When the exterior of a solid material is polished and then etched with acid, lines can be seen on its surface through a light microscope. These lines are the grain boundaries, or the lines that mark the outside edge of grains, crystal-like shapes that form as a material cools from liquid to solid. Solids that do not form grains are called amorphic, because the atoms composing them do not organize into patterns as they do in crystalline solids.
The grains in crystalline materials form similar to the way snowflake crystals do as water freezes. Before a liquid freezes, there are locations inside that are cooler than the rest of the fluid. The grain grows from these sites outward until it reaches another grain and stops. When all of the liquid between grains growing toward one another has frozen into a solid, a grain boundary forms as growth stops.
Good examples of crystalline solids are metals and metal alloys. Metallurgists, who deal with designing properties into metals, find that the grain boundary is important in changing the functioning of metals for various applications. The size and shape of grains and their boundaries can be changed through heating and cooling the metal at different rates, or by cold working the grains, thinning them by compressing them under impact at room temperature.
In order to change a metal’s properties, it is exposed to enough heat so that the grain boundaries dissolve and reform, a process called annealing, where the slower the cooling rate, the larger the grain size formed. When a metal part is stressed, the defects and holes in the atomic layers of the metal, called dislocations, move from within the grain toward its grain boundary. If the metal is cooled quickly, the grains have less time to grow, they become smaller, and dislocations meet up with resisting boundaries, adding strength to the metal — small-grained iron alloys, for example. If the metal cools slowly, the grains are larger, because dislocations have more time to move toward the boundary without causing the start of a larger hole or crack. Large grains are seen in metals, such as copper and aluminum, that are ductile, extend easily and are slow to crack.
The grain boundary is the area on the surface of a grain that is more vulnerable to both corrosive attack by chemical pollutants and forced crack growth that, in time, can result in the failure, or breakage, of a metal part. Metals with small grains tend to be stronger than larger-grained metals but have an increased opportunity for cracking at their boundaries, tending to make them brittle and causing them to break without warning. Cracks in ductile metal parts, such as aluminum alloys used in jets, with few dislocations at their grain boundaries, grow slowly. They can be tracked safely over time to predict how much life remains in a metal part, or how much time the part has before it can no longer function properly.