Plastic deformation recrystallization

Plastic deformation, unlike elastic deformation, is not accurately predicted from atomic or molecular properties. Rather, plastic deformation is determined by the presence of crystal defects such as dislocations and grain boundaries. While it is not the purpose of this chapter to discuss this in detail, it is important to realize that dislocations and grain boundaries are influenced by things such as the rate of crystallization, particle size, the presence of impurities, and the type of recrystallization solvent used. Processes that influence these can be expected to influence the plastic deformation properties of materials, and hence the processing properties. 

Annealing in metals can first lead to stress relaxation in which stored internal strain energy due to plastic deformation is relieved by thermally activated dislocation motion (see Figure 5.18). Because there is enhanced atomic mobility at elevated temperatures, dislocation density can decrease during the recovery process. At still higher temperatures, a process known as recrystallization is possible, in which a new set of... 

Dislocations multiply in a facile manner during a plastic deformation process, and several mechanisms for this have been observed by electron miscroscopy. Dislocations are destroyed by the processes of recovery and recrystallization during annealing after plastic deformation. Since dislocations cause low-yield stresses in metals and other solids, solid strengthening is accomplished either by eliminating dislocations or by immobilizing them. 

Recrystallization occurs when a crystalline material is plastically deformed at a relatively low temperature and then heated [1]. The as-deformed material possesses excess bulk free energy resulting from a high density of dislocations and point-defect debris produced by the plastic... 

Post-mortem analysis has also been carried out. Microstructural observation at the cross-section showed that significant grain growth, recrystallization and plastic deformation due to thermal stress occurred. Surface analysis indicated that the deuterium concentration was less than 0.1%... 

In general, metals can be worked extensively, either at room temperature or at high temperatures. This is so mainly because of the availability of a large of number slip systems for plastic deformation. This allows us to use metal drawing techniques to obtain filamentary metals. Metallic fibers are, generally, not spun from a molten state, although this can be done in some cases (see Section 5.2). When metals are cold worked (i.e. below the recrystallization temperature), they... 

Mechanical working of a metal is plastic deformation performed to change dimensions, properties, and/or surface conditions. Plastic deformation below the rectys-tallization temperature is called cold working. Plastic deformation above the recrystallization temperature, but below the melting or burning point, is called hot working. 

The main structural influences can be described as follows. Any plastic deformation decreases the transition temperature by fining the structure. The higher the degree of deformation, the lower is the DBTT. An as- rolled tungsten sheet shows a DBTT of about 100-200 °C. Fine wires or foils are even ductile at room temperature. Any aimealing increases the transition temperature, and recrystallized samples show a maximum, mainly due to the coarsening of the structure (360 °C). 

Fig. 8.4 Structures of compacts from high-pressure agglomeration after processing equal feed materials (recrystallized KCI, Potash) at low (left, brittle disintegration) and elevated (right, plastic deformation) feed temperatures in identical roller presses with the same operating parameters. (For more information see text.).

Thermomechanically affected zone (TMAZ) In this region, the FSW tool has plastically deformed the material, and the heat from the process will also have exerted some influence on the material. In the case of aluminum, it is possible to obtain significant plastic strain without recrystallization in this region, and there is generally a distinct boundary between the recrystallized zone (weld nugget) and the deformed zones of the TMAZ. 

As a result of this process, a joint is produced in solid state. Because of various geometrical features of the tool, the material movement around the pin can be quite complex. During FSW process, the material undergoes intense plastic deformation at elevated temperature, resulting in generation of line and equiaxed recrystallized grains. The fine microstructure in friction-stir welds produces good mechanical properties of the joints, both static and dynamic. 

Cold working — plastic deformation below recrystallization temperature. 

In steels, moderate plastic deformation and low annealing temperature are beneficial in developing the recrystallization texture. Planar anisotropy increases with increasing deformation and higher annealing temperature in copper sheets, whereas the inverse is found in brass sheets [28]. 

With progressing recrystallization the microstructural rearrangements will reduce the non-compatibility of the initial plastic deformation and therefore eliminate the. source of residual stresses. Beyond the minimum of Young s modulus, where recrystallization really starts, the residual stresses may have disappeared and leave the field open for other... 

As a consequence of the above mentioned effects, but in contrast to many other metallic materials, the fracture toughness of Mo and W is strongly reduced with increasing degree of recrystallization. With increasing plastic deformation, the fracture toughness increases (see Sect. 3.1.9.4), combined with a transition from intercrystalline to transcrystalline cleavage and to a transcrystalline ductile fracture [1.147,158,159]. 

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