Development of Nickel-Rich Nickel–Titanium–Hafnium Alloys for Tribological Applications

Nickel-rich (54–56 at.% Ni) NiTi-based alloys have gained increased attention for their high hardness, corrosion resistance, strength, and wear resistance, leading to their development for high-performance bearings and other wear applications. An investigation of a broader compositional range of NiTiHf alloys, in terms of Ni and Hf content, is presented in this study. Their Vickers micro-hardness, 3 ball-on-rod rolling contact fatigue, and compression performances are benchmarked against early compositions identified for bearing applications, e.g., Ni₅₅Ti₄₅ and Ni₅₄Ti₄₅Hf₁. The results show that by varying heat treatments and alloy composition, certain NiTiHf alloys can exhibit long life (~ 10⁷ cycles) at 10–20% greater contact stress levels (up to 2.2–2.4 GPa), together with up to 20% greater hardness (up to 770 HV) and 30% larger compressive yield strengths (up to 3.4 GPa) than the original bearing compositions. These improvements are attributed to the ability to develop alloys with high-volume fractions of fine precipitate phases. Interestingly, a variety of combinations of different precipitate strengthening phases, depending on alloy composition and heat treatment, were able to achieve improvements relative to Ni₅₅Ti₄₅ and Ni₅₄Ti₄₅Hf₁, demonstrating the versatility of the NiTiHf system.