The real grain boundary - Effects of temperature and chemistry

The present article is 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 designations applied to grain boundaries, as well as the issues developed in the present context.

Initially, the joint is subjected to temperature effects that can change its equilibrium structure, or even cause the joint to melt at a temperature below the material's normal melting temperature.

This is followed by a discussion of the processes involved in transporting point defects, vacancies and inserted or substituted atoms, alloying elements or impurities to the grain boundary, since their density is generally greater at grain boundaries than in the matrix. It is the phenomenon of intergranular segregation that affects not only the structure and energy of a grain boundary, but also its defects and their behavior. Differences in chemistry between joints can totally erase their geometrical differences.

Finally, the preferential precipitation of a second phase at the grain boundaries of an initial phase (or mother phase), which is closely linked to segregation and difficult to distinguish from the latter in its early stages, also plays an important role in the properties of the material.

Considering the effects of temperature and chemistry on grain boundary structures brings us closer to the real material. This approach must be complemented by mechanical considerations [M 4 013] in order to understand, or even control, the role of grain boundaries in the behavior of the polycrystalline assembly.