Microstructural Evolution and Mechanical Properties of TC18 Titanium Alloy with Bimodal Structure

The microstructural evolution and mechanical properties of TC18 alloy after solution treatment (ST) at 770-830 °C plus aging at 510-630 °C were investigated. The results show that the volume fraction of the primary α (α_p) phase decreases with the increasing ST temperature while the size of secondary α (α_s) phase increases with the incremental aging temperature. A good combination of strength and ductility of the alloy is obtained under ST at 800 °C/1 h plus aging at 530 °C/6 h, with tensile strength of 1295 MPa and the elongation at fracture of 8.2%. The strength of TC18 with bimodal structure is mainly governed by secondary α_S precipitation that effectively blocks dislocation slip, whereas the ductility is tuned through the compatible deformation between primary α_p and retained β matrix. Higher fraction of α_p and coarser α lamella in β matrix which could decrease the strain concentration at α_p/β_trans interface are beneficial to ductility. Moreover, the EBSD analysis shows that plastic deformations in both α_p and α_s are orientation dependent. Grains favorable for prismatic and basal slip sustain more plastic strain while grain with <0001> aligned to loading direction are difficult to deform.