Materiediffusion

Materiediffusion is the process by which atoms or molecules move within a material or across interfaces due to gradients in concentration, chemical potential, temperature, or mechanical conditions. In solids, it enables mass transport over time and underpins many processing and performance phenomena, such as alloying, phase transformations, heat treatment, and diffusion bonding.

In solids, diffusion occurs mainly through two mechanisms. Substitutional diffusion involves atoms jumping into adjacent vacancies in the crystal lattice, which requires a vacancy concentration and is typically relatively slow. Interstitial diffusion occurs when small atoms move through interstitial sites between host atoms and is often much faster. Diffusion can also occur along defects, such as grain boundaries and dislocations, which provide shorter pathways and can dominate transport in polycrystalline materials.

The rate of diffusion is described by Fick’s laws. The first law states that the diffusive flux J is proportional to the negative gradient of concentration, J = -D ∇C. The second law governs how concentration changes in time, ∂C/∂t = D ∇^2C. The diffusion coefficient D depends strongly on temperature and material, typically following an Arrhenius form, D = D0 exp(-Q/RT), where Q is the activation energy. Diffusion is also influenced by crystal structure, defect density, and microstructure, and can be driven by other factors such as stress or chemical potential differences.

Measurement and related concepts include tracer diffusion using isotopes, radiotracers, and profiling techniques like SIMS. In practice, diffusion is essential for processes such as homogenizing alloys, carburizing and nitriding, diffusion bonding, sintering, and semiconductor doping, making it a central concept in materials science and engineering.