Kirkendal Effect and Moving Interface Problems

For the interdiffusion of two species A and B with unequal diffusivities the speed at which the A/B interface moves v) can be calculated by 

Fick s first and second laws must be modified when describing a system that manifests the Kirkendal effect. Fick s first law becomes (written in terms of species A) 

Net movement of mass to right causes effective brass/copper interface to move left 

FIGURE 4.17 Kirkendal effect iUustTated for a brass/copper diffusion couple. Since the diffusion of zinc into copper is mnch faster than the diffusion of copper into brass, a net flow of mass occurs from the brass side to the copper side. As a result, the location of the interface between the brass and the copper moves toward the brass side. 

As A and B diffuse across the eutectic interface into the P and a phases, respectively, the interface will move (just as we discussed previously in the context of the Kirkendal effect). This will occur even if the difftisivities of A and B in a and P are equal, because the local concentration gradients on the two sides of the interface will likely be different. (Remember, the flux depends on the diffusivity and the concentration gradient.) 

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