Superior mechanical properties and strengthening–toughening mechanisms of gradient-structured materials

The tensile and impact properties and strengthening–toughening mechanisms of gradient-structured low-alloying steel were investigated, where the grain size and density of retained geometrical necessary dislocations (GNDs) change continuously. A plausible strength–ductility synergy was achieved by the gradient structures, featuring as an inverse linear rule between the strength and ductility. This condition was attributed to the continuous work hardening from the surface to the core and caused by the sustaining accumulation of GNDs. A simultaneous enhancement of yield strength and impact toughness was obtained at a specific gradient structure compared with the coarse grain counterpart. The characterization of the fracture morphology of the sample with high strength and toughness showed the prominent ductile fracture by microvoid coalescence, which was contrast to the predominant brittle cleavage fracture of the sample with high strength but low toughness. This study is of great importance to the development of advanced structural materials with exceptional strength–toughness combination by architecturing proper gradient structure.