Probing the optimal grain size for the strength-ductility synergy in titanium via deformation dynamics

In this study, fully recrystallized pure titanium with grain sizes ranging from 0.5 to 26 μm was prepared, and its mechanical properties were evaluated. The results indicated that the product of yield strength (∼550 MPa) and uniform elongation (∼14 %) peaked at a grain size of 1.8 μm, achieving optimal strength-ductility synergy. Strain rate sensitivity, activation volume, and mobile dislocation density of the samples were determined through strain rate jump tests and repetitive stress relaxation. The findings demonstrate that the sample with a grain size of 1.8 μm exhibits heightened strain rate sensitivity during deformation, indicating greater resistance to plastic instability. Moreover, its smaller activation volume (∼27b³) suggests that grain boundary-mediated dislocation activity predominates the deformation mechanism. Microscopic characterization reveals a significant gradient in the density of geometrically necessary dislocations (GNDs) within the grain boundary affected region (Gbar). The grain size is approximately twice the width of Gbar, which maximizes the strain gradient effect at this size condition and enhances back-stress hardening. Additionally, the low exhaustion of mobile dislocations during deformation is attributed to the high density of grain boundaries, ample storage capacity, and effective dislocation accumulation, further enhancing Taylor hardening. © 2025 Elsevier B.V.