EXPERIMENTAL-STUDY OF MECHANICAL HYSTERESIS OF NITI DURING FERROELASTIC AND SUPERELASTIC DEFORMATION

Shape memory alloys are known to exhibit a range of novel thermomechan ical behaviour associated with the unique thermoelastic martensitic tr ansformation; Such behaviour includes the superelasticity associated w ith stress-induced martensitic transformation at relatively high tempe ratures and the ferroelasticity associated with a martensite reorienta tion process at low temperatures. Both the stress-induced martensitic transformation and the martensite reorientation are energy-dissipative processes, i.e., hysteretic between the forward and reverse processes . This work was aimed at studying the hysteretic behaviour of the ferr oelasticity and superelasticity observed in a polycrystalline NiTi all oy by carrying out simple shear deformation tests through both major a nd subloop cycles. It was found that subloops are always closed and en closed inside the major loop and that the stress hysteresis of a sublo op is only dependent on the strain amplitude of the subloop, regardles s of the position of the subloop inside the major loop.