The diffusion coefficient of aluminium atoms in migrating grain boundaries in Ni-Al alloys is observed to be three orders of magnitude higher than the corresponding values in stationary boundaries of the same material. This result may be understood if it is assumed that the diffusion in migrating boundaries occurs by non-localized diffusion involving long-range jumps of the diffusing atom. Grain boundary diffusion in stationary boundaries seems to occur by the movement of the diffusing atom from one site of low potential energy to a neighbouring site (localized diffusion).
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