Regions where the movement of atoms is easier are known as paths of high diffusivity (Figure 2.2.24). As an example of highly diffusive paths we can cite grain boundaries and the surface of the material itself. Atoms can diffuse more quickly along grain boundaries than through the interior of the material because they are less restricted along the boundaries. The boundary regions are full of vacancies and this helps facilitate the displacement of diffusing atoms. From an energetic point of view, the activation energy (Q) necessary for grain boundary diffusion is lower than that of volume diffusion.
Due to the rapid movement of free surface atoms, surface diffusion plays an important role in metallurgical processes. Grain boundary diffusion is of great interest because, in a polycrystalline metal, the sum of the areas of the grain boundaries is, many times, larger than the surface area of the body. Along with this, the grain boundaries form a network throughout the sample. In this way, when we measure the diffusion in a polycrystalline metal, the result represents the set of volumetric diffusion effects (Dv), by grain boundary (Dgb) and by surface (Ds), all together representing the apparent diffusion coefficient. In general, the smaller the grain, larger the area of boundaries and the greater importance of the boundaries to the process.
The Arrhenius diffusion equation [equation (2.9)] shows that diffusivity depends on activation energy Q and temperature T. As was seen above, the effects of bulk diffusion and grain boundaries are superimposed in the experimental curves. However, in low temperatures (for diffusion to occur) activation energy Q is the predominant factor. Therefore, a region of lower activation energy Q will permit greater diffusivity of atoms when compared to a region that requires more activation energy. Therefore, grain boundary diffusion predominates at lower temperatures. Greater mobility of atoms occurs at higher temperatures and volume diffusion becomes predominant as activation energy becomes of secondary importance.
