1. L-03 ENGINEERINGMATERIALS DISLOCATION, SLIP SYSTEMS AND TWINING

3. In fact, even with relatively sophisticated techniques, it is difficult to refine metals to a purity in excess of 99.9999%.

4. At this level, on the order of 1022 to 1023 impurity atoms will be present in one cubic meter of material.

6. Crystal Defects just keep in mind that crystalline defects are not always bad. There are basic classes of crystal defects: point defects, which are places where an atom is missing or irregularly placed in the lattice structure. Point defects include lattice vacancies, self-interstitial atoms, substitution impurity atoms, and interstitial impurity atoms 4

7. 5 Figure 4.1 Two-dimensional representations of a vacancy and a self-interstitial.

8. 6 Figure 4.2 Two-dimensional schematic representations of substitutional and interstitial impurity atoms.

9. Crystal Defects linear defects, which are groups of atoms in irregular positions. Linear defects are commonly called dislocations. planar defects, which are interfaces between homogeneous regions of the material. Planar defects include grain boundaries, stacking faults and external surfaces. It is important to note at this point that plastic deformation in a material occurs due to the movement of dislocations (linear defects). 7

10. Crystal Defects Millions of dislocations result for plastic forming operations such as rolling and extruding. It is also important to note that any defect in the regular lattice structure disrupts the motion of dislocation, which makes slip or plastic deformation more difficult. These defects not only include the point and planer defects mentioned above, and also other dislocations. 8

11. Crystal Defects Dislocation movement produces additional dislocations, and when dislocations run into each other it often impedes movement of the dislocations. This drives up the force needed to move the dislocation or, in other words, strengthens the material. 9

14. Point Defects Interstitial impurity atoms are much smaller than the atoms in the bulk matrix. Interstitial impurity atoms fit into the open space between the bulk atoms of the lattice structure. An example of interstitial impurity atoms is the carbon atoms that are added to iron to make steel. Carbon atoms, with a radius of 0.071 nm, fit nicely in the open spaces between the larger (0.124 nm) iron atoms. Vacancies are empty spaces where an atom should be, but is missing. They are common, especially at high temperatures when atoms are frequently and randomly change their positions leaving behind empty lattice sites. In most cases diffusion (mass transport by atomic motion) can only occur because of vacancies. 12

15. Linear Defects - Dislocations Dislocations are another type of defect in crystals. Dislocations are areas were the atoms are out of position in the crystal structure. Dislocations are generated and move when a stress is applied. The motion of dislocations allows slip – plastic deformation to occur. The TEM (Transmission Electron Microscope-image resolutions of 1 - 2 Angstroms) allowed experimental evidence to be collected that showed that the strength and ductility of metals are controlled by dislocations. 13

17. Thus, it becomes possible to design and tailor the mechanical properties of materials—for example, the strength or toughness of a metal–matrix composite.14 Underlying-بنیادی

18. materials may experience two kinds of deformation: elastic and plastic. Plastic deformation is permanent, and strength and hardness are measures of a material’s resistance to this deformation. On a microscopic scale, plastic deformation corresponds to the net movement of large numbers of atoms in response to an applied stress. During this process, inter atomic bonds must be ruptured and then reformed. 15

19. DISLOCATION In crystalline solids, plastic deformation most often involves the motion of dislocations, linear crystalline defects. Dislocations and Plastic Deformation. a type of linear crystalline defect is known as dislocation. Edge and screw are the two fundamental dislocation types. 16

20. 17 the nature of a dislocation (i.e., edge, screw, or mixed) is defined by the relative orientations of dislocation line and Burgers vector. For an edge, they are perpendicular(Figure 4.3), whereas for a screw, they are parallel (Figure 4.4); they are neither perpendicular nor parallel for a mixed dislocation. 3. Virtually all crystalline materials contain some dislocations that were introduced during solidification, during plastic deformation, and as a consequence of thermal stresses that result from rapid cooling.

21. 18 Figure 4.3 The atom positions around an edge dislocation; extra half-plane of atoms shown in perspective.

22. 19 Figure 4.4 (a) A screw dislocation within a crystal.

23. Edge Dislocations The edge defect can be easily visualized as an extra half-plane of atoms in a lattice. The dislocation is called a line defect because the locus of defective points produced in the lattice by the dislocation lie along a line. This line runs along the top of the extra half-plane. The inter-atomic bonds are significantly distorted only in the immediate vicinity of the dislocation line. 20

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25. 22 Figure7.3 Representation of the analogy between caterpillar and dislocation motion.

26. 23 Figure 7.2 The formation of a step on the surface of a crystal by the motion of (a) an edge dislocation and (b) a screw dislocation. Note that for an edge, the dislocation line moves in the direction of the applied shear stress for a screw, the dislocation line motion is perpendicular to the stress direction.

27. 7.4 SLIP SYSTEMS Dislocations do not move with the same degree of ease on all crystallographic planes of atoms and in all crystallographic directions. Ordinarily there is a preferred plane, and in that plane there are specific directions along which dislocation motion occurs. This plane is called the slip plane; it follows that the direction of movement is called the slip direction. This combination of the slip plane and the slip direction is termed the slip system. The slip system depends on the crystal structure of the metal and is such that the atomic distortion that accompanies the motion of a dislocation is a minimum. 24

28. SLIP SYSTEMS For a particular crystal structure, the slip plane is the plane that has the most dense atomic packing—that is, has the greatest planar density. The slip direction corresponds to the direction, in this plane, that is most closely packed with atoms—that is, has the highest linear density. Consider, for example, the FCC crystal structure, a unit cell of which is shown in Figure 7.6a. There is a set of planes, the {111} family, all of which are closely packed. A (111)-type plane is indicated in the unit cell; in Figure 7.6b, this plane is positioned within the plane of the page, in which atoms are now represented as touching nearest neighbors. 25

29. 26 Figure 7.6 (a) A {111}<110>slip system shown within an FCC unit cell. (b) The (111) plane from (a) and three <110>slip directions (as indicated by arrows) within that plane comprise possible slip systems.

30. SLIP SYSTEMS Slip occurs along<110>-type directions within the {111} planes, as indicated by arrows in Figure 7.6. Hence, {111}<110> represents the slip plane and direction combination, or the slip system for FCC. Figure 7.6b demonstrates that a given slip plane may contain more than a single slip direction. Thus, several slip systems may exist for a particular crystal structure; the number of independent slip systems represents the different possible combinations of slip planes and directions. For example, for face-centered cubic, there are 12 slip systems: four unique {111} planes and, within each plane, three independent<110> directions. 27

31. SLIP SYSTEMS The possible slip systems for BCC and HCP crystal structures are listed in Table 7.1. For each of these structures, slip is possible on more than one family of planes (e.g., {110}, {211}, and {321} for BCC). For metals having these two crystal structures, some slip systems are often operable only at elevated temperatures. Metals with FCC or BCC crystal structures have a relatively large number of slip systems (at least 12). 28

32. SLIP SYSTEMS These metals are quite ductile because extensive plastic deformation is normally possible along the various systems. Conversely, HCP metals, having few active slip systems, are normally quite brittle. With regard to the process of slip, a Burgers vector’s direction corresponds to a dislocation’s slip direction, whereas its magnitude is equal to the unit slip distance (or interatomic separation in this direction). Of course, both the direction and the magnitude of b will depend on crystal structure, and it is convenient to specify a Burgers vector in terms of unit cell edge length (a) and crystallographic direction indices. 29

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