In-situ microstructural evolution and deformation mechanisms of metastable beta titanium fabricated by laser powder bed fusion under flexural stress

Compared to other loading conditions, studies on deformation mechanisms of additive manufacturing (AM)-produced beta-type Ti alloys under bending remain limited. This study investigates a metastable beta-type Ti-25Nb-3Zr-3Mo-2Sn (TLM, wt.%) alloy fabricated via laser powder bed fusion (L-PBF) during in-situ three-point bending. In-situ observations using scanning electron microscopy (SEM) combined with electron backscatter diffraction (EBSD) and ex-situ transmission electron microscopy (TEM) imaging during bending provided evaluation of microstructural changes and deformation mechanisms. These mechanisms are characterized by dislocation slip, {332}<113>(beta) deformation twin, alpha" phase, and omega phase formation during plastic bending stage. The {112}<111> slip system dominates in the compression zone, while the {123}<111> slip system governs in the tension zone during bending. The synergistic effect of twinning-induced plasticity (TWIP) and transformation-induced plasticity (TRIP) significantly enhances the ductility of L-PBF-produced TLM alloy. The deformation involves stress-induced alpha" and omega phases, with the latter can form within grains/twinning band and at twinning boundaries. Importantly, the presence of the interfacial twin boundary (ITB)-omega thin layers at twinning band boundaries exerts a pinning effect, restraining the outward extension of stress-induced alpha" phase. This mechanism suggests optimized utilization of space within twinning bands, facilitating alpha" nucleation and uniform growth, thereby providing insights into further enhancing ductility.