1.4 Dispersion-strengthened composite materials
Dispersion-strengthened composite materials (DCM) is a material in the matrix which are uniformly distributed fine particles of the second substance. In such materials at loading all the load sees the matrix in which the many and not practically soluble therein particles of a 2-phase structure is created, efficiently prevent plastic deformation.
It is known that viscous, devoid of the fragility of the material before destruction is undergoing significant deformation. Moreover, plastic deformation (shift) in the real crystalline starting materials at voltages that are less than theoretically calculated, about 1000 times.
Deformation is the change in the size or shape of the body under the action of external forces or physico-chemical processes occurring in the body. Such a low strength compared to the theoretical is explained by two possible factors:
1) in plastic deformation of dislocation are actively involved;
2) affects the scale factor of the ratio of the length of interatomic bonds and the size of the sample.
During deformation by dislocation shear of atoms in the adjacent position occurs not simultaneously over the entire surface of slides and jumps in time. This gradual slide is due to small displacements of the atoms in the region of dislocations does not require significant voltage, that appears in testing plastic materials.
Dislocations play a fundamental role in the structure and properties (especially in strength) DCM. Therefore, recall that such a dislocation and plastic deformation of solids when sliding an edge dislocation.
Under the deployment understand the special kind of linear imperfections of a crystal lattice that violate the correct sequence of the atomic planes.
Dislocations are linear defects of a lattice, i.e. imperfections, in the crystal covering a region whose length in one direction is much greater than the size of atoms or ions. The nature of lattice distortions dislocations are divided in pure form on the boundary (linear) and spiral. In real crystals dislocations often are a combination of edge and screw dislocations. Such dislocations are called mixed. Dislocations are sources of internal stresses in crystalline solids, they create even free from external loads, the crystal field of strain and stress.
One of the basic concepts in the theory of dislocations is the concept of dislocation lines. The line of dislocation is called the imaginary line in a crystal, along which (in its small neighborhood) are concentrated maximum lattice distortion. In fact, when the occurrence of the dislocation the crystal lattice remains undistorted everywhere except for the region immediately surrounding the dislocation line. Line of deployment does not have to be straight, and may have bends, notches, i.e. any form. The peculiarity of this line is that it never ends (not ends) in crystal, and comes to the surface, closes on itself, forming a loop, or snaps to the dislocation line.
1.4.1 An edge dislocation
A symptom of an edge dislocation is the presence in one part of the crystal the extra ("dangling" or "unfinished") atomic plane (half-plane or extrapolate), which continue in another part of the crystal (figure 4, extraploate arrow).
Figure 4 – Model of an edge dislocation in a crystal
The formation of an edge dislocation can be represented as follows. For example (figure 5) that on the crystal of the force P that causes the shift of one part relative to another at a certain slip plane (mn).
Figure 5 – Scheme of the formation of an edge dislocation
Thus the shear force is not large enough to cause the simultaneous breaking of all the vertical atomic planes in the lattice (which in the limit would be tantamount to cut crystal), and causes rupture only some of the planes. If we consider only one of them, the upper part (FG) of the broken plane will be displaced in the direction of application of force and will remain unfinished, and the lower part will interlock with the adjacent planes lying in the upper part of the crystal, forming a normal of the atomic plane. As a result, in the upper part of the crystal formed by an extra half-plane (FG), which continue at the bottom of the crystal, i.e. is there an edge dislocation (denoted by ┴), with atoms located directly above the edge of the extra half plane (Fig. 15 at the top of the crystal) and experience compressive and beneath — tensile stresses.
Maximum distortion and stress in the lattice are concentrated in the presence of an edge dislocation along the edge of the extra atomic plane. This zone of maximum stresses, bounded by a dotted line in Fig. 15, is called the core of the dislocation.
