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

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.