The real grain boundary - Mechanical aspects

The present article is the second devoted to the real grain boundary: it follows on from the article [M 4 011] to which the reader must necessarily refer in order to fully understand the denominations applied to grain boundaries as well as the issues developed in the present context. It completes a first approach to chemical, point and volume defects [M 4 012] , to focus on linear defects resulting from mechanical stress transfers between grain boundaries and crystals, or directly from the effect of external stresses.

The interactions of matrix dislocations with a grain boundary bring the boundary into a non-equilibrium state. These interactions are studied mainly by transmission electron microscopy and by simulation; methods specific to the study of joints are explained with, each time, an example of application to a particular case of interaction. A joint returns to equilibrium via intergranular stress relaxation processes, studied by in situ electron microscopy and compared with processes predicted by various models.

Point defects and linear defects interact in joints, as is the case in matrix: the reciprocal effects of interactions between extrinsic dislocations and segregation are discussed, with their consequences for grain joint behavior.

Finally, the ultimate goal in materials science is to understand the repercussions of these structural phenomena on the properties of grain boundaries and hence the properties of the material as a whole. This aspect cannot be presented in all its complexity in the context of this article, and only a few pointers are given, paving the way for a whole series of studies yet to be undertaken. Understanding the links between elementary-scale mechanisms and macroscopic material behavior is still in its infancy.

Finally, these approaches to grain boundaries or homophase interfaces can easily be extended to heterophase interfaces (between two different phases or materials).

These considerations of the reactions of grain boundaries to mechanical loading, and the assemblies they form in polycrystals, may pave the way for "grain boundary engineering".