The phase composition, phase transition and phase structure transformation of the wire-cut section of functionally graded WC-Co cemented carbide with dual phase structure were investigated by XRD phase analysis. It is shown that the composition of η phase in the core zone is Co_3W_3C (M_6 C type). The structure of cobalt based solid solution binder phase is fcc type. At the cooling stage of the sintering process, the phase transition of η phase, i.e. M_6C→M_12C and the martensitic phase transition of the cobalt based solid solution binder phase, i.e. fcc→hcp are suppressed, which facilitates the strengthening of the alloy. Because the instantaneous temperature of the discharge channel is as high as 10 000 ℃ during the wire cutting process, the processed surface is oxidized. Nevertheless, the oxide layer thickness is in micro grade. In the oxide film, η phase is decomposed into W_2C and CoO, and cobalt based solid solution binder is selectively oxidized, while WC remains stable due to the existence of carbon containing liquid organic cutting medium.
Rare earth-doped WC-Co cemented carbides with low carbon-containing level were prepared by adding mischmetal (mainly containing lanthanum and cerium) in the form of cobalt pre-alloyed powder during ball-milling.After sintering, the sinter skin (exterior) was observed and analyzed by scanning electron microscopy(SEM)and energy disperse X-ray spectroscopy(EDXS). The results show that during the sintering process lanthanum and cerium are separated and cerium predominantly migrates towards the surface forming precipitates containing oxygen.Thermodynamic analysis and polarization of cerium atom caused by the strong interaction between cerium atoms and high energy carbon atoms were used in explaining the observed phenomenon.
ZHANG Li CHEN Shu XIONG Xiang-jun HUANG Bai-yun ZHANG Chuan-fu
