Global 3D-gradient-structured steel sheets with superior strength-ductility combination

Global gradient structures (GGS) from core to surface processed by twisting have been reported to possess exceptional strength-ductility combinations. However, it is hard to apply this technique to sheet (plate) samples because of their lower axial symmetry. Here, we report a novel 3D-GGS produced in a 304 stainless steel (304ss) sheet using cyclic twisting. The 3D-GGS 304ss sheets exhibited much better strength-ductility combinations than GGS 304ss rods due to their more pronounced heterostructural effects. During the tensile testing , a unique transformation-induced plasticity (TRIP) effect was activated in which reverse martensitic transformation ( α′ → γ ) was first activated, then transited to forward martensitic transformation ( γ → α′ ) after some strain. Both the reverse and forward TRIP effects contributed to higher ductility. In addition, the 3D-structures also produced hetero-deformation induced (HDI) work hardening to retain ductility. This work developed a practical approach to constructing 3D-GGS to enhance integrated performances of metal sheets and components without changing their geometries.