Classification Scheme
Classification systems for grain boundary diffusion define the mathematical treatment of atomic transport through polycrystalline materials based on the relationship between grain size and diffusion length. Use of harrison kinetics allows for the categorization of these processes into regimes designated as type A, type B, type C, or the transition states between them. Each regime requires a specific analytical approach to decouple the contribution of the bulk lattice from the grain boundary network, as the relative penetration depths dictate whether the boundaries act as isolated pipes or as part of a continuous medium.
Regime Boundary
Transition points between these categories depend on the temperature and the duration of the measurement. When the diffusion length in the lattice is much smaller than the grain width, the transport remains confined to the boundaries. This condition defines the limit where grain boundary properties are measured in isolation from the surrounding crystal structure.
Diffusion Interaction
Interactions between the grain boundaries and the adjacent lattice determine the effective penetration depth of the solute. In the intermediate B regime, leakage from the boundary into the bulk creates a complex concentration profile. Analytical models must account for this loss to prevent the underestimation of the boundary diffusion coefficient.
Transport Mechanism
High temperature applications often move the system into the A regime where the lattice and boundary contributions become indistinguishable. The resulting effective diffusion coefficient represents a weighted average of both paths. Precision in these measurements relies on knowing the grain boundary volume fraction within the sample.
