Temperature and microstructure dependence of localized tensile deformation of superelastic NiTi wires

Highlights: • Localized tensile deformation of superelastic NiTi wires having range of microstructures studied in wide temperature range. • Magnitude and recoverability of upper plateau strains depend on the test temperature and wire microstructure. • Long plateaus and loss of the strain recoverability ascribed to the martensitic transformation into twinned austenite. • Refinement of the virgin austenite microstructure by the martensitic transformation into twinned austenite. -- Abstract: Superelastic NiTi wires possessing wide range of microstructures were deformed in tensile tests up to the end of the stress plateau in a wide temperature range −20 – 200 °C, unloaded and stress free heated to ~200 °C. The recoverability of upper plateau strains was evaluated in dependence on the test temperature and wire microstructure. At low temperatures below 20 °C, the upper plateau strains are fully recoverable upon unloading and heating. With increasing test temperature, however, the recoverability of upper plateau strains becomes gradually lost. Unusually long upper plateaus (10–17%) were observed at test temperatures, at which the upper plateau stress approached the yield stress for plastic deformation of martensite. The volume fraction of the martensite phase at the end of the stress plateau (evaluated by in-situ electric resistance and synchrotron x-ray diffraction) decreases with increasing test temperature in the range where the long plateaus were observed. TEM analysis revealed high density of {114} austenite twins in wires deformed in this temperature range. These observations are rationalized by gradual increase of the activity of the stress-induced B2 → B19′ → B2^T martensitic transformation into twinned austenite and plastic deformation by dislocation slip with increasing test temperature.