Localized thermal treatment of Ni-Ti thin wires under controlled tensile load

Ni-Ti-based shape memory alloys (SMAs) are widely used in aerospace, bioengineering, and robotics due to their unique properties such as shape memory effect, superelasticity, high strength, corrosion resistance, and biocompatibility. These functional characteristics arise from phase transformations between austenite and martensite structures, and intermediate R phase formation promoted by Ni4Ti3precipitates. Traditional thermal treatments (TTs) of SMAs are typically conducted in furnaces, but direct electric resistance heat treatment (DERHT) has emerged as a promising alternative, allowing faster processing through Joule heating. Despite the advantages of DERHT, current studies often lack repeatability and precise control of heat dissipation, leading to inconsistent mechanical properties. This study presents a cost-effective and versatile setup for localized TT of Ni-Ti wires via DERHT, designed to maintain consistent tension during treatments. The power supplied to the samples is calibrated through a thermal camera, which allows for a fine tuning of the temperatures. Moreover, the system is equipped with a load cell for tensile stress monitoring. Two TT protocols were investigated: TT1 (350 °C for 30 min) for superelasticity and TT2 (500 °C for 15 min) for the shape memory effect, with an additional variant (TT1-50) under controlled tensile stress (50 MPa). Differential scanning calorimetry and mechanical testing demonstrated consistent phase transformation temperatures and mechanical behavior across samples, validating the reliability of the proposed setup. Microstructural analyses revealed variations in precipitate morphology and distribution as a function of treatment conditions. The results confirm that the DERHT method, combined with controlled tensile stress, offers precise tuning of functional properties and provides a valuable tool for the efficient and repeatable local functionalization of Ni-Ti wires in advanced applications.