Work hardening: The process of plastically deforming a sample by rolling or drawing the material at low temperatures (less than half the melting temperature, Tm), which increases the number of dislocations and the amount that they are entangled, resulting in reduced ductility and increased hardness and strength.
Annealing is a heat treatment process that brings about a softer or more relaxed state in worked materials. There are three main stages - recovery, recrystallisation and grain growth, which are temperature dependent processes.
Stage 1 - Recovery - some restoration of original properties (eg hardness, ductility, resistivity) is achieved by the rearrangement of dislocations at temperatures around 0.3 Tm, to lower the overall strain energy. The dislocation density is lowered slightly, and the strength, grain shape and grain size are largely unchanged.
Stage 2 - Recrystallisation - at a temperature above 0.4 Tm, new crystals begin to grow at certain points in the deformed metal and eventually absorb the deformed crystals. The new crystals are more equiaxed and contain far fewer dislocations than the deformed ones, with the dislocation density reduced approximately from 1015 m-2 to 1010 m-2.
Stage 3 - Grain growth - holding the metal at recrystallisation temperature for an extended period of time, or at a higher temperature, allows the average grain size of the metal crystals to increase. This occurs because the grain boundaries have a higher energy than a perfect lattice, so there is a driving force to reduce the area of grain boundaries by increasing the grain size.
Samples that have been annealed have very different properties to those that are work hardened.
The resistance to the movement of dislocations can be separated into a contribution from the lattice and a contribution from other obstacles, such as impurities and grain boundaries. Any increase in dislocation density will have a large effect on how easily the material can be deformed. More ... In addition an increase in grain size can lower the yield stress. More...
In the annealed sample the contribution from the lattice dominates, as the density of dislocations is low. Slip occurs at a relatively low stress, yielding almost immediately, and slip will continue until there is a large extension. It is possible to see shear bands forming on the surface - the appearance becomes more matt as the shear bands roughen the surface on a very small scale. As the sample is extended, the dislocation density increases, and extensive work hardening takes place.
In contrast, the work hardened copper already has a high dislocation density, meaning that it takes a much larger stress for slip to begin. This means that the yield point will be much higher, and so the elastic region is much longer.
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