Abnormal WC crystal growth from liquid Co flux occurs via eta phase decomposition

The growth mechanism of large WC crystals from a liquid Co-based flux is identified. This is achieved by systematically varying the growth temperature and Co content from 1200 to 1400 degrees C and 70-83 at.% respectively. Crystal growth was characterised using metallography and X-ray diffraction. The WC grains were bimodally distributed, consisting of a smaller (10-20 mu m) population of grains, which nucleated homogenously from the liquid, and a secondary population of abnormally large grains, several millimetres in size. The abnormal grains nucleated on the (W,Co)(6)C eta phase, and subsequently consumed it through a carburisation reaction. The size of abnormal grains was enhanced by adopting the eutectic composition, such that the first solid phase to form was the eta phase, whilst at the same time cooling slowly through the eta -> WC transformation temperature, at similar to 1300 degrees C. This growth mechanism could be exploited for a variety of metal carbides with similarly sluggish diffusion rates.

Automated summary

The growth mechanism of large WC crystals from a liquid Co-based flux has been identified, where abnormal grains nucleate on the (W,Co)(6)C eta phase and consume it through a carburisation reaction. By adopting the eutectic composition, the size of abnormal grains is enhanced. This growth mechanism could be applied to various metal carbides with similar diffusion rates.