Microstructure-property relationship in Zr-alloyed Ni-rich NiTi alloys: Enhancements in high-temperature stability and superelasticity

This study delves into the influence of the Zr element on the thermal stability and superelasticity of Ni-rich NiTi(–Zr) alloys. The inclusion of Zr heightened the hardness, reshaped the morphology of the Ni₄Ti₃ phase, and promoted the formation of nanodomains. The Zr-containing alloys demonstrate remarkable thermal stability, as evidenced by the hardness of 3Zr alloy remaining at 540.6 HV after 100 h of exposure to 600 °C. This marks a substantial 63.9% improvement compared to the 0Zr alloy. After treatments at 400 °C and 500 °C, the 3Zr alloy precipitated the H-phase. Following heat treatment at 600 °C for 100 h, the microstructure of the 0Zr alloy primarily consisted of Ni₃Ti₂ and Ni₃Ti phases, whereas the 3Zr alloy was mainly composed of Ni₄Ti₃, Ni₃Ti₂, and Ni₃(Ti, Zr)₂ phases. The 3Zr alloy showcases exceptional superelastic properties, enduring a cyclic load of 1200 MPa for 5 times while maintaining a recovery rate exceeding 90%. The microstructure of the loaded 3Zr alloy consisted of Ni₄Ti₃ and nanodomains, exhibiting stable structural characteristics. Relevant mechanisms are discussed, laying the foundation for the application of Ni-rich NiTi alloys.