Effect of VC addition on diffusion mechanism at the WC/Co interface using a VC doped diffusion couple was investigated. In the diffusion couple with VC doped to WC half, (WC + VC)/Co, the η and η’ phases were formed between WC/Co interface after 1623 K for 14.4 ks treatment. In contrast, the η, η’, W and W2C phases were observed in the couple with VC doped to Co half, WC/(Co + VC). The phase change on the C-Co-W ternary isothermal phase diagram was inferred from the microstructure of diffusion couple. The (WC + VC)/Co couple showed the phase change through WC-η, η-η’, η’-Co along with these tie-lines on the C-Co-W isothermal phase diagram. The WC/(Co + VC) couple showed more complicated phase change as compared with the (WC + VC)/Co. The phase change was through the (WC + W2C + η), (W2C + η), (W + η) coexisting region, and along with tie-lines of η-η’ and η’-Co.
WC-Co ultrafine-grained cemented carbide with the combined addition of Ti(C,N) and Cr3C2 was studied for microstructure and mechanical properties. In particular, the strength (transverse rupture strength) of the alloy was examined in detail comparing it with other kinds of cemented carbides. The WC grain size of the alloy with the single addition of Ti(C,N) or Cr3C2 became finer with increasing the amount of the additive. The combined addition of Ti(C,N) and Cr3C2 made the WC grain size further finer and the microstructure more homogeneous. The hardness of Ti(C,N)-Cr3C2 added alloys increased with decreasing the average grain size of WC or the mean free path of the binder phase, but the fracture toughness decreased. The strength of Ti(C,N)-Cr3C2 added alloy varied depending on the amount of the additive. It was noted that the strength of 3 vol%Ti(C,N)-0.5 vol%Cr3C2 alloy was the best and reached 4.6 GPa on average and 5.0 GPa at the maximum. It was observed that such a high strength alloy was shattered after the transverse rupture strength test and so that it was difficult to detect the fracture origin. Based on the limiting strength that is generated in normal-grained cemented carbide, it was considered that a high strength level exceeding the limiting strength of normal-grained alloy was achieved in the ultrafine-grained cemented carbide obtained in this study. The WC-Co ultrafine-grained cemented carbide with Ti(C,N)-Cr3C2 was superior in strength compared to conventional cemented carbide. New applications can be expected to take advantage of these characteristics.