Thus, the edge dislocation line extends indefinitely in the slip plane along the edge of the extra atomic plane in the direction perpendicular to the slide direction (direction of force), i.e. the line of dislocation passes through the point F perpendicular to the plane of the drawing. In other words, for a pure edge dislocation line of a dislocation perpendicular to the direction of application of shear forces and the direction of the slide.
1.4.2 Screw dislocation
A symptom of a screw dislocation is the transformation of parallel atomic planes (in defect-free crystal) to a single atomic plane in the form of a helical surface (like a spiral plane) and the presence on the surface of the crystal a kind of dislocation of the atomic steps (ledge).
A schematic representation of a screw dislocation is shown in figure 6. Her education you can imagine, if you mentally do this in the crystal section (in the plane ABCD), and then by the force P to move one part of the crystal in this plane relative to the other part of one interatomic distance down so that one cut edge of each atomic plane of the lattice perpendicular to the plane of the cut, coincided with the other cut edge of the underlying plane lattice. Cut surfaces can thus be joined exactly, then it is impossible to distinguish, at a plane within Cree-stall was made the cut. The position location will be determined only by the position of the edge of the section plane, in particular, as a result of its intersection with the crystal surface is formed of an atomic step (ABE).
Figure 6 – Scheme of the formation of a screw dislocation
The maximum torsional and shear distortion in the lattice (kernel location) will take place in the crystal region (bounded by the dashed line in figure 6), located at the base of the stairs, and line of deployment will be the line of the sun. Thus, for a pure screw dislocation it is parallel to the direction of application of force P shear and the direction of the slide. If you mentally put yourself in the lattice point E (figure 6) and around the dislocation line of the sun the contour EKLMNAr we go to point L, shifted downwards by one interatomic distance in a perfect crystal no screw dislocation would fall back to point"), making another of the same turnover, move down another one interatomic distance, etc.
In other words, in the presence of a screw dislocation the atomic planes of the lattice, as already mentioned, transformed into the likeness of a screw spiral surface, hence the name screw dislocation (figure 7).
Figure 7 – Schematic representation of the atomic planes in a screw dislocation
1.4.3 the Effect of dislocations on the properties of crystalline bodies
The concept of dislocations was introduced in the 30-ies of XX V. Ya. I. Frenkel, D. I. Taylor, E. Arowana etc. Theory of dislocations that are subsequently developed by many scientists, has proved extremely fruitful and allowed us to explain the peculiarities of many important properties of crystalline solids and processes with their participation. Theoretical predictions concerning the impact of this type of imperfections of the lattice on the properties of crystalline bodies, was brilliantly confirmed in practice. Moreover, 50 years of the presence in crystals of dislocations was proved by direct observation. In particular, an edge dislocation in the form of extra atomic planes has been observed in crystals of certain substances under the electron microscope with higher resolution (figure 8).
Figure 8 – Atomic plane in the crystal of one of the compounds of platinum (the box highlights the region in which the atomic planes are curved due to the presence of dislocations)
It should be noted that to observe the dislocation lines in crystals under a conventional microscope using the so-called method of decorating the dislocations. Foreign atoms introduced into the crystal impurity substances tend to concentrate in the defective regions of the crystal and, in particular, at the edge of the broken planes, i.e. along the line of an edge dislocation. Their accumulation along similar decorated with atoms line allows you to see the location of dislocations in the crystal.
One of the properties, is critically dependent on the presence of dislocations, is the strength of crystal bodies. Knowing the structure and energy of the chemical bonds between atoms in a crystal, we can calculate the force required for the deformation and fracture of perfect (i.e., not con-had been defects) of the crystal, i.e., its theoretical strength. Experience shows that those tensions, which occur deformation and destruction of the real crystals, i.e., their actual strength, be in the 10-104 times less than calculated theoretically. Currently, it is proved that the reason for the high plasticity and low strength is the existence in a real crystal, easily mobile defects — dislocations.
