polygonization

Polygonization is the subdivision of existing grains into smaller regions, called subgrains, separated by low-angle grain boundaries. It occurs during or after cold working and subsequent low-temperature annealing, typically between 200 and 400 degrees Celsius depending on the metal. The process is driven by the relief of internal stress accumulated during deformation, and it represents an intermediate stage between the distorted structure left by cold work and the fully recrystallized structure that forms at higher temperatures.

When metal is cold-worked, dislocations accumulate unevenly throughout the grains, creating regions of higher and lower strain energy. Polygonization allows these dislocations to rearrange and climb, collecting into organized arrays at the boundaries between subgrains. This reduces the overall energy of the system without requiring the complete dissolution and reform of grain boundaries that characterizes true recrystallization. The subgrains remain separated by boundaries that are only slightly misoriented, typically 1 to 15 degrees apart.

Detection and practical significance

Polygonization can be observed under optical microscopy in etched samples, though electron microscopy provides clearer definition of the subgrain structure. It is particularly evident in metals with high stacking fault energy, such as iron and nickel alloys, and less prominent in copper and its alloys where recrystallization dominates. The phenomenon is important in controlling mechanical properties: polygonized structures retain much of the hardness from cold work while gaining some ductility back, offering a middle ground between fully work-hardened and fully annealed material.

The term itself reflects the appearance of the subgrain patterns, which often show faceted or polygonal boundaries when viewed in cross-section. This is distinct from recrystallization, where grain boundaries migrate continuously and new grains with random orientations nucleate and grow. Polygonization is sometimes called recovery, though recovery is the broader category encompassing various processes that reduce stored energy in deformed metals without changing the overall grain structure.

In practice, polygonization is often an unwanted intermediate stage. If the goal is to restore ductility and remove all traces of work-hardening, annealing must proceed past the polygonization temperature to complete recrystallization. However, for applications requiring a controlled balance of strength and toughness, partial polygonization achieved by precise temperature control can be useful. The process is temperature and time dependent, proceeding faster in materials with high atomic mobility and in those already containing mobile dislocation sources.