Anisotropic grain growth based on the atomic adsorption model in WC-25 pct Co alloy

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I. INTRODUCTION

CEMENTED carbides with a base composition of WCCo are widely used for cutting tools, wear resistant parts, shock resistant parts, and molding materials since they possess high hardness, wear resistance, and thermal resistance. In general, decreasing WC particle size increases mechanical properties. However, starting powder particles tend to coarsen due to the large surface area between carbide particles. Furthermore, the alloy is susceptible to abnormal grain growth (AGG) during liquid-phase sintering, which is accelerated at high temperature.[1–6] Small addition of transition metal carbide is known to be effective for suppressing grain coarsening, VC being one of the most effective inhibitors.[7,8] The exact mechanisms of AGG and of grain-coarsening inhibition in WC are not well understood despite the empirical remedy. There is a growing amount of evidence, however, which indicates that both phenomena originate from the crystal anisotropy. The inhibiting role of TaC was attributed to reduction of the anisotropy in the interfacial energy of WC and binder.[9] Recently, Jaroenworaluck et al.[10] showed segregation of V along the interface of WC and Co. Anisotropic grain growth is frequently associated with AGG in ceramics and WC-base alloys.[11] Although other factors such as crystal defects, grain boundary energy, and grain boundary mobility are quoted, the surface energy anisotropy is held to be the most responsible reason for AGG.[12,13] Faceting of solid/liquid interface has been observed in BaTiO3 and Al2O3,[14,15] which is concurrent with anisotropic grain growth. In Ni and Ni alloys, faceting was attributed to the anisotropy in crystal surface energy.[16] H.S. RYOO, Postdoctoral Student, Department of Metallurgical Engineering, and S.K. HWANG, Professor, Department of Metallurgical Engineering and Center for Advanced Aerospace Materials, are with Inha University, Incheon 402-751, Korea. B.K. KIM, Principal Research Engineer, is with the Materials Forming Laboratory, KIMM, GyeongNam 641010, Korea. H.S. CHUNG, Professor, is with the Department of Molecular Science and Technology, Ajou University, Suwon 442-749, Korea. Manuscript submitted August 9, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS A

There are two classical models to account for the mechanism of AGG in WC, the role of h phase[17,18] and coalescence of neighboring grains.[18–21] Morphological evolution of WC grains, however, does not fit these models well. For example, coalescence of two faceted prism planes, as proposed by Eun,[18] has not been experimentally observed. The two-dimensional nucleation model, as proposed by Park et al.,[6] has a difficulty in explaining the role of h phases. Coalescence of spherical grains was proposed by Kingery,[22] but this is unsuitable for faceted grains. A specific aspect of the shape of abnormally grown WC grains is the dimension ratio R, the ratio of the sidewise length of the triangular prism face to the height of the prism, which is higher than 4.[18,23,24] The models proposed so far do not