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 degrees C. This marks a substantial 63.9% improvement compared to the 0Zr alloy. After treatments at 400 degrees C and 500 degrees C, the 3Zr alloy precipitated the H-phase. Following heat treatment at 600 degrees 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.