Functional Fatigue of NiTi Shape Memory Alloy: Effect of Loading Frequency and Source of Residual Strains

Due to latent heat release and the consequent thermomechanical coupling, NiTi SMAs are highly rate sensitive. In this study, the effect of loading frequency on the functional characteristics of superelastic NiTi is explored under fatigue loading. The variation and interplay between the elastocaloric temperature change, forward/reverse transformation stresses, residual strain accumulation and hysteresis were studied for single crystals compressed along the 〈011〉 and 〈001〉 loading orientation. It is shown that the functionality of NiTi is better at higher loading frequency due to reduced hysteresis and the functional degradation is linked to transformation induced plasticity in the austenitic domains. TEM investigation reveals transformation induced parallel dislocations not only on the widely reported ⟨ 100 ⟩ \{011\} _B2 slip system but also on the ⟨ 1 1 ¯ 1 ⟩ \{011\} _B2 slip system. The source of these dislocations is traced to the type II and type I internal twins of the martensite correspondent variant pair (CVP) via lattice correspondence, respectively. Moreover, it was observed that these dislocations pile-up at the precipitate-B2 interface and activate slip and/or shear into the precipitates. This is yet another functional degradation and dissipative mechanism that has not been reported before.