Simultaneous improvement of strength and plasticity: Additional work-hardening from gradient microstructure

Enhancing strength-ductility synergy of materials has always been a hot but difficult topic in material science, for most structural materials, steels in particular, it is inevitable to sacrifice ductility when increasing strength, and vice versa. In this study, by introducing a linear gradient in grain size into Fe-Mn-C twinning-induced plasticity (TWIP) steel, which is one of the promising structural steels in automobile industry, it is interesting to find that a simultaneous improvement of strength and plasticity (SISP) has been successfully achieved. It is believed that this evasion of strength-ductility trade-off may be mainly attributed to the formation of geometric necessary dislocations during tensile deformation, which contributes to an additional work-hardening especially in the later deformation. Such extraordinary strain hardening, which is inherent to the gradient structures and is absent in homogeneous materials, helps enhance the strength and delay the necking. This represents a novel strategy for the strength-ductility improvement which emphasizes the importance of work hardening and thickness of gradient layer (not a narrow sharp gradient) in material design. Inspired by this, other methods on optimizing structural design of the high-performance materials may be developed.