Achieving strength-plasticity synergy in trace B-modified near β titanium alloy fabricated by laser directed energy deposition

Achieving a balance between strength and ductility in additively manufactured near β titanium alloys is challenging. In this study, adding trace B to Ti-5Al-5Mo-5V-3Cr-1Zr significantly enhanced constitutional undercooling and suppressed grain growth, resulting in significant refinement of β grains. During heat treatment, TiB acted as a heterogeneous nucleation site, promoting the coarsening and compositional changes of the primary α phase (α_p) at elevated temperatures. At subsequent aging, compositional changes of α_p led to the precipitation of fine secondary α phase, resulting in a dual-sized α phase microstructure. The dual-sized α phase microstructure exhibits a tensile strength of 1059 MPa and an elongation of 15.6 %, whereas the uniform α phase microstructure exhibits a tensile strength of 1139 MPa and an elongation of 2.9 %, exhibiting a slight reduction in strength but a significant improvement in elongation. This can be attributed to: (1) refined grains, which enhanced deformation compatibility and reduced stress/strain localization. (2) Nano-twinning in coarse α_p is fully activated. (3) The fine grains and coarse primary α phase strongly impede crack propagation. This study demonstrates that trace B enables the regulation of refined β grains and dual-sized α phase in additively manufactured near β titanium alloys, achieving a balance of strength and ductility.