polygonize

Polygonization is the subdivision of existing grains into smaller polygonal cells during mechanical working or subsequent annealing of metals. When a metal is deformed, dislocations accumulate unevenly within each grain, creating regions of high and low dislocation density. During heating, these dislocations rearrange themselves into ordered, low-angle grain boundaries that partition the original grain into many small subgrains, typically 1 to 10 micrometers across. Each subgrain is bounded by a network of edge dislocations that create gentle misorientation between adjacent cells.

Polygonization occurs most readily in metals with moderate to high stacking fault energy, particularly pure metals like aluminum, copper, and iron. It happens during recovery, the early stage of annealing that precedes full recrystallization. The process is driven by reduction of stored elastic energy: the rearrangement of dislocations into organized walls lowers the overall dislocation density and the strain they introduce. Temperature and time determine the extent; higher temperatures accelerate the process, and longer holding times allow more complete subdivision.

Recognition and Consequences

Polygonized grains appear as distinct cells under optical microscopy, often revealed by electron backscatter diffraction (EBSD) which can measure the small misorientation angles between subgrains. The phenomenon creates a visible checkerboard or cellular structure within the original grain boundaries. Polygonization hardens the metal slightly through the subgrain boundary structure, but leaves more stored energy than fully recrystallized material, making the metal more prone to further grain growth if heated above the recrystallization temperature.

The term itself reflects the blocky, angular appearance of subgrains when viewed in cross-section. Polygonization must be distinguished from recrystallization, in which the original deformed grains are replaced entirely by new strain-free grains; in polygonization, the original grain structure remains recognizable, only subdivided. The process is particularly important in controlling the mechanical properties of heavily worked metals and in managing the microstructure of commercial alloys during intermediate annealing steps in multi-pass forming operations.