Effect of heat treatment and cooling rate on phase transformation behavior of a Ti-rich NiTi shape memory alloy

Abstract This study investigates the influence of heat treatment and cooling rate on the phase transformation behavior of a commercially obtained Ti–rich NiTi shape memory alloy (nominal composition Ni–47 at% and Ti–53 at%). The microstructural evolution, phase constitution, and transformation characteristics were systematically analyzed using scanning electron microscopy coupled with energy-dispersive spectroscopy (SEM–EDS), X-ray diffraction (XRD), and differential scanning calorimetry (DSC). The as-received alloy exhibited a fully austenitic B2 structure and superelastic behavior at room temperature. Solution treatment at 1000 °C followed by water quenching resulted in a well-defined single-step B2 ↔ B19′ martensitic transformation. In contrast, subsequent annealing at 800 °C combined with controlled furnace cooling at rates of 0.5–1.0 °C min⁻ 1 induced multi-step transformation sequences involving B2 → R → B19′ phases, accompanied by broadened transformation hysteresis and shifted transformation temperatures. These results demonstrate that tailored thermal processing and cooling rate control provide an effective pathway to engineer transformation sequences and tune the functional behavior of Ti–rich NiTi shape memory alloys, a compositional range that remains relatively less explored in the literature.