Growth mechanism of round-edged NbC grains in Co liquid

To investigate the growth mechanism of round-edged grains in a liquid matri x, 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 grai ns changed from faceted with rounded edges to spherical, and thc ir growth rate increased. In contrast, as the liquid volume fraction increased, the g rowth rate decreased, regardless of B addition, suggesting that the growth of the NbC grains was controlled by the diffusion of atoms through the liqu id. To quantitatively evaluate the growth behavior and mechanism of the NbC grains of different shapes, we measured the growth-rate constant, growth e xponent, and activation energy. The measured grain growth kinetics confirme d that the growth of the NbC grains was diffusion-controlled regardless of whether their shape was spherical or faceted with rounded edges. The observ ed 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 t han of rounded edges and the grain shapes in growth and in dissolution. (C) 2001 Published by Elsevier Science Ltd on behalf of Acta Materialia Inc.