To investigate the effects of TiCN/WC ratio on grain shape and grain growth during the liquid phase sintering of ((100-x)TiCN+xWC)-30Co, specimens were sintered above the eutectic temperature for various times under a carbon saturated condition.
In agreement with previous observations, (Ti,W)(C,N) grains with a core/shell structure, in which the core was TiCN phase and the shell was (Ti,W)(C,N) solid solution, formed. With increasing WC content, the shape of (Ti,W)(C,N) grains changed from faceted one with rounded corners to spherical one because the anisotropy in interfacial energy changed. Faceted grain with rounded corners showed faster growth than spherical ones in contrast to conventional understanding that spherical grains grow faster than faceted ones with a similar composition. Core/shell structure, impurity segregation and interfacial energy seem to affect the grain growth behavior.
When TiCN/WC ratio was 25/75, (Ti,W)(C,N) grains with a rounded shape and WC grains with a faceted shape coexisted in the same liquid. With increasing sintering time, the average size of (Ti,W)(C,N) grains increased continuously and large abnormal WC grains appeared. A plot based on the kinetic equation $r^3-r^3_0$ confirmed that the rounded (Ti,W)(C,N) grains normally grew under a diffusion controlled process. Considering the bimodal grain size distribution, faceted WC grains exhibited abnormal grain growth. These results demonstrate that grain growth behavior in a liquid matrix depends on the shape of grains.
