Analytical electron microscopy of discontinuous solid state reactions

Abstract

The discontinuous solid state reactions represent a group of diffusional solid state phase transformations during which the solute redistribution accompanying the formation of a new phase occurs on a nanometer scale just at the moving reaction front. Such solute changes can be sufficiently revealed by means of analytical electron microscopy (AEM). A number of systematic investigations made using AEM during the last decade in discontinuous precipitation, dissolution, and coarsening are reviewed and critically analysed. Measurements of the solute concentration profiles in the direction parallel to the reaction front are strongly emphasised, as they allow verification of theoretical models of discontinuous reactions and consideration of discontinuous reactions as local phenomena in the form of a number of events occurring in individual cells. Such an attempt removes the divergence between the tracer diffusivity values obtained by the sectioning technique and the chemical diffusivity data obtained from the discontinuous precipitation and dissolution reactions. A discussion is also presented on how the results obtained via AEM can be used for an optimisation of the theory of discontinuous solid state reactions. The implementation of alternative electron optic techniques for the determination of the solute redistribution accompanying the discontinuous reactions is demonstrated.