ASSESSMENT OF THE INFLUENCE OF R-PHASE FORMATION ON THE MATERIAL BEHAVIOR OF NiTi USING A MICROMECHANICAL MODEL

Shape memory alloys (SMA) show several interesting features in their material behavior which are due to martensitic phase transformation. In NiTi, this transformation covers the three crystallographic phases of austenite, martensite, and R-phase. This publication analyzes the influence of the R-phase formation on the overall material behavior by means of a micromechanical model. The model is based on energy minimization with the assumption of a certain energy dissipation when martensite is formed. To simplify the formulation, the dissipation associated with the transformation between austenite and R-phase is neglected, since the geometrical change in unit cell geometry is relatively small in this case. Numerical simulations show that especially the slope reduction in the stress-strain curve, which is known from experiments, can be explained by R-phase formation.