Strengthening and toughening an ultra-high strength medium Mn steel by fibrous ferrite bridging mechanism

Achieving exceptional strength and ductility is a critical requirement for most materials. In this work, an ultra-strong medium Mn steel with hard martensite as the matrix and a large number of uniformly distributed long and coarse soft fibrous ferrites embedded within it had been designed. The steel demonstrated ultra-high yield and tensile strengths of 1310 and 1516 MPa, as well as 8.3% uniform and 14.7% total elongations. Such excellent strength-ductility synergy is attributed to the fully activated fibrous ferrite bridging mechanism, in which the micro-voids at the grain boundaries of martensite serve as the bridging cracks while fibrous ferrites act as the bridging ligaments. During the uniform deformation stage, the uniform distribution of fibrous ferrites and the synergetic deformation between fibrous ferrites and martensite reduce the growth rate of micro-voids, ultimately generating numerous uniformly distributed micropores. This stage makes full use of the strength of martensite, leading to high strength. During the necking stage, countless micro-voids expand while tearing fibrous ferrites. Most of the long and coarse fibrous ferrites participate in the bridging mechanism and undergo significant plastic deformation. A fibrous ferrite can even activate the bridging mechanism multiple times. This stage takes full advantage of the ductility of fibrous ferrites, leading to high elongation. Eventually, the increased bridging region (extrinsic) and fracture surface area (intrinsic) are responsible for the material's excellent capacity to absorb energy. Research results promote the development of medium-Mn dual-phase steels with superior ductility and strength.