An analysis of grain boundaries and grain growth in cemented tungsten carbide using orientation imaging microscopy
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I. INTRODUCTION
CEMENTED tungsten carbide, WC-Co, represents a class of cermet materials consisting of tungsten carbide (WC) grains embedded in a ductile metal matrix (usually cobalt, Co). It is well known in the field that WC grains acquire a characteristic triangular platelet prism shape after sintering owing to the highly anisotropic nature of WC crystals.[1,2,3] It is also known that WC and Co powders are irregularly shaped equal-axial particles before sintering. Although the dramatic changes of grain morphologies from unsintered to sintered states are well known, there have been very few studies in the past on the details of the mechanisms of these changes and how they affect densification and grain growth. The interest in and the need for understanding these details grew in recent years, however, because it is a particularly important issue for nanocrystalline and ultrafine grained WCCo materials.[4,5,6] For many different materials including WC-Co, grains grow rapidly during nanosintering to such an extent that the material is no longer nanocrystalline once it achieves full density. Thus, the goal of the sintering of nanosized powders to not only achieve full densification but also retain nanoscaled grain sizes remains a difficult task. Specifically, the grain growth of WC during sintering can be treated as consisting of two parts: during heatup and during isothermal hold at liquid phase sintering temperatures. Although grain growth during liquid phase sintering, which has been well researched and documented, is important for the final microstructure of the material, the grain growth during heatup and the early stages of sintering must be controlled in order to maintain nanoscale or ultrafine grain sizes in the final microstructure. Previously published studies VINEET KUMAR, Graduate Research Assistant, and ZHIGANG ZAK FANG, Assistant Professor, are with the Department of Metallurgical Engineering, University of Utah, Salt Lake City, UT 84112. Contact e-mail: [email protected] S.I. WRIGHT, Director of Technology, and M.M. NOWELL, Application Scientist, are with TSL-EDAX, Draper, UT 84020. Manuscript submitted August 9, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS A
showed that the rapid grain growth is closely tied to densification and the change of morphology of grains from equalaxial aggregated particles to prism-shaped grains with well-defined surfaces and grain boundaries.[4] It was further demonstrated by the previous study that the shape changes and the increase in size of WC grains occur almost instantaneously during heatup prior to reaching the designated liquid phase sintering temperature. In order to control grain growth of ultrafine or nanosized materials, a fundamental understanding of the evolution of microstructure during heatup, particularly changes in shape, crystallographic orientation, and grain boundaries of WC grains and their relationship to grain growth, is inevitable. Although it has been widely speculated that initial rapid grain growth is accomplished by coalescence of WC grai
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