Shape memory and superelastic nitinol wires: A combined mechanical, electrochemical, and ion release study for biomedical use

This work presents a study in detail of commercial shape memory (SM) and superelastic (SE) nitinol wires, including phase transformation analysis, mechanical testing, and electrochemical evaluation in physiological solutions. Phase transformation behavior was investigated by differential scanning calorimetry (DSC) and X-ray diffraction (XRD), confirming a phase transformation B2 → R → B19′ for both alloys. The SE wire exhibited lower transformation temperatures (Ms = −73.01 °C, Af = 53.29 °C) compared to the SM variant (Ms = −45.53 °C, Af = 67.87 °C), with a predominantly B2/austenitic state at room and physiological temperatures under the thermal history relevant to the present experiments enabling stress-induced martensitic transformation. Microstructural characterization by microscopy and XRD revealed acicular microstructures with Ni 4 Ti 3 and Ni 3 Ti 2 precipitates in both materials. Vickers microhardness testing revealed a higher hardness for the SE (429 HV0.2) relative to SM (259 HV0.2). Uniaxial tensile testing at −60 °C, 23 °C, and 100 °C established that SM wires achieve superior ductility in the martensitic state (32% strain at −60 °C), whereas SE wires exhibit complete pseudoelastic recovery with strain-dependent hysteresis. Electrochemical characterization via OCP, potentiodynamic polarization, and EIS in artificial saliva and phosphate-buffered saline demonstrated that the SM alloy presents superior corrosion resistance, characterized by nobler potentials, lower passive current densities, and higher polarization resistance. Ion release quantification by ICP-OES confirmed negligible titanium leaching and limited nickel release (0.90–0.98 µg·cm⁻²·day⁻¹) for SM, with SE below detection limits. These findings suggest that thermomechanical processing influences functional, mechanical, and electrochemical behavior, and that differences in material condition have a stronger effect than the test electrolyte on corrosion resistance. Thus, material selection must balance mechanical functionality with electrochemical stability for biomedical applications.