Influence of niobium on the thermal and microstructural behavior of TiNiCu shape-memory alloys

NiTi-based Shape Memory Alloys (SMAs), such as Nitinol, are widely used in biomedical applications due to their shape memory effect, superelasticity, and biocompatibility. While copper addition enhances mechanical strength and compositional tolerance, it introduces brittleness, limiting processability. To mitigate this limitation, niobium additions can be used to improve ductility but may increase the hysteresis of the thermoelastic martensitic transformation. The present work analyzes the effect of Nb x (x = 0, 10, 15 at%) in addition to the thermal hysteresis and the microstructure of a Ti 50 Ni 30 Cu 20 alloy. The results showed that Ti 50 Ni 30 Cu 20 (reference alloy) is constituted by a martensitic B19 matrix and Ti 2 (Ni,Cu) particles. Further Nb addition leads to a hypoeutectic microstructure consisting of B2 austenitic matrix and a ductile enriched niobium phase (β-Nb); the interdendritic lamellae was refined with 10 at% Nb, but 15 % resulted in a hyperfine morphology. It also resulted in a lower fraction of intermetallic Ti 2 (Ni,Cu). In the three alloys studied, only the B2↔B19 transformation was present upon heating and cooling. A reduction in all phase transformation temperatures and thermal hysteresis was also observed. Vickers microhardness decreased with the addition of 10 at% Nb, but it increased again with the introduction of 15 at% of this element, due to the microstructure refinement. The reduction of the Ti 2 (Ni,Cu) fraction, the suppression of the B19↔B19’ transformation, and the ultrafine hypoeutectic microstructure show that the addition of Nb to NiTiCu alloys has the potential to increase the processability of these materials and promote a new range of applications. • TiNiCu shape memory alloys were fabricated with niobium additions up to 15 at%. • B19↔B19’ transformation was suppressed in all compositions (0, 10 and 15 at% Nb). • Nb addition affected the transformation temperatures and the thermal hysteresis. • A hypoeutectic lamellar structure was found in the 15 at% Nb, increasing hardness.