Micromechanical Model of the Two Way Memory Effect in NiTi Alloys

The aim of this paper is to present a micromechanical modelling of the two way memory effect in NiTi shape memory alloys. The martensitic transformation is governed either by stress or temperature and its evolution is described by the volume fraction of martensite for each variant. The dependence of the resolved shear stress acting on the habit plane and along the direction of shape change on both the volume fraction and the temperature is postulated from thermodynamics. Such a relationship is established by dividing the resolved shear stress into a first part associated to the chemical and elastic energies and a second irreversible part reflecting mainly the energy dissipated during the interface movement. The polycrystal behaviour is then derived from the single crystal one assuming a uniform stress state over the whole sample. Regarding the two way memory effect, the irreversible part of the stress is taken dependent on the activity of the variants during the training, i.e. it increases for non-induced variants; simulated results are presented which are consistent with experimental works