So in a perfect crystal without dislocations under the influence of external forces occurred plastic deformation, i.e., shear along any slip plane required to simultaneously break all of the atomic planes across the glide plane. This requires great effort, which is equivalent to the high strength of perfect crystals. A different failure mechanism takes place in a real crystal containing dislocations. The essence of it boils down to is that in the presence of an edge dislocation shear of one part of crystal relative to the other is not due to the simultaneous rupture of all atomic bonds in the slip plane, and by a gradual (relay) gap separate relations in the course of movement of an edge dislocation by slip, which does not require much effort. After the initial deployment under the influence of small efforts will begin to move, their movement is accelerated, the number increases (the propagation of dislocations), which leads to plastic deformation-tion of the crystal. The consequence of the mechanism of fracture is reduced, the strength of real crystals is compared with the ideal.
It is known that increase in temperature increases the ductility of any material. This effect is also largely due to the behavior of the dislocations with increasing temperature decreases the voltage required to move dislocations and, in addition, facilitated their ability to move the creep in the other slip plane, causing the slip begins to occur on many planes. In MgO, for example, the voltage required for the movement of dislocations in the plane (100) at room temperature, 50 times more than on the plane (110), and at 1000°C this difference is reduced to 2-3 times.
Changing in one way or another the number and properties of dislocations, it is possible to influence the strength of crystal bodies. First of all, we can assume that crystals having the minimum number of dislocations will have increased strength. Indeed, the so-called whisker crystals ("whiskers") of certain substances that are free from dislocations, with loads of experience only elastic deformation in the hundreds or even thousands of times stronger than ordinary crystals. Even at elevated temperatures the crystals without dislocations do not experience plastic deformation. The strength of whiskers approaches the theoretical strength of the order of tens of thousands of MPa. For example, the obtained whisker crystals of MgO with a diameter of 1-3 microns with a tensile strength approaching 25•103 MPa. Currently, the obtained whisker crystals of various compounds: metals, graphite, sulfides, silicon carbide, oxides (MgO, BeO, A12O3, etc.), etc. Receive heavy-duty whiskers has a great industrial value. Reinforcement of these crystals other materials allows to obtain high-strength engineering materials, with often a high fire resistance and chemical resistance.
However, obtaining crystals without dislocations is not the only way of hardening materials. It turns out that increased strength have not only crystals without dislocations, but the crystals with high dislocation density. For example, a long-known method of hardening of metals due to their mechanical treatment (impact load) in cold condition ("work hardening" of metals). As a result of plastic deformation during the "hardening" of the dislocation density sharply increases and strength increases. This is because the negative impact on the strength of the material is determined not by the presence of dislocations and their mobility. If this ability in any way to limit, the strength of the material will increase.
There are several possibilities for braking the motion of dislocations. One of them is the already mentioned method of increasing the dislocation density as a result of plastic deformation caused by machining of the material. This method is based on the fact that when you increase the number of dislocations to move the dislocation in the slip plane, riddled with many other dislocations, very difficult. Another way of braking the motion of dislocations is their fastening by dissolving in the material of foreign atoms (used, for example, when alloying metals) or the introduction of small particles of the other phase. In the first case the inhibition is due to the fact that solubility and thus the concentration of foreign atoms near the dislocations is always higher than in the undistorted part of the crystal. To detach the dislocation from the "clouds" of dissolved atoms, it is necessary to expend definite energy. Therefore, plastic deformation of the crystal in which dislocation fixed "clouds" of dissolved atoms requires a higher voltage, which is equivalent to hardening of the material. The presence of fine particles of the other phase in the material (e.g. of iron carbide in iron) also inhibits the motion of dislocations, for requires additional energy to dislocation during movement could "break" between the particles.
It should be noted that methods based on the inhibition of dislocation motion, increasing the strength of crystalline bodies, not allow, however, to approach the theoretical strength. Most radical in this respect are methods of producing defect-free crystals containing no dislocations.
To manufacture various materials an important process, which depends strongly on the presence of crystal defects, such as dislocations, is the process of crystal growth.
