In-situ tensile study of Fe and trace B effects on deformation behavior and strengthening mechanisms in LDED-fabricated Ti-6Al-4V alloys

The Ti-6Al-4V (Ti64) alloy, modified with Fe and B via laser-directed energy deposition (LDED), exhibits significant microstructural refinement and an enhanced synergy between strength and ductility. To elucidate the underlying deformation and fracture mechanisms, this work employs in-situ tensile testing, which uniquely captures real-time crack initiation and propagation compared with conventional ex-situ methods. The results reveal a distinct transition in fracture modes. Ti64 with coarse prior β grains and thick α laths undergoes double necking and transgranular fracture. Fe-refined Ti-6Al-4V-3Fe (Ti643) exhibits single necking with a mixed intergranular–transgranular zigzag fracture, while Ti-6Al-4V-3Fe-0.05B (Ti643B), further refined by in-situ TiB, shows homogeneous deformation and a dominant transgranular mode. In-situ observations highlight that TiB whiskers strongly bond to the matrix, where slip bands either bypass or penetrate TiB, producing progressive TiB fracture and interface dislocation pile-ups that enhance strain hardening. These mechanisms underpin the simultaneous improvements in strength and ductility in both Ti643 and Ti643B alloys, demonstrating the unique capability of in-situ testing to reveal fracture mode transitions and TiB–slip band interactions for the design of high-performance titanium alloys.