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Plastic
deformation causes a change in shape of a material and work must be done
on it by external forces to effect this change in shape. If the deformation
is carried out at 'low' temperatures for which work hardening processes
dominate and recovery processes are slow, the deformation process is known
as cold work. The final state of the material is thermodynamically unstable
as the free energy of the material has been increased by grain shape distortion,
dislocation generation, and the formation of point defects through dislocation
interactions during the deformation. The material will have increased hardness
and reduced ductility as a result of the cold work process.
Thermodynamics
may be used to modify the properties of a cold worked material. By taking
it to temperatures at which recovery processes such as dislocation climb
and re-crystallization can occur, the total energy stored in the material
may be reduced, and its ductility enhanced. It is frequently necessary
to use annealing processes of this type when using cold work to make large
changes in the dimensions of a sample. For example, a silver-smith raising
a bowl by hammering the metal will need to stop and anneal it from time
to time in order to recover enough ductility to prevent cracking during
fabrication. |
