Kinetic theory of precipitation processes in solid solutions: III

Abstract

Abstract Precipitation processes in solid solutions have been studied by assuming the existence of a criticalsize for clusters. The kinetics have been followed by solving simultaneous differential equations for two types of the cluster size dependence of reaction constants: (1) size-independent and (2) size-dependent (proportional to the number of solute atoms in the respective cluster), the former case having been discussed by Damask, Danielson and Dienes (1965). Most of the calculations are made with an ‘equilibrium approximation'; the validity of the approximation has been examined in detail. It has been found that the shape of reaction curves and the resultant particle size distributions are very different for the two cases. Although Damask et al. claimed that the critical size of clusters could be estimated from experimental data, this is valid only for a special case of size-independent reaction constants, which would never be realized in actual precipitation processes. It is pointed out that the critical size in the sense of Damask et al. is not the same as that in the thermodynamical theory of nucleation.