ADVANCED MATERIALS & PROCESSES | NOVEMBER 2026 28 the transformation plateau stress and recoverable strain (Fig. 3b). This suggests that temperature is the primary factor controlling the evolution of the alloy’s microstructure and functional performance during heat treatments. Figure 4 summarizes the effects of heat treatment temperature on the phase transformation and mechanical properties of the NiTiNbY alloy. As shown in Fig. 4a, the transformation temperatures exhibited a systematic decrease with decreasing heat treatment temperature. For specimens heat treated below 600°C, no thermal martensitic transformation was observed down to -150°C, suggesting that the transformation temperatures were depressed below the measurement range. Such strong suppression of thermal martensitic transformation is beneficial for maintaining a stable austenitic phase and enabling superelastic behavior at cryogenic temperatures. This result is attributed to the increased stability of the austenitic matrix associated with residual cold-work-induced defects and dislocations, which hinder the lattice distortion required for martensite formation[3]. As the heat treatment temperature increases, progressive annihilation and rearrangement of defects reduce austenite stabilization, thereby enabling the recovery of martensitic transformation at higher temperatures. The corresponding mechanical properties are presented in Figs. 4b and 4c. Increasing heat treatment temperature resulted in noticeable changes in the transformation plateau stress and recoverable strain. Consistent with the reduced stability of the austenite phase, the transformation stress decreased with increasing heat treatment temperature, indicating a lower resistance to stress-induced martensitic transformation. Despite this reduction in transformation stress, the alloy maintained a high level of recoverable strain following heat treatment at temperatures below 600°C, demonstrating robust superelastic performance. While Fig. 3 shows that the mechanical properties are relatively insensitive to heat treatment time, which is advantageous for shape-setting processes and manufacturing quality control, Fig. 4 demonstrates that heat treatment temperature is a much more influential parameter governing both transformation behavior and mechanical performance. By adjusting the heat treatment temperature, the transformation temperature, plateau stress, stiffness, and superelastic recovery can be effectively tailored, enabling optimization of the property balance for specific application requirements. The ability to achieve high plateau stress while retaining substantial recoverable strain over a wide range of heat treatment conditions further highlights the versatility of the NiTiNbY alloy for demanding structural, medical, and cryogenic applications. Superelasticity at Extreme Low Temperatures. One of the most significant findings is shown in Fig. 5, where the NiTiNbY alloy maintains superelastic behavior over a remarkably wide temperature range, including extremely low temperatures. The stress-strain responses retain clear loading and unloading plateaus together with substantial strain recovery, indicating that reversible Fig. 4 — Effect of heat treatment temperature on (a) phase transformation temperatures and (b) and (c) mechanical properties. UTS: ultimate tensile strength; YS: yield strength; E%: elongation; UPS: upper plateau stress; and LPS: lower plateau stress. (a) (b) (c) FEATURE
RkJQdWJsaXNoZXIy MTYyMzk3NQ==