Grain refinement and phase transformation mechanisms in Ti–4Mo and Ti–7Mo alloys processed by high-pressure torsion

This study investigates the microstructural evolution and mechanical properties of Ti-xMo (x = 4, 7 wt.%) alloys during high-pressure torsion (HPT) processing. After hot rolling and annealing at 900 °C, Ti-4Mo exhibits a dual-phase basketweave structure, while Ti-7Mo forms a lamellar structure due to differences in β-phase stability. During HPT, progressive grain refinement and stress-induced phase transformations are observed. Microstructural analysis reveals that the β→ω transformation completes entirely, while the α→ω transformation occurs partially, with Ti-7Mo showing greater propensity for ω-phase formation. The microhardness of both alloys increases with deformation. After 5 revolutions of HPT, the hardness at the edge of the Ti-4Mo alloy increased by 157 Hv, while that of the Ti-7Mo alloy increased by 213 Hv, achieving a higher hardness (∼ 486 Hv). This is attributed to its finer grain size and higher ω-phase content. These findings highlight the crucial role of Mo content in enhancing mechanical properties through grain refinement and phase transformation during severe plastic deformation.