Metals, Return of
Metals, Return of
the process of partial restoration of the structural integrity and properties of deformed metals by heating them to below the recrystallization temperature. It is accomplished by redistributing and reducing the concentration of pinholes and dislocations that are not associated with the formation and movement of grain boundaries (crystallites). Return includes elementary processes with varying activation energy Q, which take place in various temperature ranges. This is connected with the great diversity of types and character of distribution of defects of crystalline structure, introduced by work hardening during deformation.
There are two stages in the return of metals. The first stage, relaxation, is the reduction of the concentration of pinholes and their annihilation and flow to boundaries and dislocations, as well as the redistribution of dislocations by means of slip in their planes, without the formation of new boundaries. The process takes place on heating to a temperature equal to 0.05-0.2 of the melting point (Tm); in this case Q of relaxation is 0.1-0.7 electron volts (eV). The second stage is polygonization—that is, the redistribution of dislocations by slip and diffusion, which is accompanied by their partial annihilation and the formation within the crystallites of regions (polygons) free from dislocations and separated from one another by narrow-angle dislocation boundaries. The process takes place on heating to (0.3-0.4) Tm; in this case polygonization Q is 160.210 × 10-21 to 240.315 × 10-21 joules (1.0-1.5 eV). Upon heating after large deformations, polygonization is, as a rule, an initial stage in recrystallization.
The structural changes in return of metals are observed at the polygonization stage by electron-microscope analysis of thin foils using transmission electron microscopy and, in individual cases, in an optical microscope after etching. Important information about return is given by decrease in width of lines on X-ray pictures and by forms of X-ray reflections. In cases of return there is a decrease in hardness, strength, electrical resistance, coercive force, and solubility in acids, and an increase in plasticity. The extent of restoration of properties during return varies: the electrical resistance is appreciably restored even at the relaxation stage, and the mechanical properties and coercive force are restored at polygonization. In metals and alloys with high packing defects energy, the degree of restoration of properties on return is greater than with materials where the energy of these defects is low. The higher the temperature to which the metal is heated and the longer the heating time, the greater the restoration of properties. At each given temperature the rate of the return process falls off according to a law expressed by an exponential function.
Return is used to increase the plasticity of work-hardened materials and the thermal stability of their structure and properties.
REFERENCESVozvrat i rekristallizatsiia metallov: [Sb. statei]. Moscow, 1966. (Translated from English.)
Gorelik, S. S. Rekristallizatsiia metallov i splavov. Moscow, 1967.