, and it was shown that a large isothermal strain was found if the transformation was accompanied by a small stored elastic energy.