Investigation of (Mo,W)C Mixed Carbides by Electron Probe Microanalysis and Kossel Technique

Abstract

Tungsten as an element has such a unique combination of outstanding properties that it is very difficult to find a substitute with similar properties. In alloys the task is somewhat simpler: tungsten in high speed steels has been successfully replaced at least partially by molybdenum in the past decades. Attempts to replace tungsten by molybdenum in the hardmetal industry, which takes up more than 50% of the total tungsten production, have been discouraged for a long time by the belief that molybdenum in hardmetal is harmful because of its tendency to form brittle subcarbide (Mo₂ C)-needles or brittle complex compounds. It was only recently that Rudy and coworkers¹ showed that tungsten atoms in WC can be replaced by molybdenum atoms without changing the crystal structure. Apparently a partial substitution of W in WC by Mo results in only gradual and minor changes in physical and mechanical properties. Hardmetals sintered with these mixed (Mo, W)C-carbides show nearly the same properties as straight WC-based hardmetals. The microstructure of Mo-substituted (Mo, W)C-Co hardmetals is distinctly different from that of unalloyed conventional WC-Co hardmetals. In (Mo, W)C-Co hardmetals the carbide grains have a zoned structure, which is easily observable in the etched microsection shown in Fig. 1. In order to gather further information on the homogeneity of composition and crystal structure within one single carbide grain, a combination of electron microprobe analysis (EPMA) and microbeam diffraction technique had to be employed.