6 - Kinetics of mass transport and phase transformations

Abstract

Atom movements and reactions occur in solids to drive them toward thermodynamic equilibrium. The resulting time-dependent mass transport processes produce changes in chemical composition, in physical amounts, and in the geometry and size of phases present. Kinetic change in solids invariably occurs by diffusional mass transport. Atomic diffusion can occur in response to externally imposed concentration gradients or to internal composition variations due to inhomogeneous processing. The former is readily modeled by the diffusion equation together with appropriate initial and boundary conditions. Simple solutions in terms of complementary error and gaussian functions enable the concentration of the diffusing species to be determined as a function of position and time. Phase transformations may be divided into several categories. The least complex involves a change in physical state, for example, solidification, with no alteration of phase chemical composition. Decomposition from a single phase to a mixture of two phases, each with different composition and structural morphology, is at the other extreme of complexity. Concurrent heterogeneous nucleation and diffusional growth processes make phase transformations complex events. In the conversion of glass to glass-ceramics these effects are decoupled, enabling finer tuning of the transformation. The generic similarity of nucleation-growth phenomena in the crystallization of amorphous films and the transformation of steel should not go unnoticed. Similar transformations occur in the precipitation hardening treatment for nonferrous metals and in recrystallization effects that occur in previously deformed metals. The Avrami equation provides a fruitful way to analyze the kinetics for all such nucleation-growth transformations. Mass transport effects are also of interest because they play a central role in phase transformations of all kinds, high-temperature annealing of metals, electrical transport in ceramics, mechanical behavior at elevated temperatures, and assorted heat treatments to strengthen or toughen materials.