A Novel Dual-Heterogeneous-Structure Ultralight Steel with High Strength and Large Ductility

Abstract

A dual-heterogeneous structure with bimodal grain size distributions of both austenite and B2 phase was developed in a novel ultralight steel by a simple rolling and annealing process. Tensile test results show that the steel annealed at 900℃ (denoted as A900) exhibited the superb synergy of a high specific yield strength of 160.3 MPag^-1cm³, a large ductility of 39.3% and a persistently high work hardening rate in the range of 1-3.5GPa, which can be mainly attributed to the heterogeneous structures caused multi-level deformation mechanisms at different deformation stages. Detailed microstructural analysis of the deformed A900 samples unveils that with an increase of strain, the plastic deformation of individual austenite grains was governed by different factors, i.e., grain size and Schmid factor (SF) at 4.2% and 9% strains and local stress at 25% strain, while the deformation of individual B2 grains was mainly regulated by grain size at 4.2%, grain size and SF at 9% strain and local stress in the late deformation stage, which leads to the consistently high work hardening rate and large elongation. In addition, owing to the hardness difference, a high density of geometrically necessary dislocations was observed near the interfaces of austenite/B2, which also contribute to the consistently high heterogeneous deformation-induced strengthening effect and thus the outstanding mechanical properties. The present work sheds light on the dual-heterogeneous structure of the ultralight steel that can be feasibly employed and guide the optimization of mechanical performance of a wide range of alloys for lightweight engineering.

Keywords: Ultralight steel, Dual-heterogeneous structure, Specific strength, ductility, heterogeneous deformation-induced strengthening