Effect of annealing temperature on the microstructure and mechanical properties of Ti551 alloy forgings

To clarify how annealing temperature regulates the microstructure–texture–property anisotropy in hot-forged titanium alloys, a near-α titanium alloy forged at 920 °C was selected as the research object and annealed in the range of 910~950 °C. Texture evolution was characterized by electron backscatter diffraction(EBSD) combined with α/β pole figures, and room-temperature tensile and impact properties were evaluated along the longitudinal direction(LD) and normal (thickness) direction (ND). The results show that the microstructure at 910~920 ℃ is dominated by primary α (α_p) with a relatively low fraction of secondary α (α_s). With increasing annealing temperature to 930~950 ℃, the microstructure gradually transforms into a bimodal structure, and orientation-consistent blocky regions of α_s become more pronounced. As the annealing temperature increases, the intensity of the {0001} pole figure of α phase increases progressively, while the {110} pole figure of β phase also exhibits a trend toward stronger orientation concentration. In terms of mechanical properties, the ultimate tensile strengths along LD and ND remain close with minor fluctuations, whereas the directional differences in yield strength and ductility are more sensitive. Anisotropy is more pronounced in the equiaxed regime but is effectively improved in the bimodal regime. The as-forged condition shows relatively large anisotropy, which is alleviated after annealing. The absorbed impact energy is generally higher along LD than along ND; however, the LD–ND difference converges markedly with increasing annealing temperature. A favorable strength–toughness balance with reduced anisotropy can be achieved at 950 ℃. These trends are attributed to enhanced transformation-variant selection and basal-texture strengthening caused by α_p dissolution and β-grain growth during sub-β-transus (T_β) to near-T_β annealing, as well as the combined constraint effects of multiscale interfaces/grain boundaries in the bimodal microstructure on deformation accommodation and crack propagation paths.