Enhanced flexibility of austenitic Ti-Ni wires for endodontic instruments: The role of R-phase nanodomain construction on mechanical properties

In the evolution of Ti-Ni root canal files, the researches for heat treatment have mainly focused on adjusting the phase composition from austenite to martensite. Increasing the volume fraction of martensite enhances the flexibility of root canal files, but this is accompanied by a decrease in cutting efficiency. While root canal files composed of austenite phase have excellent cutting efficiency, their flexibility is relatively limited. In our study, this low flexibility of austenitic files can be effectively addressed by R-phase nanodomains. Introducing Ni clusters into nanocrystalline Ti₄₉Ni₅₁ alloys reduces the nucleation barrier of R-phase nanodomains. These R-phase nanodomains can nucleate and grow under a low external stress, thereby achieving elastic modulus as low as 23.2 GPa in the fully austenitic state. Grain nanocrystallization induces R-phase formation by restricting the strain energy release of martensite, effectively lowering the strain threshold for generating R-phase nanodomains.‌ Ni clusters, as a precursor to Ni₄Ti₃ precipitation, further promote the formation of R-phase nanodomain structure by introducing internal strain fields. In summary, the Ti-Ni alloys with R-phase nanodomains can achieve high flexibility while maintaining excellent cutting efficiency of B2 phase, effectively resolving the contradiction between cutting efficiency and flexibility. ‌This provides a novel approach to adjust the mechanical properties of Ti-Ni alloy for endodontic applications.