Effect of Cr:Fe ratio on the mechanical properties of (Cr,Fe)₇C₃ ternary carbides in abrasion-resistant white cast irons

(Cr,Fe)(7)C-3 ternary carbides constitute the majority of eutectic carbides in abrasion-resistant white cast irons. Density functional theory models have predicted these carbides to have a combination of metallic, covalent and ionic bonding, in proportions depending on the carbide's Cr:Fe ratio. However, experimental research to validate these predictions has been lacking. This study investigates the characteristics of the carbides as a function of Cr:Fe ratio, which was manipulated from Fe-rich to Cr-rich by varying the Cr:C ratio of the bulk alloy. The carbides' crystalline structure, hardness, Young's modulus, fracture toughness and abrasion performance have been assessed through techniques including nano-indentation, HR-TEM and the inner circumference abrasion test (ICAT). Fe-rich M3C formed at very low bulk Cr:C ratio was found to have an orthorhombic crystal structure. In all other alloys, with Cr:C ratios above 2.7, M7C3 was formed and found to have a hexagonal structure. Hardness, Young's modulus and calculated fracture toughness of M7C3 all increase with Cr:Fe ratio, from (Fe-5,Cr-2)C-3 up to a maximum for (Cr-4,Fe-3)C-3 (in 18Cr-6.8Cr:C WCI). This gave the highest hardness (22.9 GPa) and Young's modulus (315 GPa), but also the highest fracture toughness (4.5 MPa.m0.5). The peak fracture toughness at carbide composition of (Cr-4,Fe-3)C-3 in this study is consistent with the prediction of DFT models in the literature; while the peak hardness at the same carbide composition shows a marginal deviation from the predictions. Abrasion performance generally increased with carbide hardness and fracture toughness, with one exception: (Cr-4.3,Fe-2.7)C-3. Although (Cr-4.3,Fe-2.7)C-3 showed marginally lower inherent fracture toughness than (Cr-4.0,Fe-3.0)C-3, the higher Cr:Fe carbides imparted the highest abrasion performance, associated with modified eutectic morphology.