**4.1 Grain boundary migration**

If the adjacent grains differs in defects density, the defect-poor one bulges into the defectrich one; see fig. 24. It results in the removal of grains with many dislocations. It also enables the spontaneous crystallization and the growth of new grains - "nuclei" (either defectless or with few dislocations) inside the defect-rich grain; these fine new grains are called 'subgrains' as well.
