A polycrystal, held at temperature for some time t, the average grain diameter grows. A grain grows at the expense of its neighbors: small grains disappear and big ones get bigger. Total number of atoms is conserved.

The cause for grain growth is readily understood. Atoms at a grain boundary are poorly packed, and have higher energy than atoms in the lattice. As the grains grow in size and their numbers decrease, the net of amount of grain boundary reduces, and thereby the free energy of the system reduces. But how does each atom know about this global agenda of reducing the energy of the system? If you cannot wait for an answer, jump to the last paragraph of this lecture.