Concentration of transformation-induced plasticity in pseudoelastic NiTi shape memory alloys: Insight from austenite-martensite interface instability

Transformation-induced plasticity (TRIP) in pseudoelastic polycrystalline NiTi shape memory alloys (SMAs) is investigated through tensile tests combined with in-situ infrared and digital image correlation (DIC) observations, and a new TRIP concentration phenomenon is revealed. To this end, an instability evolution model for solid-solid phase transformation is established to explain the multi-scale mechanisms of TRIP in SMAs. It is shown that, during the stress-induced transformation, the local stress at the grain scale evolves discontinuously inside austenite-martensite mixtures, presenting the maximum magnitude at the austenite side of the transformation interfaces. This local stress field triggers TRIP although the macroscopic stress still evolves "smoothly"with a relatively low magnitude. As the temperature increases, a further plasticity in austenite is required to lower the discontinuity at the austenite- martensite interfaces. Due to the accumulation of the heat generated in transformed regions, the temperature at the moving transformation interface continuously increases till reaching the maximum at the end of the transformation. Therefore, TRIP is concentrated at the final transformation band fronts, manifesting as concentration peaks in the residual strain map. The concentration of TRIP also shows a strong rate/frequency dependence: when the loading rate increases, TRIP accumulates to a higher average density throughout the transformation region while the concentration is significantly relieved.