Exceptional strength-plasticity synergy in β-Ti alloy via HPT and short-period annealing

A new metastable β-type Ti-3.5Al-5Mo-4 V (Ti-B20) alloy with a high strength has been recently developed. However, the plasticity of the alloy after solution or aging treatment is poor due to its large initial β grain size, and conventional deformation-induced grain refinement is quite limited. In the present work, a high-pressure torsion (HPT) method was used to manufacture a Ti-B20 alloy with large deformed grains. The Ti-B20 alloy subjected to HPT was annealed in a β-phase field (850 ℃) for different periods. The deformation mechanism during HPT processing and the recrystallization behavior of the deformed alloy were systematically revealed. The results show that the deformation mode during HPT processing is mainly dislocation slip and shear band splitting of the original grains and martensitic phase (α″). During recrystallization annealing, the shear bands and high-density dislocations generated by the HPT deformation of the Ti-B20 alloy significantly promote nucleation and growth of recrystallized grains, which shortens the time to complete recrystallization. After recrystallization annealing, exceptional strength-ductility synergy with a high tensile strength of 941.9 MPa and a high plasticity of 39.7% is achieved in the fine-grained (14.68 µm) alloy, which is the highest elongation reported in the Ti-B20 alloy at present. The fine grains with a low dislocation density provide more space for dislocation slip and accumulation while maintaining high strength. This work provides a new possibility for preparing fine-grained β-titanium alloys with synergistic strength and plasticity.