• Ali, Yahia
• Lin, Weikang
• Gates, J. D.
• Jokari-Sheshdeh, Maziar
• Gallo, Santiago Corujeira

Abstract

<jats:title>Abstract</jats:title><jats:p>(Cr,Fe)<jats:sub>7</jats:sub>C<jats:sub>3</jats:sub> 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 M<jats:sub>3</jats:sub>C 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, M<jats:sub>7</jats:sub>C<jats:sub>3</jats:sub> was formed and found to have a hexagonal structure. Hardness, Young’s modulus and calculated fracture toughness of M<jats:sub>7</jats:sub>C<jats:sub>3</jats:sub> all increase with Cr:Fe ratio, from (Fe<jats:sub>5</jats:sub>,Cr<jats:sub>2</jats:sub>)C<jats:sub>3</jats:sub> up to a maximum for (Cr<jats:sub>4</jats:sub>,Fe<jats:sub>3</jats:sub>)C<jats:sub>3</jats:sub> (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<jats:sub>4</jats:sub>,Fe<jats:sub>3</jats:sub>)C<jats:sub>3</jats:sub> 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<jats:sub>4.3</jats:sub>,Fe<jats:sub>2.7</jats:sub>)C<jats:sub>3</jats:sub>. Although (Cr<jats:sub>4.3</jats:sub>,Fe<jats:sub>2.7</jats:sub>)C<jats:sub>3</jats:sub> showed marginally lower inherent fracture toughness than (Cr<jats:sub>4.0</jats:sub>,Fe<jats:sub>3.0</jats:sub>)C<jats:sub>3</jats:sub>, the higher Cr:Fe carbides imparted the highest abrasion performance, associated with modified eutectic morphology.</jats:p>

Topics

• density
• impedance spectroscopy
• morphology
• theory
• carbide
• hardness
• transmission electron microscopy
• density functional theory
• iron
• fracture toughness
• white cast iron