The growth mechanism of NbC grains in a Co matrix has been investigated. Cemented alloys of NbC-Co with or without B were sintered under carbon saturated condition.
With increasing B content, the shape of NbC grains changed from faceted one with rounded edges to spherical one, and their growth rate increased. In contrast, with increasing liquid volume fraction, the growth rate decreased, regardless of B addition, implying that the growth of NbC grains was controlled by diffusion of atoms through liquid.
In order to evaluate quantitatively the growth behavior and mechanism of NbC grains of different shapes, growth-rate constant(K), growth exponent(n) and activation energy were measured under the assumption that the growth equation followed a kinetic equation, $r^n-r_o^n$=Kt, where r is the average grain size.
The measured K values of $2.7×10^{-14}~5.42×10^{-14}㎤/sec$ were remarkably smaller than theoretically predicted values of $2.3×10^{-12}~3.42×10^{-12} ㎤/sec$. The growth exponent n was 3, whatever the grain shape was. The activation energy of grain growth was 28 Kcal/mol for an alloy with no boron addition, and 32 Kcal/mol for an alloy with 3 wt% boron addition. These values are in good agreement with previous data for diffusion controlled growth of various carbides. For an alloy with 2.25 wt% boron addition, apparent activation energy was 68 Kcal/mol, much higher value than the others. This result is thought to be artifact and result from grain shape change with temperature which appears to affect grain growth kinetics considerably.
Based on the measured grain growth kinetics in function of grain shape, two possible mechanisms for growth of faceted grains with rounded-edges were proposed. One is the creation of ledges at rounded edges, which can act as kink sites for atom attachment on the flat interface. The other is the combination of diffusion and interface reaction control of growth. This mechanism assumes a kinetic balance between diffusion rate and interface reaction rate at flat surface. It is, however, difficult to justify the assumption at the moment. Nevertheless, the two mechanisms can explain the variation of grain growth rate with grain shape.
