Effect of deformation and heat treatment parameters on the microstructure and properties of forged TB18 titanium alloy

TB18 titanium alloy is a near-β-titanium alloy with high strength and good plasticity. However, its microstructure and mechanical properties are affected by its deformation and heat treatment parameters. The effects of deformation pass number, deformation temperature, and heat treatment temperature on the microstructure and mechanical properties of TB18 titanium alloy were conducted by free forging experiments. The results show that the deformation pass had limited effect on the mechanical properties of the sample after heat treatment. Three types of microstructures were obtained after heat treatment: equiaxed or needle-like primary α (αp) + lenticular secondary α (αs), long or short rod-like αp + lenticular αs, and Widmannstatten α (αWGB) + grain boundary α (αGB) + lenticular αs. The equiaxed or needlelike αp + lenticular αs microstructure had the optimal elongation and reduction of area, owing to the transgranular fracture caused by the nucleation and propagation of cracks in the αp phase. The long or short rodlike αp + lenticular αs microstructure had the optimal tensile and yield strengths, which was ascribed to the interface strengthening caused by the dispersed αs phase. The αWGB + αGB + lenticular αs microstructure had the optimal fracture and impact toughness values, which was attributed to the deflection of cracks at crossed grain boundaries during propagation, increasing the crack path length and consuming more energy. The best comprehensive mechanical properties were obtained when deformed in the (α + β) two-phase or β-single-phase regions plus heat treatment in the β-single-phase region.