Effects of high-pressure torsion treatment on the microstructural aspects and electrochemical behaviour of austenitic NiTi shape memory alloy

In this study, microstructural aspects and electrochemical properties of high-pressure torsion (HPT)-treated austenitic NiTi alloy were investigated. Microstructural characterizations were performed by high-resolution transmission electron microscopy (HRTEM) and X-ray diffraction (XRD) analysis. Potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) tests were used to evaluate the electrochemical properties in simulated body fluid (SBF) solution. It was found that HPT process increased the martensitic phase from 8.22 to 71.31 wt%. HRTEM results revealed a combination of amorphous/nanocrystalline austenitic and martensitic phases of NiTi after HPT. The lower value of corrosion current density of HPT-treated NiTi (2.5 x 10(-6)A/cm(2)) was observed compared with the untreated sample (7.6 x 10(-6)A/cm(2)). EIS tests confirmed higher corrosion resistance of the HPT-treated sample. EIS analysis showed a decrease and increase in the impedance value of the specimens at different immersion times indicating the formation and degradation of oxide films over time on the specimens' surface. However, the impedance value of the HPT-treated sample was more than that of the untreated sample. Localized corrosion is observed at the surface of the untreated specimen due to the absence of a dense oxide protective film but no evidence of localized and pitting corrosion on the surface of HPT-treated sample was observed.