An analysis of grain boundaries and grain growth in cemented tungsten carbide using orientation imaging microscopy

Abstract

Tungsten carbide grains are usually prism-shaped triangular platelets in cemented tungsten carbide materials, owing to the highly anisotropic nature of tungsten carbide grains. The misorientation distribution function (MODF) shows a preferred misorientation relationship between WC/WC crystals. The misorientation relationship is characterized as 90 deg misorientation about the axis. The carbide-carbide boundaries with 90 deg rotation about the axis are low-energy boundaries and play an important role in grain growth and densification during sintering. In this study, the evolution of carbide-carbide boundaries with 90 deg rotation about the axis during the sintering process is determined quantitatively using orientation imaging microscopy (OIM). The evolution of boundaries is correlated with grain growth. The rapid grain growth at early stages of sintering is attributed to coalescence of grains along the 90 deg low-energy boundaries. The origin of these boundaries and their role in grain growth during sintering are also discussed. [101 0] [101 0] [101 0]

Some aspects of the structure of cobalt and nickel binder phases in hardmetals

“…This imbalance in knowledge is in part due to the relative ease of producing polished and etched 2-D cross-sections for identification of individual WC grains, while obtaining a good finish on the small, soft regions of the Co binder is much more difficult. Increasingly, electron backscatter diffraction (EBSD) has been used to characterise the WC grain structure [8][9][10], distinguishing individual grains consistently by their crystallographic orientation and yielding large sets of data on WC grain size and distribution. EBSD has also enabled some information to be gained on deformation mechanisms within the WC [11,12], complementing that obtained by transmission electron microscopy (TEM) [3,13].…”

“…WC grain size (circle equivalent diameter) distributions from measurements of grain size on EBSD maps of Ni binder (subsets 0,3,5,8,9,10).…”

Characterization and comparison of grain boundary character distributions in cemented carbides with different binder phases

“…The crystallographic configuration of 90°/[10 À10] boundary can be speculated to has a significant effect on the performances of cemented carbides, mainly because the (0 0 0 1) basal and (10-10) prismatic facets of carbide crystals are different in either surface energy [8] or micro-properties [2]. Earlier works suggest that the 90°/[10 À10] boundaries can either pre-exist in the raw powder and be eliminated during sintering [10,11], or be evolved by the coalescence of low-energy crystallographic facets [8]. The outcome of current work illustrates that 90°/[10 À10] boundaries are the most frequently occurring carbide/carbide boundaries in cemented carbide samples with either cobalt or nickel as the binder phase (see Fig.…”

“…Among these boundaries, the 90°/[10 À10] boundary, with the basic crystallographic feature of a 90°rotation about the [10 À10] axis [7], and with low interfacial energy as well as high work of separation [8], is attracting special attention [6,[9][10][11][12][13]. Note that the hexagonally close packed lattice structure of tungsten carbide has a c/a ratio of 0.976 (here c = 0.2837 nm and a = 0.2906 nm) [14] which is quite close to 1, so the 90°/[10 À10] boundary can be referred to as R2 boundary in coincidence site lattice (CSL) notation [7], where the low R value represents high reciprocal density of coinciding sites and symmetrical configuration of dense planes [15].…”

Effect of densification mechanism on the Σ2 grain boundary plane distribution in WC–Co composites

“…The GBPD is expressed in terms of five macroscopically observable parameters, including three Eulerian angles to describe the lattice misorientation across the boundary, and two spherical angles to describe the orientation of the grain boundary plane normal. There are increasing studies about the S2 boundary distribution using EBSD [3,4] and the FPA method [5,6]. However, because of the difficulty of obtaining the very large data sets needed to apply the FPA method to a hexagonal material, it has not previously been possible to compare the GBPDs of WC-Co samples prepared in different ways.…”