Identification and accurate characterisation of the metastable CoFCC phase in WC–Co cemented carbides

Abstract

Composed of hard tungsten carbide (WC) particles and a soft cobalt (Co) matrix, WC–Co cemented carbides exhibit significant differences in the material removal rates of these phases during metallographic preparation. This disparity, combined with the susceptibility of the soft Co phase to deformation-induced martensitic phase transformation from its face-centered cubic (Co_FCC) to hexagonal close-packed (Co_HCP) structure, poses substantial challenges for microstructural characterisation. The resulting ambiguity complicates the identification of the pristine Co phase and raises concerns about the presence of preparation-induced artefacts. In this study, we present a detailed comparative analysis aimed at minimizing ambiguities in the characterisation of the pristine Co phases in a series of WC–Co cemented carbides. We quantitatively report on the controllability of various preparation parameters under multiple conditions, for plane-polished cross-sections used in (e.g.) EBSD analysis and for thin-sections such as used in (e.g.) TEM analysis. We report on the interplay between material removal and the deformation-induced martensitic Co_FCC–Co_HCP phase transformation during metallographic preparation, identifying “GO” and “NO GO” regimes for the unequivocal identification of the pristine Co phase in WC–Co cemented carbides. The optimal metallographic preparation method for the “GO” regime involves an Ar⁺ ion polishing energy density of ∼10 MJ/m² and a duration of ∼80 min. This work establishes a robust workflow for accurately determining the pristine Co phase, providing a pivotal aspect for the characterisation of microstructure–property relationships in WC–Co cemented carbides.