Diffusion can be described as the random movement of particles through space, usually due to a concentration gradient. Diffusion is a spontaneous process and is a result of the random thermal motions between two particles.
Most of the processes in metals and alloys proceed by means diffusion and self-diffusion.
Diffusion in alloy is a process of transfer of atoms of different alloy components, resulting in changing chemical composition of some of the alloy regions.
Self-diffusion is a process of transfer of atoms of a certain element among themselves.
Diffusion mechanism is determined by the energy barrier that an atom must overcome in order to change its location. Some diffusion (self-diffusion) mechanisms are presented in the picture.
Metallic atoms diffuse mostly by the vacancy mechanism and elements with small atom sizes (H, N, C) diffuse by the interstitial mechanism.
Besides diffusion within the crystal, grain boundary (surface) diffusion may occur. As the grain boundaries regions are saturated with crystal lattice imperfections, energy barrier (activation energy) here is relatively low, therefore the diffusion rate along these regions is much higher, than the volume diffusion rate.
The classical laws, describing the diffusion process, are Fick’s law
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