Microstructure development and strengthening behaviour in hot-extruded Ti-Mo alloys with exceptional strength-ductility balance

A series of strong and ductile Ti-Mo alloys containing 2.5, 5, 7.5 and 10 wt% Mo was experimentally investigated by a two-stage blended-elemental powder metallurgy process. This process consisted of a spark plasma sintering stage for powder-consolidation and in-situ-homogenisation, and a hot extrusion stage for densification and microstructure enhancement. Microstructures in the extruded alloys changed from α dominant to β dominant with molybdenum addition. Simultaneously, a grain refinement effect was observed due to suppressed α precipitation and increase β retention. A significant strengthening effect was observed with molybdenum addition. In comparison with similarly processed commercially pure titanium, Ti-10Mo exhibited a yield strength improvement of 290 % to 1382.8 MPa, and an ultimate tensile strength improvement of 239 % to 1496.5 MPa. Meanwhile, Ti-5Mo exhibited a yield strength improvement of 211 % (1007.9 MPa) without any substantial reduction in ductility, resulting in an exceptional tensile toughness of 361.3 MJ.m⁻³. A quantitative analysis of strengthening mechanisms reveals considerable strengthening effects from solid solution strengthening, dislocation strengthening, and grain refinement effects. These factors arise from the novel multi-modal microstructures formed by thermomechanical processing at super-transus temperatures. The newly achieved balance of strength and ductility appears to be unrivalled amongst all previously reported binary Ti-Mo alloys.