MICROMECHANICAL MODEL OF THE 2 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 tr ansformation is governed either by stress or temperature and its evolu tion is described by the volume fraction of martensite for each varian t. The dependence of the resolved shear stress acting on the habit pla ne and along the direction of shape change on both the volume fraction and the temperature is postulated from thermodynamics. Such a relatio nship is established by dividing the resolved shear stress into a firs t part associated to the chemical and elastic energies and a second ir reversible part reflecting mainly the energy dissipated during the int erface movement. The polycrystal behaviour is then derived from the si ngle crystal one assuming a uniform stress state over the whole sample . Regarding the two way memory effect, the irreversible part of the st ress is taken dependent on the activity of the variants during the tra ining, i.e. it increases for non-induced variants. simulated results a re presented which are consistent with experimental works.