Driving Force
The boundary between one grain and its neighbour is a defect in the crystal structure and so it is associated with a certain amount of energy. As a result there is a thermodynamic driving force for the total area of boundary to be reduced. If the grain size increases, accompanied by a reduction in the actual number of grains, then the total area of boundary will be reduced.
In comparison to phase transformations the energy available to drive grain growth is very low and so it tends to occur at much slower rates and is easily slowed by particles or solute atoms.
Read more about this topic: Grain Growth
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