Discontinuous reactions in solids

Discontinuous reactions are solid state moving boundary phase transitions characterised by a discontinuous or abrupt change in orientation and composition between the matrix phases in the reactant and product aggregate across the migrating boundary or reaction front that provides a short circuit path of diffusion. The reactions include discontinuous precipitation, discontinuous coarsening, discontinuous dissolution, and diffusion induced grain boundary migration. All these reactions may account for a substantial change in microstructure, composition, and material properties, and hence, deserve adequate scientific attention for a better understanding. The present review provides a comprehensive discussion on the current status of understanding about nucleation and growth mechanisms, genesis and driving force, product morphology and distribution, kinetic growth models, and related experimental techniques, and above all, the unresolved questions concerning these discontinuous reactions. In addition, exhaustive lists have been provided to document the important literature on the concerned subjects, whenever possible. Finally, a particular emphasis has been placed on analysing the recent findings about dynamic behaviour of grain boundaries, scope of determination of Arrhenius parameters by kinetic analysis, and orientation/structural dependence of boundary migration and diffusion.