Growth mechanism of round-edged NbC grains in Co liquid

To investigate the growth mechanism of round-edged grains in a liquid matrix, cemented alloys of NbC-Co with or without B were sintered under a carbon-saturated condition. As the B content increased, the shape of the NbC grains changed from faceted with rounded edges to spherical, and their growth rate increased. In contrast, as the liquid volume fraction increased, the growth rate decreased, regardless of B addition, suggesting that the growth of the NbC grains was controlled by the diffusion of atoms through the liquid. To quantitatively evaluate the growth behavior and mechanism of the NbC grains of different shapes, we measured the growth-rate constant, growth exponent, and activation energy. The measured grain growth kinetics confirmed that the growth of the NbC grains was diffusion-controlled regardless of whether their shape was spherical or faceted with rounded edges. The observed reduction in growth rate with an increasing facet fraction of the grains can be explained in terms of the much slower growth rate of facet planes than of rounded edges and the grain shapes in growth and in dissolution.