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Comprehensive explanation required as to the atomistic level processes which lead to
1. strengthening of metals by cold working or strain hardening
2. regaining of ductility of strain hardened structures by recovery, recrystallisation and grain growth
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This solution answers two questions on atomistic level processes about strengthening metals through cold working and strain hardening and recrystalization.
Strengthening Mechanisms of Metals:
Cold-working in a metal involves the plastic deformation at a temperature low enough that the strain-hardened condition is retained by storing the strain energy expended in the plastic deformation in the form of dislocations and other imperfections. The strain hardening occurs at low temperatures where recovery is very slow or does not occur at all. It is a very convenient mechanism of metal strengthening. Strain hardening is the increase in strength cased by an increase in the dislocation density resulting from a given plastic strain. The new dislocations that are created from the cold-working must interact with the dislocations that already exist. As more dislocations are created, it becomes more difficult for them to move through the high density dislocation cluster that now exists within the metal. This causes the strain hardening.