5 - Diffusion in Metals and Alloys

Abstract

Transport in materials via random individual atomic migration steps (jumps) is called ‘diffusion’. General understanding and detailed theory of diffusional transport represent a classical field of material science. While in liquids and gases, diffusion rate ranges up to millimeters or even centimeters per second, transport in solid materials is rather slow. In densely packed metals near to the melting point, one can expect about a micrometer per second and this rate drops down to about a nanometer per second at half the melting temperature. At room temperature atomic migration is practically frozen, except for few exceptional cases of small interstitial impurities. Measurement of diffusion in solids needs therefore sensitive techniques and well-developed microscopy. Not surprising that a scientific proof of diffusion in solid metals came rather late in history of science (Roberts-Austen, 1896).