Low hysteresis and high cyclic stability in a Ti50Ni45.2Cu1Fe3.8 shape memory alloy

Shape memory alloys, exhibiting shape memory effect and superelasticity, are the key components of actuators and sensors. However, the hysteresis and cyclic instability associated with the martensitic phase transformation, limit their use in the long-duration precise control. We report a quaternary TiNiCuFe shape memory alloy with ultra-low hysteresis and high cyclic stability. The discovery of this alloy is guided by investigating the synergistic effect of Fe and Cu on the martensitic phase transformation. The optimized alloy Ti50Ni45.2Cu1Fe3.8, shows a hysteresis measured by electric resistance, as low as 0.3 K (the hysteresis by differential scanning calorimetry is about 3.5 K), and a shift of phase transformation temperature about 0.05 K (after 60 cycles). Compared to the state-of-the-art TiNiCuPd alloy with a Cu content higher than 10 at %, the Cu content in our alloy is reduced to only 1 at %, thereby suppress the brittleness and improve the workability. In addition, the precious metal, Pd is replaced by the low-cost Fe, which may facilitate the wide applications of shape memory alloys. The origin of the optimized hysteresis and cyclic stability, is ascribed to the possible phase coexistence of B19 and R that is realized by manipulating the phase transformation paths with Fe and Cu. This singular microstructure may accommodate more phase transformation strain and consequently mitigate the incompatibility of the austenite/martensite interfaces.