Grain size effect on stress hysteresis of nanocrystalline NiTi alloys

In this paper, the grain size effect on the stress hysteresis of a nanocrystalline NiTi alloy during superelastic cycling was investigated by testing two types of samples with 39% and 75% cold-drawn area reduction. The TEM microstructure and DSC analysis reveal that the sample with 75% area reduction experiences a crystallization process upon heating and contains few dislocations after annealing. The activation energy for grain boundary migration of the NiTi alloy was analysed and found to be 353.6 ± 15.3 kJ/mol. The hysteresis analysis shows that the stress hysteresis of the samples after annealing increases with decreasing grain size. We propose that the energy dissipation per volume of grain boundary is larger than that of the grain core in the nanocrystalline NiTi alloy, which contributes to this grain size effect. However, the sample with 39% area reduction does not experience the crystallization process upon heating, and many dislocations are encountered in the samples after annealing. The stress hysteresis of the samples with 39% area reduction after annealing decreases with increasing dislocation density. The results clarify the difference between the grain size effect and the dislocation density effect on the stress hysteresis of nanocrystalline NiTi alloys.