A grain boundary represents a significant dislocation - perhaps for sound, or for electrons. But you can have different phases or orientations of crystallites (hence polycrystalline) in a single grain - if every variation of the crystallites forms a significant boundary, then each crystallite forms a grain.
A grain boundary represents a significant dislocation - perhaps for sound, or for electrons. But you can have different phases or orientations of crystallites (hence polycrystalline) in a single grain - if every variation of the crystallites forms a significant boundary, then each crystallite forms a grain.