Preparation Induced Phase Transformation of Co : An experimental evaluation of effects from ion polishing and processing parameters on phase composition in WC-Co using EBSD

Sammanfattning

Cemented carbides (WC-Co) are widely used in demanding applications due to their unique combination of harness and toughness. The Co binder phase can exist in either face-centered cubic (fcc) or hexagonal close-packed (hcp) crystal structure. Although the fcc structure is typically retained after sintering, it may transform to the more brittle hcp structure due to external influences, including sample preparation. This study investigates the extent of which Ar+ ion polishing, used prior to Electron Backscatter Diffraction (EBSD) analysis, affects the martensitic phase transformation of Co, from fcc to hcp and how this is influenced by processing parameters: carbon content, ball milling duration and sintering temperature. The results showed that the ion polishing parameters significantly influence both EBSD indexability and the detected phase composition. X-Ray Diffraction (XRD) measurements on unpolished samples did not indicate the presence of hcp-Co, while ionpolished samples showed minor amounts, suggesting that the hcp-Co observed after ion polishing is largely a surface artifact introduced during the sample preparation. The incidence angle and energy were found to significantly affect EBSD indexability and detected phase composition, where a higher angle improved indexability. This highlights the importance of optimized preparation procedures. It is unknown whether the hcp-Co was introduced by the ion polishing or the mechanical polishing. Furthermore, the results showed that sintering temperature had the strongest influence on the phase composition. Ball milling duration was found to mainly influence the results at low sintering temperatures. Carbon content had barely any effect on the phase composition alone, however it became important in combination with the other parameters. The combination of high sintering temperature, long milling time and low carbon content yielded the lowest fraction of hcp-Co. Magnetic saturation measurements indicated that this parameter combination resulted in a high level of solute tungsten in the binder phase. However, sintering temperature was found to not influence magnetic saturation, thus the dissolvement of atoms in the binder is not the sole stabilizer of the fcc-Co phase. Overall, the study demonstrates that EBSD-based phase identification of Co in cemented carbides is highly sensitive to sample preparation, and careful optimization of ion polishing conditions is required to obtain reliable microstructural information.