100%(1)1 out of 1 people found this document helpful
This preview shows page 17 - 21 out of 98 pages.
(tensile force perpendicular to the plane). Hence even asimple 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 XX1isa slip plane. Slip results in plastic flow and forms the basis for estimating ductility. Slip indifferent crystal structures occurs as follows. There as 12 possible slip systems in the f.c.cstructure these generally occur in close packed planes such as (111) and (110). Similar slipsystems can be defined for b.c.c, c.p.h and other structures.3.4.2 VacanciesVacancies are simply empty atom sites as shown in Figure 3.10.The lattice vacancies are astable feature of metals at all temperatures above absolute zero. By successive jumps ofXX’PPPPPTPSXX’14
3.4.4 DislocationA dislocation may be defined as a disturbed region between two substantially perfect parts ofa 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 duringsolidification, during plastic deformation, and as consequence of thermal stresses that resultfrom rapid cooling.The importance of dislocations to the metal user is that dislocation interactions within a metalare a primary means by which metals are deformed and strengthened. When metals deformby 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 themost useful engineering materials. The metallic bond is such that strains to the crystal latticeare accommodated by dislocation motion. Many metals can tolerate significant plasticdeformation before failing. Two simple types of dislocation are:1. Screw dislocationFigure 3.12: Screw dislocation2. Edge dislocationFigure 3.13: Edge dislocation16