In an interdiffusion experiment, a sample A (mostly composed of A atoms) is welded to a sample B (mostly composed of B atoms) and annealed at a temperature high enough to observe an evolution of the concentration profile. According to eqn [12], the flux of component i in the reference crystal lattice is proportional to its concentration gradient:

J = — D, ГС, [18]

where the so-called intrinsic diffusion coefficient Di is a function of the phenomenological coefficients and the thermodynamic factor:

LV

D = <f — C F [19]

An interdiffusion experiment consists of measure­ment of the intrinsic diffusion coefficients as a func­tion of local concentration. The resulting intrinsic diffusion coefficients are observed to be dependent on the local concentration. Within the TIP, while the driving forces are locally defined, the L-coefficients are considered as equilibrium constants. It is not easy to ensure that the experimental procedure satisfies these TIP hypotheses, especially when concentration gradients are large, and the system is far from equilib­rium. When measuring diffusion coefficients, one implicitly assumes that a flux can be locally expanded to first order in chemical potential gradients around an averaged solid solution defined by the local con­centration. Starting from atomic jump frequencies and applying a coarse-grained procedure, a local expansion of the flux has been proved to be correct in the particular case of a direct exchange diffusion mechanism.62

An interdiffusion experiment is not sufficient to characterize all the diffusion properties. For example, in a binary alloy with vacancies, in addition to the two intrinsic diffusion coefficients, another diffusion coefficient is necessary to determine the three inde­pendent coefficients LAA, LAB, and LBB.