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 degrees C plus aging at 510-630 degrees C were investigated. The results show that the volume fraction of the primary alpha (alpha(p)) phase decreases with the increasing ST temperature while the size of secondary alpha (alpha(s)) phase increases with the incremental aging temperature. A good combination of strength and ductility of the alloy is obtained under ST at 800 degrees C/1 h plus aging at 530 degrees 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 alpha(S) precipitation that effectively blocks dislocation slip, whereas the ductility is tuned through the compatible deformation between primary alpha(p) and retained beta matrix. Higher fraction of alpha(p) and coarser alpha lamella in beta matrix which could decrease the strain concentration at alpha(p)/beta(trans) interface are beneficial to ductility. Moreover, the EBSD analysis shows that plastic deformations in both alpha(p) and alpha(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.