B2=>B19'=>B2T martensitic transformation as a mechanism of plastic deformation of NiTi

Based on the results of systematic thermomechanical loading experiments on superelastic and actuator NiTi wires, in-situ synchrotron x-ray diffraction and TEM observation of lattice defects in deformed wires, it is proposed that NiTi alloys transforming martensitically at high temperatures and high stresses undergo a sequential B2=>B19‘=>B2^T martensitic transformation into twinned austenite. As this transformation can be viewed as an irreversible sequential rearrangement of atomic configurations involving martensitic transformation and deformation twinning, it effectively represents a mechanism of plastic deformation of NiTi alternative to dislocation slip in austenite and/or martensite phases. The crystallographic and thermodynamics aspects of this deformation mechanism are outlined and experimental evidence supporting its activity in selected thermomechanical loads on superelastic and actuator NiTi wires is presented. It is shown how the activity of this deformation mechanism depends on the temperature and microstructure of the wire. On one side, this mechanism introduces unrecoverable strains in cyclic superelastic and actuator loads and ultimately destroys the shape memory and superelastic functionality of NiTi at high temperatures. On the other side, it enables shape setting and large mechanical deformation of NiTi at relatively low temperatures.