THE IMPORTANCE OF RECRYSTALLIZATION

Many metallic materials are produced initially as large castings which are then further processed in the solid state by forging, rolling, extrusion etc. to an intermediate or final product.  These procedures, which may be carried out hot or cold, and which may involve intermediate anneals, are collectively termed thermomechanical processing.  Recovery, recrystallization and grain growth are core elements of this processing. 

The mechanical properties and behaviour of a metal depend on the dislocation content, the size of the grains and the orientation or texture of the grains.  Of these, the dislocation content and structure are the most important.  The mechanical properties depend primarily on the number of dislocations introduced during cold working and their distribution.  As this increases from ~10¹¹m^-2, typical of the annealed state, to ~10¹⁶m^-2, typical of heavily deformed metals, the yield strength is increased by up to 5-6 times and the ductility decreased.  If the strain hardened material is subsequently heated to around 1/3 of its melting temperature, dislocation loss and rearrangement (recovery) occur and this is manifested by a decrease in strength and increased ductility. There is an enormous literature on the magnitude of these changes the reader is referred to the appropriate volumes of Metals Handbook.

The grain size and texture after annealing are determined mainly by the recrystallization process, and there are numerous examples of the need to control grain size.  For example, a small grain size increases the strength of a steel and may also make it tougher.  However, a large grain size may be required in order to reduce creep rates in a nickel-based superalloy for use at high temperatures.  Superplastic forming, in which alloys are deformed to large strains at low stresses, is becoming an important technological process for the shaping of advanced materials.  Great ingenuity must be exercised in producing the required fine grain size and preventing its growth during high temperature deformation.  The control of texture is vital for the successful cold forming of metals, a particularly important example being the deep drawing of aluminium or steel to produce beverage cans.