Microstructure and mechanical properties of Ni-rich NiTi alloys with Zr and Hf additions

This study investigates the microstructure and mechanical properties of Ni 55 Ti 42 (Hf, Zr) 3 (at.%) alloy, with a focus on the evolution of its microstructure after heat treatment. The solution-treated Ni 55 Ti 42 (Hf, Zr) 3 alloys consist mainly of matrix(B2), Ni 4 Ti 3 (at.%) phase and nanodomains. Their hardness exceeds 680 HV, and tensile strength surpasses 1800 MPa. Ni 55 Ti 42 (Hf, Zr) 3 alloys exhibit the Elinvar effect, maintaining nearly constant modulus above 220°C due to the balance between matrix softening and hardening from nanodomain transformation into the H-phase. The Ni 55 Ti 42 (Hf, Zr) 3 alloy showed excellent structural stability after heat treatments at 400°C and 500°C for 100 h. However, after heat treatment at 600°C for 100 h, the hardness of the four alloys significantly decreased, with only the hardness of the Ni 55 Ti 42 Hf 3 alloy remaining at 600.2 HV. This is attributed to the slower coarsening kinetics of Hf-containing alloys compared to those with Zr. These findings highlight the potential of Ni 55 Ti 42 (Hf, Zr) 3 alloys for applications in various engineering fields.