New deformation mechanism and strength-ductility synergy in pure titanium with high density twin

•The high-density twinning Ti sample exhibits an enhancement of both strength and ductility, particularly with a remarkable increase in uniform elongation of nearly 50 % compared to its coarse-grained counterpart.•An unreported significantly disparities not only in the extent of dislocation density enhancement between the matrix and twin layers, but also in the dislocation types.•In addition to the preferred crystal orientations and potential dislocation transmutation mechanisms, an increase in the c/a ratio near the twin boundaries is proposed as the underlying mechanism to the activation of dislocations within twins, thereby enhancing the work hardening ability. The simultaneous optimization of strength and ductility in high-performance metallic materials has long been a challenge for researchers, characterized by an inherent trade-off between the two properties. Despite a vast body of research aimed at overcoming this challenge, achieving a desirable balance between strength and ductility remains elusive. Here, we present a novel approach that involves the introduction of high-density twin boundaries into pure Ti while maintaining a nearly unchanged grain size. This approach leads to a significant improvement in yield strength, ultimate tensile strength, and uniform elongation of pure Ti. In-situ electron backscatter diffraction (EBSD) analysis reveals a substantially higher density of dislocations in twins compared to the matrix, which translates into a remarkable improvement in strain hardening rate and enhanced ductility at high stress levels. The finding from the In-Grain Misorientation Axes (IGMA) distribution method indicate that the high density of dislocations is triggered by the activation of non-basal ⟨c+a⟩ slipping. Furthermore, it is reveaaled that, in addition to the preferred crystal orientations and potential dislocation transmutation mechanisms, an increase in the c/a ratio near the twin boundaries also contributes to the activation of ⟨c+a⟩ dislocations within twins. Our findings offer a promising route for developing high-performance HCP (Hexagonal close-packed) metallic alloys by introducing high-density twins.