Achieving enhanced strength-ductility synergy by combining synergetic effects of multiscale gradient structures and bimodal grain structures in commercially pure titanium

The strength-ductility trade-off in metallic materials remains a critical challenge in materials science. However, optimizing microstructural design can improve mechanical properties by activating synergistic interactions among multiple strengthening and deformation mechanisms. This study proposes a novel strategy to achieve enhanced strength-ductility synergy in commercially pure titanium (CP Ti) by introducing an overall multiscale gradient structure (GS) coupled with a localized bimodal grain structure. A multiscale GS with bimodal grain structures in CP Ti is generated by explosive hardening, followed by annealing at different temperatures. According to uniaxial tensile test results, the yield strength of GS Ti reaches 460-609 MPa, which is 43.75%-90.3% higher than that of coarse-grained Ti, while the uniform elongation ranges from 5.8% to 12.2%, with a maximum improvement of 13%. Thus, GS Ti with bimodal grain structures achieves enhanced strength-ductility synergy. A quantitative correlation is established between the gradient structure and yield strength. The strengthening mechanism of GS Ti is revealed: the hetero-deformation-induced stress and grain refinement are the main strengthening factors of yield strength in GS Ti; the synergy effects including both GS/bimodal grain structures and strain delocalization contribute to the strain hardening, further improving ductility of GS Ti.