Thermal cycling stability of Ni₅₅Mn₂₅Ga₁₈Ti₂ high-temperature shape memory alloy

Research on high-temperature shape memory alloys has attracted much attention due to the control requirements of the high-temperature drive (>100 ℃) and the overheating warning in high voltage transmissions, nuclear power, aerospace, automotive, oil exploration, and other engineering fields. High-temperature shape memory alloys refer to those with reverse martensitic transformation starting temperature (A_s) higher than 100 ℃. A wide range of high-temperature shape memory alloys exists, including Ti‒Ni‒Pd/Pt, Ni‒Ti‒Hf/Zr, Cu‒Al‒Ni, Ni‒Mn‒Ga, Ru-based, β-Ti-based, and Co-based systems. Besides the high transformation temperatures and good mechanical and shape memory properties, the thermal stability of microstructures and properties at high temperatures and after thermal cycling transformations is also an important basis for evaluating the practicability of high-temperature shape memory alloys. Dual-phase Ni‒Mn‒Ga‒Ti high-temperature shape memory alloys were chosen because of their better ductility compared with single-phase Ni‒Mn‒Ga alloys. In this paper, the as-quenched Ni₅₅Mn₂₅Ga₁₈Ti₂ high-temperature shape memory alloy was prepared. Specimens are then thermal-cycled at a temperature between the room temperature and 480 ℃ for 5, 10, 50, 100, and 500 times. The thermal stability of the microstructure, martensitic transformation temperatures, and mechanical and shape memory properties were studied by X-ray diffraction analysis, scanning electron microscopy, simultaneous thermal analyzer, and room-temperature compression analysis. Results show that there are no obvious changes in the phase structure and microstructure of the Ni₅₅Mn₂₅Ga₁₈Ti₂ high-temperature shape memory alloy after 500 thermal cycles. All as-quenched and thermal-cycled specimens show dual-phase structures with non-modulated tetragonal martensite and Ni-rich face-centered-cubic γ phase. With the increase of thermal cycling times, the forward martensitic transformation temperatures are almost kept constant, and the reverse martensitic transformation temperatures and the hysteresis are observed to be steady when the thermal cycles exceed five times. After 500 thermal cycles, the compressive strength and compressive stain slightly change, and the shape memory strain drops but remains over 1.4%. The Ni₅₅Mn₂₅Ga₁₈Ti₂ high-temperature shape memory alloy shows high thermal cycling stability.