Martensitic transfor mation involved mechanical behaviors and wide hysteresis of NiTiNb shape memory alloys

On the basis of thermoelastic martensitic transformation involved, the mechanical behaviors of NiTiNb shape memory alloys were experimentally studied and their wide hysteresis effect was theoretically discussed. It was found that even at the same temperature NiTiNb shape memory alloys perform different mechanical behaviors when under different states. In addition, the yielding behavior for the reorientation of martensite is dependent on the martensitic transformation temperature of NiTiNb alloys. The alloy with lower M_S temperature exhibits lower yield stress for the reorientation of martensite. According to kinetic and thermodynamic analysis of thermoelastic martensitic transformations, a relationship between the relaxation of elastic energy and the hysteresis expansion was established. It is suggested that the mechanism for wide hysteresis of NiTiNb shape memory alloys is predominantly attributed to the Nb element dissolved in the matrix of NiTi(Nb) phase, rather than the precipitated β-Nb phase. This mechanism gains support from the present experimental study that the NiTiNb alloys with low Nb content exhibit wide transformation hysteresis upon a proper pre-deformation.