Influence of grain size distribution on mechanical properties and HDI strengthening and work-hardening of gradient-structured materials

Gradient structures (GS) with various grain size distributions are successfully architectured in a commercial low-alloying steel by using the pre-torsion and thermal annealing routes, and the corresponding mechanical properties and hetero-deformation induced (HDI) strengthening and work-hardening are investigated. Results demonstrate that an optimal grain size distribution from ~5.0 ± 2.8 μm at the surface to 11.1 ± 4.5 μm at the center leads to the maximum HDI strengthening of ~325 MPa. The HDI work-hardening intensifies continuously with the decreased average grain size of the GS due to the formation of abundant geometrical necessary dislocations (GNDs) whose density decreases abnormally from the fine-grain surface to the coarse-grain core caused mainly by the interactions between the short-range GNDs and long-range GNDs. Thanks to the HDI strengthening and work-hardening, the yield strength increases with a reduction of uniform ductility approximately following the linear inverse rule, in stark contrast to the banana-shaped strength-ductility trade-off of monolithic materials, and an exceptional strength-ductility synergy is achieved.