Hf Addition Enhances Structural Stability and Mechanical Properties of NiTi Alloys across a Broad Temperature Range

The potential application of Ni‐rich NiTi(Hf) alloys as structural materials for applications across a broad temperature range, with a focus on thermal–mechanical coupling, is investigated. Specifically, the microstructure and mechanical properties of Ni 55 Ti 45 and Ni 55 Ti 42 Hf 3 alloys are analyzed under various temperature conditions. The role of Hf doping in enhancing the thermal stability and strengthening behavior is examined following 100 h heat treatments at temperatures ranging from 400 to 600 °C, along with the thermo–mechanical coupling effects on mechanical properties from −150 to 600 °C. In the Ni 55 Ti 42 Hf 3 alloy, long‐term heat treatment leads to the coarsening of the Ni 4 Ti 3 phase and the formation of the H‐phase, Ni 3 Ti 2 , and Ni 3 (Ti, Hf) 2 phases. Compared to the Ni 55 Ti 45 alloy, the Hf‐doped alloy exhibits superior mechanical properties at high temperatures, particularly in terms of high‐temperature resistance. After treatment at 600 °C for 100 h, the hardness of the Hf‐containing samples remains around 600 HV. Furthermore, the high‐temperature strength of the Ni 55 Ti 42 Hf 3 alloy at 500 and 600 °C is nearly double that of the Ni 55 Ti 45 alloy. These findings highlight Hf doping as a promising strategy for the development of high‐temperature structural materials in Ni‐rich NiTi alloys.