無機(jī)非金屬材料工程專業(yè)英語-第7章-資料下載頁
2025-08-05 08:01本頁面
【正文】 rystalline metal specimen at temperatures that are low relative to its absolute melting temperature produces microstructure and property changes that include (1) a change in grain shape(2) strain hardening (3) an increase in dislocation density. Fundamentals of Materials Science and Engineering l Some fraction of the energy expanded in deformations is stored in the metal as strain energy which is associated with tensile, pressive and shear zones around the newly created dislocations. These properties and structures may revert (return) back to the precoldworked states by appropriate heat treatment. l Such restoration results from two different processes that occur at elevated temperatures: recovery and recrystallization, which may be followed by grain growth.l .Fundamentals of Materials Science and Engineering Recoveryl During recovery, some of the stored internal strain energy is relieved by virtue of dislocation motion (in the absence of an externally applied stress), as a result of enhanced atomic diffusion at the elevated temperature. l There is some reduction in the number of dislocations, and dislocation configurations (similar to that shown in Figure ) are produced having low strain energies. l In addition, physical properties such as electrical and thermal conductivities and the like are recovered to their precoldworked states.Fundamentals of Materials Science and Engineering Recrystallizationl Recrystallization is the formation of a new set of strainfree and equiaxed grains that have low dislocation densities and are characteristic of the precoldworked condition. (Fig. )l After recrystallization, the metal bees softer, weaker, yet more ductile.l Recrystallization temperature the temperature at which recrystallization just reaches pletion in 1h. Typically, it is between one third and one half of the absolute melting temperature of a metal or alloy and depends on several factors, including the amount of prior cold work and purity of the alloy. Fundamentals of Materials Science and Engineering The influence of annealing temperature on the tensile strength and ductility of a brass alloy. Grain size as a function of annealing temperature is indicated. Grain structures during recovery, recrystallization, and grain growth stages are shown schematically.Fundamentals of Materials Science and Engineering Fundamentals of Materials Science and Engineering Important conclusions:l On a microscopic level, plastic deformation of metals corresponds to the motion of dislocations in response to an externally applied a hear stress, a process termed slip. Slip occurs on specific crystallographic planes and within these planes only in certain directions. l A slip system represents a slip plane slip direction bination, and operable lips systems depend on the crystal structure of the material.Fundamentals of Materials Science and Engineering l Since the ease with which a metal is capable of plastic deformation is a function of dislocation mobility, restriction of dislocation motion increases hardness and strength. l On the basis of this principle, three different strengthening mechanisms were discussed. They are grain boundaries strengthening, Solid solution strengthening and strain hardening.Fundamentals of Materials Science and Engineering l Grain boundaries serve as barriers to dislocation motion。 thus refine the grain size of a polycrystalline metal renders it harder and stronger. l Solid solution strengthening results from lattice strain interactions between impurity atoms and dislocations. l And, finally, as a metal is plastically deformed, the dislocation density increase, as does also the extent of repulsive dislocationdislocation strain field interactions。 strain hardening is just the enhancement of strength with increased plastic deformation .
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