Achieving higher strength without sacrificing any ductility in a medium carbon steel via a controllable heterostructured composite strategy

A heterostructured (HS) composite strategy, centered on designing non-uniform precipitate distribution, has been successfully employed to enhance the mechanical properties of a model medium-carbon low-alloy steel. A Heterogeneous microstructure consisting of soft zones without cementite particles and particle-rich hard zones was engineered by an optimized quenching and tempering treatment. This differs fundamentally from the conventional quenched and tempered microstructure where cementite particles are dispersed uniformly within the ferrite matrix. The density of nanoscale cementite within the hard zones of heterogeneous tempered structure is closely related to the volume fraction of residual ferrite. When the ferrite content reaches ∼27 %, the area fraction of cementite is maximized at approximately 20.7 %, with a greater difference between the hard and soft zones, thereby enabling higher hetero-deformation induced (HDI) stresses. Finally, significant HDI strengthening and strain hardening effects enhance the optimized HS steels. Under full constraint of the hard zones, more effective accumulation of back stress within the particle-free soft zones can be produced, improving the global yield strength. Additionally, the plastically deformable hard zones relieve the high stress concentration at interfaces, and formation of extensive micro shear bands facilitate strain de-localization. © 2025 Elsevier B.V.