Simultaneously enhancing strength and ductility of HCP titanium via multi-modal grain induced extra <c+a> dislocation hardening

According to Considère necking criterion, enhancing strength of a material will decrease its elongation to failure, i.e. ductility, even if the strain hardening rate remains unchanged. Unfortunately, four traditional strengthening mechanisms including grain refinement, deformation, solid solution and 2nd-phase particle strengthening increase the yield strength by increasing the critical shear stress for slip initiation and unexceptionally reduce the strain hardening ability and ductility. Recent experiments revealed that implementation of heterostructures can produce extra hetero-deformation induced (HDI) hardening and thus reduce ductility loss while enhancing strength. However, the improved ductility was still less than that of coarse-grained counterparts. Here we fabricated a bulk heterostructured Ti with a uniform multi-modal grain size distribution in which the single individual micro-grain is surrounded and constrained by three-dimensional ultrafine grains. Tensile tests revealed the multi-modal Ti has a high yield strength of 800 MPa, ductility of 28.5%, and outstanding HDI hardening effect compared with its coarse-grained counterparts (a yield strength of 550 MPa and ductility of 26.5%). These mechanical properties of our multi-modal Ti are also superior to other literature reported data of heterostructured Ti. Microstructural characterization further reveals the uniform distribution between hard and soft domains produces maximum interface density and HDI hardening effect. Moreover, the HDI causes extra 〈c + a〉 geometrically necessary dislocations piling ups in the constrained micro-grains, which produce enough and extra strain hardening to maintain and even enhance slightly the ductility. Our work provides a strategy to simultaneously enhance strength and ductility of metals via enough and extra strain hardening capability increase.