The theory of the kinetics of phase change is developed with the experimentally supported assumptions that the new phase is nucleated by germ nuclei which already exist in the old phase, and whose number can be altered by previous treatment. The density of germ nuclei diminishes through activation of some of them to become growth nuclei for grains of the new phase, and ingestion of others by these growing grains. The quantitative relations between the density of germ nuclei, growth nuclei, and transformed volume are derived and expressed in terms of a characteristic time scale for any given substance and process. The geometry and kinetics of a crystal aggregate are studied from this point of view, and it is shown that there is strong evidence of the existence, for any given substance, of an isokinetic range of temperatures and concentrations in which the characteristic kinetics of phase change remains the same. The determination of phase reaction kinetics is shown to depend upon the solution of a functional equation of a certain type. Some of the general properties of temperature‐time and transformation‐time curves, respectively, are described and explained.

Kinetics of Phase Change. I General Theory

It is shown that the free energy of a volume V of an isotropic system of nonuniform composition or density is given by : NV∫V [f 0(c)+κ(▿c)2]dV, where NV is the number of molecules per unit volume, ▿c the composition or density gradient, f 0 the free energy per molecule of a homogeneous system, and κ a parameter which, in general, may be dependent on c and temperature, but for a regular solution is a constant which can be evaluated. This expression is used to determine the properties of a flat interface between two coexisting phases. In particular, we find that the thickness of the interface increases with increasing temperature and becomes infinite at the critical temperature Tc , and that at a temperature T just below Tc the interfacial free energy σ is proportional to (T c −T) 3 2 . The predicted interfacial free energy and its temperature dependence are found to be in agreement with existing experimental data. The possibility of using optical measurements of the interface thickness to provide an additional check of our treatment is briefly discussed.

Free Energy of a Nonuniform System. I. Interfacial Free Energy

On the basis of the nucleation theory developed by Volmer, Becker, and co‐workers, and the theory of absolute reaction rates, an expression is derived for the absolute rate of nucleation in condensed systems.

Rate of Nucleation in Condensed Systems

By finding the saddle point in the expression derived in Paper I (see reference 8) for the free energy of a nonuniform system, we have derived the properties of a critical nucleus in a two‐component metastable fluid.At very low supersaturations, we find that the properties of the nucleus approach those predicted by the classical theory that assumes the nucleus to be homogeneous with an interfacial energy that does not vary with curvature. However, with increasing supersaturation, the following changes occur in the properties of the critical nucleus. (a) The work required for its formation becomes progressively less than that given by the classical theory, and approaches continuously to zero at the spinodal. (b) The interface with the exterior phase becomes more diffuse until eventually no part of the nucleus is even approximately homogeneous. (c) The composition at the center of the nucleus approaches that of the exterior phase. (d) The radius and excess concentration in the nucleus at first decrease, the...

Free Energy of a Nonuniform System. III. Nucleation in a Two‐Component Incompressible Fluid

The kinetic model for change of phases developed by M. Avrami at the end of the thirties has been used to describe the temporal behavior of phase changes. Until today this model is studied and adapted to include broader hypotheses. However, the mathematical format presented by M. Avrami is difficult to be understood by beginners. The purpose of this work is to clarify the mathematical treatment of Avrami's work, going straightforward to the arguments that led to his main results.

/pdf/kinetics-of-phase-change-svudmywmbd.pdf

Kinetics of phase change

Unter der Annahme, das die Energie eines Mischkristalls durch einfache Addition von Nachbarbindungen berechnet werden darf, wird die Keimbildungsarbeit fur die Ausscheidung in einem ubersattigten binaren Mischkristall berechnet. Daraus last sich die Haufigkeit der Keimbildung bestimmen. Aus der Durchrechnung eines Zahlenbeispiels ergibt sich fur diejenige Unterkuhlung, bei welcher die Keimbildungshaufigkeit ihr Maximum erreicht, eine befriedigende Ubereinstimmung mit vorliegenden Messungen uber die Geschwindigkeit der Ausscheidung.

R. Becker

Papers

Die Keimbildung bei der Ausscheidung in metallischen Mischkristallen