The Kinetics and the Rate Controlling Process of the Graphite Nodule Growth

Abstract

The growth kinetics of graphite nodules is studied by means of optical microscopy. The graphite nodules showed parabolic growth kinetics and the growth rates changed significantly with pre-treatments as well as with graphitizing temperatures. In order to explain these behaviors, a model of rate controlling process was proposed in which the increments in the space occupied by the graphite nodules were assumed to be supplied by vacancy diffusion from the sources in the specimen to the graphite nodules mainly through the grain boundaries. Although this model is analogous to Birchenall’s model which assumed diffusion of Fe atoms away from graphite nodules, this differs from Birchenall’s mainly in a treatment of the driving force and of the diffusion path. This model was revealed to be consistent with the experimental results of (1) the magnitude of growth rate, (2) the change in the growth rate with pre-treatments, (3) the shape of graphite nodules along grain boundaries, and (4) other microscopic charactaristics. However the activation energies observed (200∼250 kJ/mol) could not be explained using this model.