Vacancy mechanism, of diffusion

In binary substitutional alloys, it is possible for one atomic species to diffuse faster than the other. This unequal diffusion (the Kirkendall effect [83-85]) is consistent with the vacancy mechanism of diffusion. Each component in a binary alloy (e.g., A and B) has its own diffusion coefficient, and the Kirkendall effect is due to the fact that D for A is different from D for B. An overall chemical diffusion coefficient D (interdiffusion coefficient) can be defined as... 

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