Kirkendall effect
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The Kirkendall effect is the migration of markers that occurs when markers are placed at the interface between an alloy and a metal, and the whole is heated to a temperature where diffusion is possible; the markers will move towards the alloy region. For example, using molybdenum as a marker between copper and brass (a copper-zinc alloy), molybdenum atoms will migrate towards the brass. This is explained by assuming that the zinc diffuses more rapidly than the copper, and thus diffuses out of the alloy down its concentration gradient. Such a process is impossible if the diffusion is by direct exchange of atoms.