Influence of heat treatments on the tensile properties and thermal expansion behavior of Fe-Ni wire

Although Fe-Ni wires have numerous potential applications, the effect of heat treatments on the microstructures and properties of Fe-Ni wires is currently unclear. In this study, different high-temperature annealing and low-temperature aging treatments (i.e., ① annealing at 950℃ for 3 h+quenching, ② annealing at 950℃ for 3 h+quenching+aging at 500℃ for 2 h, and ③ aging at 500℃ for 2 h) were applied to cold-drawn Fe-36.24% Ni alloy wires to comparatively investigate their influences on the microstructural evolution, tensile strength, and thermal expansion coefficient, with the aid of microstructural characterization methods, such as X-ray diffraction, optical microscopy, atomic force microscopy, and transmission electron microscopy, as well as property measurements, such as room-temperature tensile testing and thermal expansion coefficient. The experimental results clearly show that the as-drawn Fe-Ni wire, although having a high tensile strength, exhibits an unfavorable high thermal expansion coefficient. The Fe-Ni wires annealed at 950℃ (wires subjected to ① and ②) have a relatively low thermal expansion coefficient but exhibit insufficient strength. In comparison, the wire aged at 500℃ (wire subjected to ③) shows a high strength (1189 MPa) and a low thermal expansion coefficient (0.2×10^-6℃^-1) at the same time. The relative strengthening mechanisms and factors affecting the thermal expansion coefficient are discussed and analyzed in terms of the microstructures. Grain boundary strengthening and dislocation hardening are observed to be the dominant strengthening mechanisms of the Fe-Ni wires, and the solute atom-dislocation interaction is mainly responsible for the evolution of the thermal expansion coefficient. The present work clearly demonstrates that suitable heat treatments are important for the optimization of the strength/thermal expansion coefficient of Fe-Ni alloy wires, which will be helpful for material design and technology tailoring of Fe-Ni wires to develop a new alloy with enhanced performance.