Grain boundaries are lines on the surface of crystalline solids formed when a material cools from a liquid to a solid. Metallurgists can modify the behavior of metals by changing the size and shape of grains and their boundaries through heating, cooling, and compressing. Grain boundaries are vulnerable to corrosive attack and forced growth of cracks, which can cause metal parts to fail. Small-grained metals tend to be stronger but more brittle, while large-grained metals are ductile and slow to break. Cracks in ductile metal parts can be monitored to predict their remaining lifespan.
When the exterior of a solid material is polished and then etched with acid, the lines can be seen on its surface through a light microscope. These lines are the grain boundaries, or the lines marking the outer edge of the grains, crystal-like shapes that form when a material cools from a liquid to a solid. Solids that don’t form grains are called amorphous, because the atoms that make them up don’t organize themselves into patterns as they do in crystalline solids.
The grains in crystalline materials form similar to the way snowflake crystals do when water freezes. Before a liquid freezes, there are locations within it that are colder than the rest of the fluid. The grain grows outward from these sites until it reaches another grain and stops. When all of the liquid between the grains growing towards each other has frozen into a solid, a grain boundary forms when growth stops.
Good examples of crystalline solids are metals and metal alloys. Metallurgists, who are concerned with engineering the properties of metals, find that the grain boundary is important in modifying the behavior of metals for various applications. The size and shape of the grains and their boundaries can be changed by heating and cooling the metal at different rates, or by cold working the grains, thinning them by compressing them under impact at room temperature.
To change a metal’s properties, it is exposed to sufficient heat so that the grain boundaries dissolve and reform, a process called annealing, where the slower the rate of cooling, the larger the grain size is formed. When a metal part is stressed, defects and holes in the atomic layers of the metal, called dislocations, move from within the grain towards its grain boundary. If the metal is cooled rapidly, the grains have less time to grow, become smaller, and dislocations meet resistance boundaries, adding strength to the metal, such as small grain iron alloys. If the metal cools slowly, the grains are larger, because the dislocations have more time to move to the boundary without causing a larger hole or crack to start. Large grains are seen in metals, such as copper and aluminum, which are ductile, stretch easily, and are slow to break.
The grain boundary is the area on the surface of a grain that is most vulnerable to both corrosive attack from chemical pollutants and the forced growth of cracks which, over time, can cause a metal part to fail or break. Metals with small grains tend to be stronger than larger grained metals, but have a greater opportunity to crack at their boundaries, tending to make them brittle and causing them to crack without warning. Cracks in ductile metal parts, such as aluminum alloys used in castings, with few grain boundary dislocations grow slowly. They can be safely monitored over time to predict how much life is left in a metal part or how long the part has before it can no longer function properly.
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