Theoretical calculations show that the ideal defect-free crystal can grow, for example, from a solution or melt with noticeable speed only at very high paracymene (about 25-50%). However, real crystals with a sufficiently high speed even at insignificant (in the thousands or more times smaller than theoretically calculated) paracymene. Currently, this is due to the presence in the crystals of screw dislocations. Consider the mechanism of growth of a perfect crystal and a crystal containing a screw dislocation.
To on the verge of a perfect crystal with parallel atomic planes may form a new atomic layer (which is equivalent to the growth of the crystal in the direction perpendicular to the plane of the face), this face must first form a stable two-dimensional germ of a new layer in a group in a certain way oriented atoms forming step (figure 9).
1 – dimensional germ of a new layer; 2 – step growth; 3 – a single atom adsorbed on the surface of the crystal face
Figure 9 – the Growth of a perfect crystal
The activation energy of this process is extremely high, therefore, for the formation of the embryo requires a very high degree of supersaturation of the solution or melt, because below a certain critical value of supersaturation (~25-50%) chance of occurrence is extremely small. After the formation of the embryo development process faces is much easier: the atoms from solution are adsorbed on the surface of the face diffun-dirout on it and attach to the embryo formed by the step of, providing education on the verge of a new fully built atomic layer. It should be noted that for further growth facets to the emerging atomic layer again to form the germ of a new layer every time to provide a layer-by-layer growth of a perfect crystal. Since at low paracymene the probability of the nucleation is negligible, if the crystal to grow in these conditions, its size would not change significantly even during the existence of the Universe. A similar growth mechanism can really only be realized with very high paracymene.
Imagine that in the crystal there is a screw dislocation that transforms the atomic planes in a single helical surface, and the output of which is on the surface of the crystal face forms on it a kind of step. The role of this step in the process of crystal growth is extremely high, as it becomes the embryo of a new atomic layer. Crystal growth in this case can be represented as follows (figure 10).
1 – dislocation of the atomic step; 2 – a single atom adsorbed on the surface of the crystal face
Figure 10 – the Growth of a crystal with a screw dislocation
Atoms from solution or melt adsorbed on the crystal face, diffuse through it towards the step and are located along it, whereby the latter is completed and begins to move along the surface edge. Since one end of the dislocation is pinned to the surface of the crystal at the point of emergence of dislocation lines, i.e., where the end step, the latter may move due to her pastraivaysa atoms only by rotation around the exit point of the dislocation line, like the clock.
After one full turn of a step on a crystal face will be extended one new atomic layer. It is important to note that the step after that will not disappear and will remain, continuing to play the role of the embryo of a new atomic layer. Due to further rotation of the crystal face would consistently be wound new atomic layers for the growth of the crystal.
Thus, when dislocation growth mechanism is not necessary in the formation on the verge of new crystal nuclei of atomic layers energy-intensive process limiting the growth rate of the perfect crystal. The role of the germ plays the vanishing in the process of growth like a self-replicating atomic step. Therefore, similar to the dislocation mechanism of crystal growth is valid even at very small paracymene, providing rapid crystal growth. Dislocation mechanism of crystal growth leads to the formation on the growing face of peculiar spirals or stages of growth (figure 11), which can be detected by electron, vasovegetative microscopy, etc.
Figure 11 – Spiral step growth around the exit of screw dislocation at the surface of the single crystal silicon carbide (a) and its interference image (b) (increase х90)
The presence of defects like dislocations affecting other properties of crystalline bodies. Dislocation have polysensitization permeability, since each dislocation line represents the path along which diffusion (mass transfer) is faster than through the undeformed lattice. One reason for this is that the atomic rearrangement in places distorted, relative to the random arrangement of atoms near the core of the dislocation may occur more often than intense. As already indicated, the movement of dislocations is accompanied by the formation or, conversely, disappearance-of the ance of the vacancies, which should influence the conductivity of ionic crystals.