Microstructural changes and mechanical property response to aging heat treatment in hypereutectoid Ti–Au–Mo biomedical alloys

β (bcc) Ti–Au-based biomedical alloys are attractive due to their excellent biocompatibility, X-ray radiographic imaging properties, and shape memory effects. Furthermore, it was recently revealed that increasing the Au content beyond the eutectoid composition to 6 mol% Au exhibits a single β phase without the intermetallic compound Ti3Au and good superelasticity in hypereutectoid compositions. It has also been shown that specific heat treatment can produce Ti3Au with favorable mechanical properties. This study verified the feasibility of developing hypereutectoid Ti–Au–Mo ternary alloys from the viewpoint of aging heat treatment. By controlling the aging temperature and time, the precipitate could be controlled as ω, α, and/or Ti3Au. At aging temperatures higher than 773 K, aging products formed on the β grain boundaries, embrittling the alloy. Aging at 573 K resulted in a decrease in ductility and an increase in hardness with increasing aging time. These changes were more drastic at lower Au concentrations, with the smallest changes observed at 6 mol% Au. This study highlights the Mo equivalent as a measure of β phase stability, with values calculated based on empirical calculations or the binary phase diagram slope. Comparison with other Ti–Mo binary alloys with similar Mo equivalents suggests that Au causes less ω growth. These findings provide a solid foundation for the development of functional low-karat titanium-gold alloys for use in advanced medical devices.