Chromium segregation at phase boundaries in Cr-doped WC-Co cemented carbides

Atom probe tomography has shown (Cr,W)C to be present as a thin WC grain surface layer in an as-sintered Cr-doped WC-Co cemented carbide microstructure slowly cooled from the sintering temperature. The Co-rich binder phase contained a segregation of Cr in front of this surface layer. The mixed carbide layer was not observed when the as-sintered material had been heat treated at 1000 \ub0C for 3000 s and then quickly cooled. A reduced amount of segregated Cr was, however, still present in the binder phase in front of the WC grain surface. These observations are supported by predictions from thermodynamic modelling; (Cr,W)C becomes stable at temperatures below 940 \ub0C in the absence of other secondary carbides. The equilibrium segregation of Cr in the binder phase was estimated to 0.8 and 0.4 monolayer in the as-sintered and heat treated materials, respectively. The energy of segregation was calculated to 0.22 eV/Cr atom from the atom probe data. This value would result in a Cr segregation of 0.3 monolayer at the sintering temperature 1410 \ub0C, provided that the energy of segregation in the Co-rich liquid phase sintering medium is the same as in the Co-rich binder phase. This Cr segregation may explain the WC grain growth inhibiting effect of Cr additions to WC-Co cemented carbide materials