Deformation hardening (Topic: 10038)

The yield strength of all known pure metals and alloys increases with the degree of plastic deformation ("deformation hardening" or "work hardening"). This is a way to increase the strength of the non heat-treatable aluminium alloys.

The condition for the alloy to work harden is that the plastic deformation is introduced to the metal at a temperature that is lower than the alloy's recrystallisation temperature.

The mechanism behind the deformation hardening is somewhat complicated, but as a simplification one can say that dislocations "pile up" during cold deformation and will subsequently develop a cellular substructure with the dislocation "pile ups" as cell walls. This substructure will function as a barrier for the movement of new dislocations that are introduced and these will get trapped in the cell walls. The cell walls are areas with very high density of dislocations (increasing with increasing amount of cold work) and the dislocations are more or less immobile. The result is an increase in strength and a drop in ductility.

Materials that has been deformation hardened can be brought back to the initial state by recrystallising annealing or it can be back-annealed to a more wanted combination of ductility and strength.

Deformation hardening on extruded products is mainly actual after the extrusion process, e.g. in cold forming of an extruded profile (bending), or making forged products out of extruded forging stock.