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

• Published in: Shape memory and superelasticity : advances in science and technology 2022-12, Vol.8 (4), p.394-412
• Main Authors: Sidharth, R., Mohammed, A. S. K., Sehitoglu, H.
• Format: Article
• Language: English
• Subjects: Characterization and Evaluation of Materials; Chemistry and Materials Science; Degradation; Dislocations; Hysteresis; Intermetallic compounds; Latent heat; Martensite; Martensitic transformations; Materials fatigue; Materials Science; Nickel base alloys; Nickel titanides; Precipitates; Shape effects; Shape memory alloys; Single crystals; Slip; Superelasticity; Technical Article
• ISSN: 2199-384X; 2199-3858
• DOI: 10.1007/s40830-022-00397-8

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.