Directly determining the polarity of WC grains in WC-12Â wt%Co alloy by ABF-STEM

WC grains in WC-Co alloys are shaped as truncated triangular prisms, each with a basal plane of a (0001) truncated triangle and two {11̅00} prismatic planes, one narrower and one wider. This anisotropic shape is related to the polarity of the WC lattice, which comes from the difference in the spacing of the W and C layers in the 〈0001〉 projection. We directly determined the relationship between the two {11̅00} prismatic planes and the polarity of the WC lattice by using high-resolution scanning transmission electron microscopy (STEM). High-angle annular dark-field STEM and annular bright-field STEM revealed that the wider prismatic plane had W and C atomic layers with a shorter spacing, while the narrower plane had a longer spacing. The WC/Co interface on the wider prismatic side was atomically flat, while many steps formed on the surface of the narrower prismatic side. This difference in step formation at these interfaces could have come from the difference in how their WC bonds formed.