Modeling and simulations of coupled transformation and plasticity in NiTi

Description of mechanisms of the plastic deformation in high-quality NiTi superelastic wires is far from trivial, as basic understanding of these mechanisms is still missing. The mechanisms of deformation qualitatively change with temperature in these wires – from functional behaviors stemming from the reversible martensitic transformation at low temperatures (<50 °C) through large plastic deformation generated alongside the martensitic transformation at the medium temperature range (100–250 °C) and, finally, to conventional plastic deformation of austenite at high temperatures (>300 °C).

In this contribution, we present a macroscopic thermodynamical description of the observed phenomena. The conventional approach describing plastic deformation mechanism by models for metal plasticity with a phase dependent yield surface was found to be insufficient to capture the evolution of the plastic strain in experiments. It was also impossible to adapt the equations usually used for the description of the transformation-induced plasticity (Greenwood–Johnson mechanism), which assumes a proportionality between the rate of the plastic strain and the rate of the martensite volume fraction. Instead, a new mechanism of the transition from martensite to plastically deformed austenite, which can develop simultaneously alongside the usual reversible austenite-martensite transformation, was proposed. A comparison of experimental and simulated data will be presented, and it will be also discussed how the understanding of these phenomena can allow tailoring of strain heterogeneity inside NiTi components, which would open new opportunities in NiTi components design.