Intergranular diffusion in polycrystalline materials

This article follows on from those on grain boundaries [M 4 011] , [M 4 012] and [M 4 013] , to which the reader can refer to understand some of the names of these crystal structure defects – or homophase interfaces – and what their properties are other than diffusion, but often governed by it.

For an approach to the theoretical basis of diffusion and, in particular, grain boundary diffusion, the author recommends [M 55] , which provides a comprehensive description of these phenomena.

The aim here is not to review in detail the models and mathematical laws of intergranular diffusion, but to focus on the elementary mechanisms at the atomic scale, and to describe the specificity of diffusion paths according to the structure of the grain boundary. This is described either at the mechanical level, in terms of intrinsic dislocations; or at the atomic scale, in terms of structural units. The diffusional behavior of a set of grain boundaries – practical case of polycrystals – is addressed, in particular through percolation theory. Finally, although diffusion controls most material properties (phase change, oxidation, corrosion), we have chosen here to focus on the predominant role of grain boundary diffusion in superplastic creep mechanisms.

The technical and fundamental interest in diffusional phenomena at grain boundaries also calls for the development of research in the field of oxides and semiconductors. This research is still guided by the relatively well-known approach of diffusion in metals, which still serves as its paradigm.