Deformation-induced martensitic transformation and its reversibility in Ni-Ti-Nb wide hysteresis shape memory alloys

The deformation-induced martensitic transformation and its reversibility in Ni-Ti-Nb alloys have been studied by means of tensile test at various temperatures, electrical resistance measurement, TEM and HREM observations. The relationship between the stability and reversibility of deformed stress-induced martensite and its variant interface structure was analyzed. It is shown that stress and strain induced martensitic transformations occur, respectively, when Ni-Ti-Nb alloys are deformed between M_s and M^σ_s temperatures and above M^σ_s temperatures. The deformation has obvious effect on interface structure of stress-induced martensite. The interface of stress-induced martensite (SIM) is straight and well-defined as well as perfectly coherent. However, ledges and distorted layers are formed in the interface of SIMs subjected to low deformation strain and the interfacial coherence is damaged to some extent. With the increasing of deformation strain, the ledges and distorted layers gradually turn into the confused layers and the interfaces of martensite variants completely lose coherence. Accordingly, the stability of deformed stress-induced martensite increases and its reversibility decreases with increasing tensile strain. Under the appropriate deformation conditions, the deformed stress-induced martensite subjected to suitable deformation strains has both high stability and good reversibility.