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dislocation info

dislocation info - urification c ell;lzes o f s pheres h...

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)urification cell. ;lzes of spheres, : holes between Es are available, { contaminated re contaminants he spheres. We surface area of ftcation. Design ,peclmens, some Design a X-ray can be accom- Gu,t' +a% ru$"-S'" w.- CHAPTER 4 t. , lmperfections in the Atomic Arrangement lntroduction The arrangement of the atoms in all materials contains imperfections which have . a profound effect on the behavior of the materials. By controlling lattice imper- fections, we create stronger metals and alloys, more powerful magnets, improved transistors and solar cells, glassware of striking colors, and many other materials of practical importance. In this chapter we introduce the three basic types of lattice imperfections: point defects, line defects (or dislocations), and surface defects. We must remem- ber, however, that these imperfections only represent defects in the perfect atomic arrangement, not in the material itself. Indeed, these "defects" may be intention- ally added to produce a desired set of mechanical and physical properties. Later chapters will show how we control these defects through alloying, heat treatment, or processing techniques to produce improved engineering materials. Dislocations Dislocations are line imperfections in an otherwise perfect lattice. They are typi- cally introduced into the lattice during solidification of the material or when the material is deformed. Although dislocations are present in all materials, including ceramics and polymers, they are particularly useful in explaining deformation and strengthening in metals. We can identify two types of dislocations: the screw dislocation and the edge dislocation. Screw Dislocations The screw dislocation (Figure 4-1) can be illustrated by cutting partway through a perfect crystal, then skewing the crystal one atom spacing.
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80 Chapter 4 lmperfections in the Atomic Arrangement FIG U RE + | The perfect crystal (a) is cut and sheared one atom spacing, (b) and (c). The line along which shearing occurs
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