The L-coefficients characterize the kinetic response of an alloy to a gradient of chemical potential. In practice, what is imposed is a composition gradient.

Chemical potential gradients, and therefore the fluxes, are assumed to be proportional to concentra­tion gradients, (eqn [9]) leading to the generalized Fick’s laws

J = — X D t1?]

A diffusion experiment consists of measurement of some of the terms of the diffusivity matrix D, y. These terms cannot be determined one by one because at least two concentration gradients are involved in a diffusion experiment. Note, that the L-coefficients can be traced back only ifthe whole diffusivity matrix and the thermodynamic factors are known. Further­more, most of the diffusion experiments are per­formed in thermal conditions and do not involve the interstitial diffusion mechanism.

In the following section, two examples of thermal diffusion experiments are introduced. Then, a few irradiation diffusion experiments are reviewed. The difficulty of measuring the whole diffusivity matrix is emphasized.