Impact of Ti ₃ Ni ₄ Precipitation on Phase Transformation, Microstructure, and Tribological Behavior of Ni‐Rich NiTi Shape Memory Alloys: A Correlative Investigation

Ni‐rich NiTi is a distinctive functional smart alloy well‐suited for biomedical and aerospace applications, owing to its exceptional thermomechanical properties, biocompatibility, and corrosion resistance. The wear resistance and long‐term service life of NiTi alloys are strongly dependent on their phase transformation behavior, which is intrinsically linked to microstructural evolution. In this context, the present study investigates the critical role of Ti ₃ Ni ₄ precipitates in governing phase transformation kinetics, microstructure, and tribological performance through controlled aging treatments at 450 °C and 650 °C. Results reveal that Ti ₃ Ni ₄ precipitation significantly hardens the alloy, inhibiting martensitic phase transformation while substantially improving wear resistance compared to precipitate‐free conditions, despite exhibiting higher surface roughness and friction coefficient. In contrast, the absence of Ti ₃ Ni ₄ precipitates (achieved by aging at 650 °C) accelerates the thermally induced transformation rate by 78% but severely degrades wear resistance, increasing both wear rate and wear depth by 67%. The findings establish a microstructure–property framework for tailoring Ni‐rich NiTi shape memory alloys: Ti ₃ Ni ₄ ‐rich microstructure optimizes wear‐critical applications, while unprecipitated microstructure favors rapid phase transformation. This work advances the design of NiTi alloys by decoupling the antagonistic effects of precipitates on transformation kinetics and wear performance, offering actionable guidelines for application‐specific heat treatments.