Considerable structural changes caused by displacement of atoms in a lattice appear at different technological operations (blacksmithing, extrusion, rolling) in metals and alloys. The displacement of atoms depends on magnitude of effective stresses. At small deforming stresses displacements of crystal lattice atoms are small and also can be converted, i.e. after removal of load the atoms return to initial places. The strain in such cases is named elastic. As the elastic deformation is not accompanied by residual changes in structure, the properties of strained metals do not vary. It is conditionally accepted, that the residual strain at a rate of 0.2 % from primal length of a sample may be considered as the boundary between elastic and plastic strains.
At increase of stress higher than a limit of elasticity the considerable displacement of atoms concerning final equilibrium positions are watched. After removal of load the atoms do not return in the former positions. Thus the properties of materials vary. The residual strain is named plastic.
It is conditionally accepted, that as the boundary between elastic and plastic strains the residual strain at a rate of 0,2 % from primal length of a sample might be considered. A stress appropriate to this strain is accepted to name by yield point of material.
Atomic-crystalline structure of materials
The real structure of industrial alloys represents the set of a great number of separate irregular-shaped crystals - grains or crystals with different orientation. The grains consist of separate fragments, disorientated relative to each other on some degrees, and fragments - from units with disorientation angles in some minutes. Inside units the structure is close to theoretical.
The physics of a plastic deformation
The repositioning of atoms at a plastic deformation happens by a slip of one part of metallic chip relative to another. The slip is carried out under an operation of shearing stresses. The shear stresses are oriented on slip planes. The slip planes are sown densely by atoms and have small adhesion forces. If the planes are packaged densely by atoms and are located in relation to operating load so, that the shearing stresses are small, the slip on this system of planes will be absent. At a plastic deformation there are two possible versions of atoms migrations: twinning and slip.
The strain by twinning usually happens at low temperatures and at high speeds of deformation, when the slip is difficult. The turn of particular parts of a crystal in a plane of a strain in this case is watched. Thus the lattice becomes a mirroring of adjacent, undeformed lattice areas.
There are twinning bands (twins) appearing in a crystal. Within the bounds of one grain there can be some twins.
The strain of metals by slipping is watched most frequently. The displacement of atoms on slip planes during a plastic deformation appears as slip lines, which can be watched under a microscope. The more degree of a strain, the more richly slip lines place. At large strains they almost drain. At small increase one slip line is visible. Actually it is the whole group of lines located from each other on small distances. If the slip appears on several systems simultaneously, the grids from slip lines are determined on macrosection.
At a plastic deformation metals hardening is watched. Process of slip and nature of metals hardening are explained fully on the basis of the dislocation instrument of a plastic deformation.
The dislocations are the linear imperfections of a crystalline metals structure. They are usually located along edges of an incomplete plane (edge dislocation) or along a line of shift of one part of a crystal relative to another (screw dislocation).
In the dislocation theory the following assumptions are adopted:
- The slip is spreaded on a plane sequentially, not simultaneous;
- The slip starts from places of violation of a lattice, which should be or arise in metal (crystal) at its loading.
Let's consider the physics of a plastic deformation on an example of migration of a linear or edge dislocation in crystal (fig. 3.2). The considered phenomena are similar at migration of screw and other dislocations.
Let there is a linear dislocation A (perpendicular to sectional plane of crystal) on the edge of incomplete plane. Under the action of shift stress called by force Р, the atoms of an incomplete plane I-I are shifted to the right (are shown by black points) in a position I´ -I´.
The top of the whole plane II-II will remove to the right too (а-b). In the particular moment there a break of the whole plane II-II appears on a horizontal plane N-N. Thus the plane I-I is integrated with bottom of a plane II-II. Thus the whole plane (shown by a shaped line) will be formed. The top of a former whole plane II-II becomes incomplete (it is shown by a shaped line in a fig. 3.2, а). The dislocation from a position A was transferred on one interatomic distance to the right to positions В.
