chapter 5

diffusion

Mass transport by atomic motion. As temp increases the number of atoms capable to diffuse increases

interdiffusion

Diffusion of atoms of one metal into another metal.

self-diffusion

Atomic migration in pure metals.

vacancy diffusion

The diffusion mechanism in which net atomic migration is from a lattice site to an adjacent vacancy. For this type of diffusion an atom must have enough vibrational energy to break bonds with neighboring atoms and move to an adjacent vacancy.

interstitial diffusion:

A diffusion mechanism by which atomic motion is from interstitial site to interstitial site.

diffusion flux (J)

The quantity of mass diffusing through and perpendicular to a unit cross-sectional area of material per unit time.

Fick's first law

The diffusion flux is proportional to the concentration gradient. This relationship is used for steady-state diffusion situations.

diffusion coefficient (D)

The constant of proportionality between the diffusion flux and the concentration gradient in Fick's first law. Its magnitude is indicative of the rate of atomic diffusion.

steady-state diffusion

The diffusion condition for which there is no net accumulation or depletion of diffusing species. The diffusion flux is independent of time.

concentration profile:

The curve that results when the concentration of a chemical species is plotted versus position in a material.

concentration gradient (dc/dx):

The slope of the concentration profile at a specific position.

driving force:

The impetus behind a reaction, such as diffusion, grain growth, or a phase transformation. Usually attendant to the reaction is a reduction in some type of energy (e.g., free energy).

nonsteady-state diffusion:

The diffusion condition for which there is some net accumulation or depletion of diffusing species. The diffusion flux is dependent on time.

Fick's second law

The time rate of change of concentration is proportional to the second derivative of concentration. This relationship is used in nonsteady-state diffusion situations.

carburizing:

The process by which the surface carbon concentration of a ferrous alloy is increased by diffusion from the surrounding environment

activation energy (Qd):

The energy required to initiate a reaction, such as diffusion.