Kirkendall effect
The motion of the interface between two metals that occurs due to the difference in diffusion rates of the metal atoms.
Kirkendall effect: atoms move faster than the interface
The Kirkendall effect is the movement of the boundary between two metals during solid-state diffusion when the diffusion rates of the atoms differ. When two metals are brought into contact and heated, atoms from each metal migrate into the other. If one metal's atoms move faster than the other's, the physical interface shifts in the direction of the slower-diffusing metal, even though material is being exchanged across it. This counterintuitive motion reveals that diffusion in metals proceeds by individual atomic jumps, not by a simple atomic swap at the boundary.
The effect manifests as a net movement of the interface and often creates voids or porosity on the side of the faster-diffusing metal. In a copper-zinc diffusion couple heated to around 1000°C, zinc atoms diffuse into copper much faster than copper atoms diffuse into zinc. The interface shifts toward the copper, and zinc-copper voids (Kirkendall voids) accumulate on the copper side. This phenomenon was first systematically described by Ernest Kirkendall in brass diffusion studies in the 1940s, and it became one of the key proofs that diffusion in metals occurs through vacancies rather than by atoms physically jumping past each other.
Industrial significance and failure modes
The Kirkendall effect directly impacts bonding processes like diffusion welding and brazing, where solid-state joining relies on atomic migration across an interface. Excessive void formation can weaken joints, reduce electrical or thermal conductivity, and create brittle intermetallic layers. In electronics, it affects solder joint reliability and wire bonding in semiconductor packages. Designers account for it by controlling temperature, time, and the composition of diffusion couples, sometimes using intermediate layers or controlled atmospheres to limit the effect.
The rate and extent of Kirkendall shift depend on the activation energy for diffusion in each metal, the temperature, and the duration of exposure. Alloy systems with large disparities in diffusion rates (such as gold-aluminum in microelectronics) show pronounced effects. Conversely, systems like silver-copper, where diffusion rates are more balanced, experience minimal interface motion. Measuring Kirkendall shift using inert marker particles (originally tungsten wires) remains a standard method for studying diffusion kinetics in metallurgical research.