The evolution of microstructure and microhardness in a biomedical Ti–35Nb–7Zr–5Ta alloy

β-Ti alloys are promising candidates for biomedical applications due to their high strength, high corrosion and wear resistance, and low elastic modulus. This study focuses on phase evolution in a low modulus Ti–35Nb–7Zr–5Ta (TNZT) alloy, systematically examined via isochronal and isothermal annealing, and its influence on microhardness. The observations indicate that the highest microhardness value was achieved at an aging temperature of 400 °C. The microstructural evolution at this temperature was investigated via systematic isothermal annealing treatments, and the results indicate a progressive transformation from β + ω + O’ (solution treated and quenched) to β + ω + α (after isothermal annealing at 400 °C/6 h), with the dissolution of the metastable orthorhombic O’ phase and the formation of the stable α phase. The maximum hardness corresponded to a highly refined mixture of co-existing ω and α phases after prolonged annealing for 48 h at 400 °C. The coexistence of both ω and α phases after such prolonged annealing indicates that at 400 °C, ω is in metastable equilibrium, despite the concurrent precipitation of the equilibrium α phase.