The role of dislocations in the coarsening of carbide particles dispersed in ferrite

Abstract

The elevated-temperature coarsening of fine banded dispersions of VC and TiC in two simple isothermally transformed Fe-V-C and Fe-Ti-C alloys has been examined by transmission electron microscopy. The metallographic observations have shown that the coarsening process is associated with the presence of dislocations and it was found that the overall kinetics for the coarsening of VC in ferrite at 725°C obeyed a law of the formx ⁻⁵αt which is in qualitative agreement with the predictions of the theory for the coarsening of particles by dislocation pipe diffusion. The observations suggest that when particles are sufficiently small to be coherent with the matrix, they can trap dislocations through strain-field interaction and subsequently coarsen preferentially by dislocation pipe diffusion. As the particles become larger and their coherency strain fields decrease, the attractive interaction with dislocations decreases and the dislocations become free to climb away until entrapped by the strain fields of particles which have not coarsened significantly. In this way a relatively low density of dislocations can significantly increase the coarsening kinetics of dispersions of fine coherent particles and result in a broader distribution of particle sizes than is predicted by the Lifshitz-Wagner theory.