When two metals are allowed to diffuse into one another, a diffusion aone is formed, the shape of which is determined by the relative diffusivities of the two elements, the time and temperature of the diffusion, and the number of phases existing in equilibrium at the diffusion temperature.

An X-ray absorption technique has been developed to analyze such concentration gradients. This involves the preparation of a thin section from a typical diffusion couple parallel to its axis. This is then scanned with a monochromatic X-ray beam and the transmitted X-rays are measured with a Gejger counter. The intensity of the transmitted beam is determined by the composition of the area in question and the absorption coefficients of the two elements.