Meaning
Driving force for mass transport arises from the spatial variation of chemical potential rather than concentration alone. A chemical activity gradient determines the direction and rate of diffusion in complex alloys where high-temperature stability is required. This thermodynamic quantity accounts for the non-ideal behavior of atoms in a crystal lattice.
Diffusional Flux
Transport of atoms occurs to minimize the Gibbs free energy of the system. The magnitude of the chemical activity gradient scales with the velocity of moving species within the substrate. Flux equations incorporate this value to predict how elements like chromium or aluminum migrate toward an oxidizing surface.
Component Interaction
Atomic interactions change the effective concentration of a solute in a solid solution. Variations in a chemical activity gradient often arise from the presence of third-party elements that alter the bonding environment. This effect causes uphill diffusion where atoms move toward regions of higher concentration to satisfy thermodynamic equilibrium.
Process Limit
Equilibrium occurs when the chemical potential becomes uniform throughout the material volume. Although the chemical activity gradient eventually reaches zero, the local concentrations might remain disparate. Calculations for coating stability rely on this terminal state to define the service life of high-pressure turbine components.
Observations of phase separation in aging alloys confirm that atomic movement continues until no potential difference exists.
