We consider the growth and morphological stability of an intermediate phase growing in a binary diffusion couple under electromigration conditions. The growth rate of the intermediate phase depends primarily on the direction of the electromigration current. Current flow that drives the diffusing species enhances growth of the intermediate phase, while current flow in the opposite direction slows growth. The morphological stability of the interfaces between the intermediate phase and the terminal phases depends on the current direction, the relative conductivities of the phases, and the thickness of the intermediate phase. We find that, when the intermediate phase has a higher conductivity than the terminal phases, the current direction that enhances growth of the intermediate phase can also cause an instability. Alternatively, when the conductivity of the intermediate phase is lower than the surrounding phases, the current direction that slows growth can cause an instability. Instability also requires that the thickness of the intermediate phase be larger than some critical value.
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