Hence even a simple tensile force will produce a shear stress in the plane

Hence even a simple tensile force will produce a

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(tensile force perpendicular to the plane). Hence even a simple tensile force will produce a shear stress in the plane. Figure 3.9 also shows the slippage between planes. The phenomenon is called slip and XX 1 is a slip plane. Slip results in plastic flow and forms the basis for estimating ductility. Slip in different crystal structures occurs as follows. There as 12 possible slip systems in the f.c.c structure these generally occur in close packed planes such as (111) and (110). Similar slip systems can be defined for b.c.c, c.p.h and other structures. 3.4.2 Vacancies Vacancies are simply empty atom sites as shown in Figure 3.10.The lattice vacancies are a stable feature of metals at all temperatures above absolute zero. By successive jumps of X X’ P P P P P T P S X X’ 14
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3.4.4 Dislocation A dislocation may be defined as a disturbed region between two substantially perfect parts of a crystal. A dislocation is a linear defect around which some of the atoms are misaligned. Dislocations can be observed in crystalline materials using electron-microscopic techniques. Virtually all crystalline materials contain some dislocations that were introduced during solidification, during plastic deformation, and as consequence of thermal stresses that result from rapid cooling. The importance of dislocations to the metal user is that dislocation interactions within a metal are a primary means by which metals are deformed and strengthened. When metals deform by dislocation motion, the more barriers the dislocations meet, the stronger the metal. Deformation by dislocation motion is one of the characteristics of metals that make them the most useful engineering materials. The metallic bond is such that strains to the crystal lattice are accommodated by dislocation motion. Many metals can tolerate significant plastic deformation before failing. Two simple types of dislocation are: 1. Screw dislocation Figure 3.12: Screw dislocation 2. Edge dislocation Figure 3.13: Edge dislocation 16

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