Investigation on pseudoelastic training method and the generated two-way shape memory effect in NiTi shape memory alloy

With the advent of shape memory alloys (SMAs), several commercial and industrial applications were proposed due to their superior mechanical and biological properties. Among these materials, Nickel-Titanium (NiTi) alloys are widely applied and well-researched since they are characterized not only by good thermal and mechanical properties but also by excellent biocompatibility compared to other SMAs. In most of the applications, the structural components and devices made of NiTi SMAs work under cyclic thermomechanical loading and one of the major limitations facing the industrial use of this alloy is the degradation of the material when subjected to cyclic loadings (i.e., training). In this study, pseudoelastic training procedure in NiTi shape memory alloy and the resultant two-way shape memory effect are studied using in-situ electric resistivity measurement. At first, variations in the residual strain and in the electric resistivity during pseudoelastic training method are revealed. Then, by measuring the electric resistivity after training procedure (upon specified thermal cycling at stress-free condition) as well as the induced two-way shape memory strain, the effects of residual martensite and dislocation (plastic deformation) on the residual strain are investigated. The obtained results show that about 33% of the residual strain accumulated in 100 pseudoelastic cycles can be ascribed to the residual martensite and about 67% of the residual stain is attributed to the dislocations (plasticity).