Plastic deformation

Deformation is a process in which, under the influence of the loads applied to the body, its size and shape change. The process of change can be of two kinds. The first refers to the reversible (elastic), to the second - residual (plastic) deformation.

The first type is a process in which, after removing the load, the body comes to its original form. This changes the distance between atoms within the parameters of the crystal lattice.

Plastic deformation is a process in which the body, after removing the load, does not restore its original form. Such a change is accompanied by a considerable shift of the crystal parts relative to each other by a distance exceeding the distance in the crystal lattice between the atoms.

Plastic deformation always follows the elastic deformation. As a result, the total change at the moment of the load action includes two processes - reversible and residual.

The deformation of metals is of great practical importance. This is due, first of all, to the fact that the processing of the material by pressure is based on the processes of changing the shape and size of the blanks. Along with this, the emerging internal stress affects the physicochemical and mechanical properties of the material.

The plastic deformation of metals (its nature and magnitude) depends on the plasticity of the materials. This property can be evaluated during the process of relative constriction or elongation of the specimens during the tensile test. The ductility characteristics of metals also include an index of impact strength. This property shows the work of fracture during the bending of the incised specimen in relation to the sectional area of the incision site.

The ductility of the material rises with an increase in the difference between the yield point and strength. Plastic deformation in brittle materials practically does not arise. Due to the fact that they have an exponent of yield strength approaching the level of the ultimate strength, they are destroyed quite quickly. This happens, for example, with glass, porcelain, cast iron, and rocks. Meanwhile, heating the metal to a high temperature leads to the fact that the index of ultimate strength practically coincides with the value of the yield strength.

The residual change in the material of the polycrystalline structure has some peculiarities in comparison with the same process in the body of a single crystal. Cold plastic deformation is composed of changes in the shape and size of individual grains and changes in border volumes. While individual grains are deformed by twinning and sliding, their interconnection and multiplicity in the polycrystalline body introduce certain features into the process.

In connection with the fact that the slip planes are oriented in space arbitrarily, the process itself will occur in different ways. The first to begin to deform those grains whose slip planes are subjected to maximum tangential stresses. Thus nearby elements will be gradually involved in process. The deformation changes their shape - they become elongated towards the flow of the metal of the greatest intensity (along, relative to the direction of deformation).

Thus, the material acquires a fibrous structure. Extruded non-metallic inclusions cause a difference in the properties of the fibers across and along. The change in the shape of the grains is accompanied by a change in the orientation of the crystal lattices in space. After most elements have the same orientation, a deformation texture will be formed.