A novel iron matrix composite fabricated by two-step in situ reaction: Microstructure, formation mechanism and mechanical properties

Abstract The improvement of comprehensive properties including hardness, toughness, strength, and wear resistance in the steel/iron matrix composites, especially alleviate the inverse relationship between strength and toughness, is strongly required for various applications. Herein, we provide a new strategy, by two-step in situ reaction, to fabricate a novel iron matrix composite with high strength and reasonable toughness. The composite consists of a bundle-shaped Nb–NbC/Fe core-shell structure and cast iron matrix. Predominant features of the Nb–NbC/Fe core-shell structure include a metal Nb core and a NbC–Fe shell layer. The NbC–Fe shell layer is composed of two zones, where zone Ⅰ, near the Nb core, exhibits a NbC/α-Fe/NbC laminated structure; and zone Ⅱ, near the matrix, exhibits a NbC–Fe dense structure. The investigation indicates that the formation of the NbC–Fe shell layer is mainly due to the concentration fluctuation of Nb and C atoms in the Fe–Nb–C ternary system during in situ reaction. The formation of NbC particles is mainly attributed to the in situ reaction of niobium atoms and carbon atoms in the liquid phase system ([Nb] + [C]→[NbC]→NbC), while the formation of Nb2C is attributed to the diffusion-type solid phase transformation driven driven by the diffusion of carbon atoms into the Nb lattice (Nb + 2C→Nb2C). In addition, the fracture toughness of the NbC/α-Fe/NbC laminated structure layer and NbC–Fe dense structure layer is evaluated to be approximately 4.6 ± 0.2 MPa m1/2 and 4.0 ± 0.1 MPa m1/2, respectively. The yield strength, ultimate compression strength, and fracture strain of the Nb–NbC/Fe core-shell rod-reinforced iron-based composite reach 1623 MPa, 1970 MPa, and 11.3%, respectively, showing high strength and reasonable toughness compared to those of the cast iron (420 MPa, 830 MPa and 21.8%, respectively). Thus, this work provides a new strategy for alleviate the inverse relationship between strength and toughness of iron matrix composite.