Chemically induced grain boundary migration

Solute drag, solute trapping and diffusional dissipation of Gibbs energy

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, ...

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Discontinuous reactions in solids

On the driving force for diffusion induced grain boundary migration

Growth theories for the lamellar eutectoid transformation are reviewed from a free energy point of view in order to form a background for an examination of the various treatments of growth during discontinuous precipitation. The shortcomings of Cahn’s theory of discontinuous precipitation are pointed out and the treatment based upon the individual consideration of growth conditions for the two phases is discussed, with particular emphasis on how the total free energy available to drive the reaction is divided into many parts which are used for various purposes. New sets of calculations are presented in order to demonstrate the effect of free energy losses due to boundary friction and volume diffusion. The effect of coherency stresses in the parent grain is shown to decrease the free energy loss due to volume diffusion and may thus provide a driving force for grain boundary movement.

On theories of growth during discontinuous precipitation

Grain boundary precipitation of M23C6 has been studied in a 20% Cr-35% Ni stainless steel with two grain sizes during creep deformation at 700°C as well as during an ordinary ageing treatment at 700°C. A special etching technique was applied which showed how the grain boundary precipitation gave rise to depletion of alloying elements in a zone of uniform thickness, independent of the carbide distribution, and with a gradual decrease of the depletion towards the grain interior. At some places the carbide precipitation and grain boundary migration co-operated and in these cases there was a sharp change in alloying content across the grain boundary. This process was more frequent in creep tested samples and the degree of co-operation was larger in the coarse-grained material where even a few cases of lamellar, eutectic-like precipitation was observed. Such a grain size dependence is expected theoretically and is caused by the large difference in diffusivity between carbon and the other alloying elements. It is proposed that the various degrees of co-operation between carbide precipitation and grain boundary migration are all examples of discontinuous precipitation. The various proposed mechanisms for grain boundary migration during discontinuous precipitation are discussed on the basis of the present results.

R. Lagneborg

Papers

Discontinuous precipitation of M23C6 in austenitic steels