| Abstrakt: |
A series of VC + Ni cermets was prepared where the carbon to metal (C / M) ratio of the carbide was varied from 0.86 to 0.75. Transmission electron microscopy and diffraction were used to determine the C/M ratio of the carbide and binder microstructure. At high C/M ratios, the carbide was principally V8C7or V8C7+ V6C5, while at low C/M ratios the carbide was V6C5or V6C5+ V4C3. The vanadium content of the binder increases as the C/M ratio of the carbide is decreased. The mechanical behavior of the cermets was evaluated by crack resistance and hardness measurements. In inverse crack resistancevshardness plots, the VC + Ni cermets exhibit three types of behavior (labeled Type I, II, and III) in order of decreasing fracture toughness for a given hardness. Auger electron spectroscopy and scanning electron microscopy show that the three behavior types are characterized by differences in the dominant fracture mode. The fracture mode is principally transgranular fracture through the binder for Type I, transgranular cleavage of carbides for Type III, and intergranular or mixed fracture mode for Type II. The differences in fracture mode and mechanical behavior are induced by changes in microstructural features, as well as changes in the binder composition and formation of second phases. These results suggest that binder strengthening is a viable means to improving cermet performance only in systems when the cleavage energy of the carbide is sufficiently high to withstand the stresses generated by plastic flow in the binder. |
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