Constitutive model for Ni-Ti-Fe shape memory alloys exhibiting pronounced R-phase transformation

Specific processing, heat treatment or addition of a ternary element may induce a two-stage transformation sequence in Ni-Ti shape memory alloys. A typical example of intermediating phases is the R-phase, a rhombohedral distortion of the cubic austenitic phase exhibiting much lower transformation strain and thermal hysteresis than martensite. In specific alloys, e.g., in Ni-Ti slightly enriched by Fe, the temperature and stress intervals in which R-phase is stable are quite broad. Hence, the occurrence of R-phase must be usually taken into account when dealing with the thermomechanical response of these alloys.

Based on extensive experimental testing of a Ni-Ti-Fe polycrystalline material, we have developed an enhanced constitutive model for Ni-Ti-based SMA featuring the R-phase transition. The model recognizes R-phase as a distinct phase with its own transformation strain, respects the coupled influence of stress and temperature on any phase transformation, and covers reorientation (reconfiguration) of both martensite and R-phase with applied stress. The experimental tests were simulated to demonstrate the constitutive model's capabilities, and the results are directly compared with experimental data. A finite element simulation of a Ni-Ti-Fe tightening ring is also presented.