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The grain boundary diffusion coefficient is the diffusion coefficient of a diffusant along a grain boundary in a polycrystalline solid. It is a physical constant denoted D b {\displaystyle D_{b}} , and it is important in understanding how grain boundaries affect atomic diffusivity. Grain boundary diffusion is a commonly observed route for solute migration in polycrystalline materials. It dominates the effective diffusion rate at lower temperatures in metals and metal alloys. Take the apparent self-diffusion coefficient for single-crystal and polycrystal silver, for example. At high temperatures, the coefficient D b {\displaystyle D_{b}} is the same in both types of samples. However, at temperatures below 700 °C, the values of D b {\displaystyle D_{b}} with polycrystal silver consistently lie above the values of D b {\displaystyle D_{b}} with a single crystal.