In the same solid, three types of diffusion can be observed:
1- surface;
2- grain boundary;
3- volume.
Volume Diffusion, also called lattice diffusion, will be of obvious importance in the case but if there were a grain boundary in the x direction, there will be grain boundary diffusion. If there is a crack in direction x, the atoms will be transported to the inside by surface diffusion. A schematic of the three types of diffusion are illustrated in Figure 2.2.8.
A diffusing atom moves more easily along the grain boundary than within the lattice; therefore, less activation energy Q is needed for diffusion on a grain boundary when compared to the value needed for volume diffusion. Along with this, seeing as the quantity of material transported by any of the three types of diffusion can be calculated by Fick’s first law (Equation 27), for the same compositional gradient, the quantity of material transported will depend on the effective area where the atoms spread. Seeing that the effect of a grain boundary or path of a surface diffusion is measured in interatomic distances (around 1/4 nm), the areas of these paths are much smaller than the areas of volume diffusion. An atomist explanation of why the diffusivity along crystalline defects (surface and grain boundary) is larger than the volume, is that there is greater availability of space for atomic movement around the defects (the atoms of a grain boundary present a lower level of coordination compared to atoms in the middle of a crystal lattice). The mechanisms of grain boundary diffusion and surface diffusion are important in phenomenons like sintering and metal oxidation.
The animation in Figure 2.2.8 shows the concentration profile for a material (orange, to highlight) that undergoes volume, grain boundary, and surface diffusion of materials (including cracks).
