ADVANCED MATERIALS & PROCESSES | NOVEMBER 2026 27 RESULTS AND DISCUSSION Microstructural Development. As shown in Fig. 1, although this material was carefully designed as a single- phase alloy, the homogenized microstructure still contains a small amount of Nb-rich secondary phases and Ti-oxide inclusions. After hot rolling, the Nb-rich phase is elongated in the rolling direction as a result of its relatively high deformability, whereas the brittle Ti-oxide particles fracture into fine fragments and form stringer-like distributions. The alignment of both phases along the rolling direction reflects the severe plastic deformation imposed during processing and contributes to the final microstructure that affects the alloy’s mechanical behavior. For example, the presence of Nb-rich phase can negatively affect the superelasticity by plastic deformation and Ti-oxide inclusions are direct cause of fatigue failure in most cases. High Transformation Stress. The stress-strain curves in Fig. 2 demonstrate that NiTiNbY exhibits substantially higher transformation plateau stress than binary NiTi subjected to the same cold drawing and heat treatment conditions. While both alloys display characteristic superelastic behavior, the NiTiNbY alloy requires considerably higher stress to induce martensitic transformation. This FEATURE elevated plateau stress translates directly into greater force output and load-bearing capability while retaining significant recoverable strain. The increase in transformation stress is particularly attractive for applications requiring compact actuation systems or structural components capable of sustaining higher loads without permanent deformation. The results indicate that the alloying strategy effectively strengthens the parent phase while preserving reversible transformation behavior. In addition to its elevated transformation stress, the NiTiNbY alloy exhibits a significantly higher Young’s modulus of approximately 70 GPa, compared with about 45 GPa for the binary NiTi alloy. This ~55% increase in stiffness provides greater resistance to elastic deformation and improved load-bearing capability, making the alloy particularly attractive for applications requiring high structural rigidity, force transmission, and dimensional stability while maintaining superelastic functionality. Heat Treatment Optimization. Figure 3 illustrates the influence of post-deformation heat treatment on the superelastic response of the NiTiNbY. It can be seen that this alloy is relatively insensitive to heat treatment time, with only modest variations observed in the stress-strain response at the same heat treating temperature (Fig. 3a). Conversely, heat treatment temperature has a much stronger influence on mechanical behavior, significantly affecting Fig. 1 — SEM images of NiTiNbY after (a) homogenization and (b) hot rolling. Fig. 2 — Stress-strain curves of NiTiNbY and a binary NiTi after the same cold drawing and heat treatment conditions. Samples with a diameter of 0.25 mm were heat treated at 400°C for 5 minutes. Fig. 3 — Stress-strain curves of (a) samples heat treated at 375°C for various times and (b) heat treated at different temperatures for 5 minutes. Wire diameter is 0.3 mm. (a) (b) (a) (b)
RkJQdWJsaXNoZXIy MTYyMzk3NQ==