Microstructural characterization of chromium carbide-reinforced iron matrix composites prepared by combination of casting and heat treatment

To improve the mechanical properties of iron-matrix alloy, chromium carbide-reinforced iron-matrix composites with polycrystalline structure were produced by a diffusion-controlled reaction under casting and two-step heat treatment. Scanning electron microscopy reveals that the structure of composites consists of numerous chromium-chromium carbide pillars and a number of chromium carbides those are dispersed in the matrix. The chromium-chromium carbide pillars are constituted by a core of residual chrome wire and a shell of chromium carbide. The chromium carbide shell forms a dense region and chromium carbide in the matrix generates a diffusion region. X-ray diffraction patterns indicate that the composites are composed primarily of M23C6, M7C3, M3C2, Fe3C, Cr and α-Fe phases. The presented Cr phase in XRD graph is due to incomplete reaction and the observed Fe phase is derived from the matrix. Transmission electron microscope for the dense region of specimen-4 confirms that the Cr23C6 is a cubic structure, the Cr7C3 and Cr3C2 correspond to be an orthorhombic structure. Crystallography orientation relationship between the Cr3C2 and Fe3C has been indexed to be (010)Cr3C2//(020)Fe3C and [001]Cr3C2//[001]Fe3C. Thermodynamic calculation results show that Gibbs free energy of chromium carbides is negative except for the cementite, which indicates that the stability sequence of carbide phases is Cr3C2>Cr23C6>Cr7C3>Fe3C.