Under the action of shear stresses the dislocation will displace sequentially in a slip plane, until the surface of a crystal will be reached (fig. 3.2, б). In result there will be a shift in a crystal on a slip plane on one interatomic distance, though all atoms in this plane did not displace simultaneously. It also explains the fact, that the real resistance to shift is much less then theoretical
The experiments display, that the dislocation density during a plastic deformation is increased from 107 см/см3 (in annealed metal) up to 1012 … 1013 см/см3 in extreme strained state. Thus, during a plastic deformation new dislocations appear.
For security of final shifts it is necessary to transfer up to the boundary of a crystal or grain a great number of dislocations. Therefore not only dislocations, available in metal, participate in such process but also newly generated.
The process of a plastic deformation is accompanied by hardening of metal that is stipulated by increasing of resistance of crystalline structure to migration of dislocations. Except of hardening, the plastic deformation results in lowering of toughness, corrosion resistance, electro- and thermal conduction.
The set of all changes in structure and properties of distorted metal is named as a work hardening. Practically the strengthening is the most important result of a strain.
The resistance of metal to dislocations migration increases under an operation of the following factors:
а) Turn of slip planes relative to operating force. Thus the slip planes tend to settle in parallel to principal direction of a strain. That is the angle between operating force and slip plane is gradually decreased, and the stress in a slip plane reaches the critical value at large loadings. Thus, some portion of hardening deposited by a strain, is explained by geometry of the strain itself;
б) Increase of a dislocation density. It results in considerable distоrtions of a spatial crystal lattice. They arise at interaction of dislocations with each other (their intersection) with other imperfections of a crystal lattice;
в) Brake of dislocations action on obstacles: the boundaries of units, grains, actuations of other phases. As in different grains of substantial metals the alignment of crystallographic planes is various, the boundaries of grains are an insuperable barrier to dislocations. The dislocations accumulate on the boundary of a grain and create large internal stresses. These stresses, in turn, actuate dislocations in adjacent grains. Thus, through the boundary of grains the migration of dislocations is transmitted by a relay way;
г) Effect of admixtures and atoms of alloying elements. The hardening of metals and alloys at a plastic deformation results in exhaustion of the metal toughness and restricts capabilities of deriving of composite details for one machining step. On the other hand, with the help of a work hardening it is possible to increase structural strength practically of all present metals and alloys.
The plastic deformation causes turn of slip planes. As a result of it the metal acquires particular alignment of crystalline structure, so-called texture. The grain-oriented metal has the brightly expressed anisotropy of properties.
Besides distоrtions of atomic-crystalline structure in plastically deformed metal the microstructure varies also. The strain calls subdivision of grains, which are stretched along a force direction. The metal acquires a filamentary structure, the internal residual stresses appear in it.
Distorted metal is in extremely nonequilibrium and thermodynamically unstable state. This condition at a room temperature for the majority of metals and alloys can be saved very long.
Effect of heating on properties of distorted metals
At heating the plastically deformed metals gradually restore the structure and properties and pass in a stable state. This transition can be dissected on two stages: return and recrystallization.
The return is watched at rather low-level heating temperatures (about 20 % from melting point) and is divided in turn on two phases: recovery and polygonization.
The recovery is accompanied by diffusive migration and mutual cancellation of dot imperfections. Thus the elastic distоrtions of a lattice are eliminated and physical properties of metal are restored partially.
Polygonization happens at more heating. It is accompanied by changes in a block structure of grains owing to reallocation and elimination of linear imperfections - dislocations. It results in further removal of elastic distоrtions of lattice and fuller restoring of physical properties. The strength properties at return do not vary, and the toughness is substantially restored.
The recrystallization flows after return at more heat called temperature of recrystallization. It is possible to define approximately this temperature for commercially pure metals under
the formula of A. Bochvar:
T recrystalization=0,4T melt
Cold and hot plastic deformation
All explained above relates to a cold plastic deformation, which is accompanied by a work hardening bounding its marginal capabilities. For restoring a toughness of distorted metal it is necessary to conduct recrystallization annealing.
Hot plastic deformation (or non-cutting shaping) is carried out at temperatures above than temperature of recrystallization. In this case the process of recrystallization follows a strain and fractionally or completely removes effect of hardening. Therefore in a hot condition the metal is not hardened and its forming is possible with considerable degrees of a plastic deformation.
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