grain growth

Grain boundaries cost energy — they are disordered seams, and a material would rather have fewer of them. So if you hold a fine-grained metal hot enough for atoms to shuffle, the grains slowly coarsen: big grains eat small ones, the total boundary area shrinks, and the average grain size grows. This is grain growth, the microstructure's way of lowering its energy by cutting down on boundaries, much as soap bubbles in a foam merge so large bubbles swell while small ones vanish.

The mechanism is grain-boundary migration driven by curvature. A curved boundary tends to move toward its center of curvature (atoms hop across from the shrinking side to the growing side), so small grains, which are more sharply curved, shrink and disappear while large grains grow. For ideal normal grain growth the mean diameter follows a parabolic law, roughly D^2 - D0^2 = k t, with k rising steeply with temperature (an Arrhenius factor), so grains coarsen far faster when hotter. Sometimes a few grains grow abnormally large while the rest stay fine — abnormal grain growth, or secondary recrystallization.

Grain growth matters because coarse grains are usually weaker (Hall-Petch again) and can ruin toughness, so it is something to control: overheating during welding or heat treatment coarsens grains and degrades properties. It is deliberately fought by pinning boundaries with fine second-phase particles (Zener pinning) or solute drag, and deliberately used when large grains are wanted (the Goss texture in transformer steel is grown by controlled abnormal growth). Honest distinction: grain growth is not recrystallization. Recrystallization replaces a deformed structure with new strain-free grains and is driven by stored deformation energy; grain growth is the slower coarsening of already-strain-free grains, driven only by boundary energy.