Phase Transformation Kinetics

Abstract

In the above, a phase transition was viewed as resulting from changing a control parameter, such as T or p, across a phase boundary, say a coexistence line, of the system’s phase diagram. In practice, such a transformation, of say a liquid phase into a crystal phase, will however require a considerable amount of molecular scale re-organization, and this will always take a finite amount of time. The precise way in which the old (liquid) phase is transformed into the new (crystal) phase will hence involve some dynamic (time-dependent) phenomena which cannot be described within the equilibrium statistical mechanical framework used until now. In the present chapter some of the kinematic aspects related to the physical transformation of one phase into another will thus be considered. Needless to say that a good understanding of these kinematic aspects is essential for the correct interpretation of what is seen in the laboratory during a phase transition. For instance, in some cases the transformation is so slow that the new phase cannot be formed, during the finite time of observation, without some help from the experimentalist. In other cases, the amount of reorganization required is so important that the system gets arrested in a glass-like configuration somewhat intermediate between the two equilibrium phases involved in the transition. Unfortunately, several of these aspects are, even today, not well understood. Here, only the classical theories of nucleation and of spinodal decomposition will be considered.