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 Ni₅₅Ti₄₅ and Ni₅₅Ti₄₂Hf₃ 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 Ni₅₅Ti₄₂Hf₃ alloy, long-term heat treatment leads to the coarsening of the Ni₄Ti₃ phase and the formation of the H-phase, Ni₃Ti₂, and Ni₃(Ti, Hf)₂ phases. Compared to the Ni₅₅Ti₄₅ 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 Ni₅₅Ti₄₂Hf₃ alloy at 500 and 600 °C is nearly double that of the Ni₅₅Ti₄₅ alloy. These findings highlight Hf doping as a promising strategy for the development of high-temperature structural materials in Ni-rich NiTi alloys.