ch05-Imperfections-in-Solids

ch05-Imperfections-in-Solids - Chapter 5 Imperfections in...

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Chapter 5 Imperfections in Solids Thanks to Jeff DePue, Greg Dykes, Alex Eaton Thanks to Jeff DePue, Greg Dykes, Alex Eaton
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Chapter 5 - 2 ISSUES TO ADDRESS. .. What types of defects arise in solids? Can the number and type of defects be varied and controlled? How do defects affect material properties? Are defects undesirable? CHAPTER 5: IMPERFECTIONS IN SOLIDS What are the solidification mechanisms?
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5.1 Introduction Every single solid has defects and imperfections. Sometimes the imperfections are purposely created and used for a specific purpose. Types of defects: Point Defects (One or two atomic positions) Linear Defects (One Dimensional) Interfacial Defects (Boundaries)
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Chapter 5 - 4 Solidification - result of casting of molten material 2 steps Nuclei form Nuclei grow to form crystals – grain structure Start with a molten material – all liquid 5.1 Imperfections in Solids Crystals grow until they meet each other Adapted from Fig. 5.19 (b), grain structure crystals growing nuclei liquid
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Chapter 5 - 5 Polycrystalline Materials Grain Boundaries regions between crystals transition from lattice of one region to that of the other slightly disordered low density in grain boundaries high mobility high diffusivity high chemical reactivity Adapted from Fig. 5.12, Callister & Rethwisch 3e.
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Chapter 5 - 6 Solidification Columnar in area with less undercooling Shell of equiaxed grains due to rapid cooling (greater T ) near wall Grain Refiner - added to make smaller, more uniform, equiaxed grains. heat flow Grains can be - equiaxed (roughly same size in all directions) - columnar (elongated grains) Adapted from Fig. 5.17, ~ 8 cm
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Chapter 5 - 7 Imperfections in Solids There is no such thing as a perfect crystal. What are these imperfections? Why are they important? Many of the important properties of materials are due to the presence of imperfections.
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Chapter 5 - 8 Vacancy atoms Interstitial atoms Substitutional atoms Point defects Types of Imperfections Dislocations Line defects Grain Boundaries Area defects
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Chapter 5 - 9 I. Point defects
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Chapter 5 - 10 Vacancies : -vacant atomic sites in a structure. Self-Interstitials : -"extra" atoms positioned between atomic sites. 5.2 Point Defects in Metals Vacancy distortion of planes self- interstitial distortion of planes
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Boltzmann's constant (1.38 x 10 -23 J/atom-K) (8.62 x 10 -5 eV/atom-K) N v N = exp - Q v k T No. of defects No. of potential defect sites Activation energy Temperature Each lattice site is a potential vacancy site Equilibrium concentration varies with temperature! Equilibrium Concentration:
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This note was uploaded on 03/08/2010 for the course CHEN 313 taught by Professor Seminario during the Fall '07 term at Texas A&M.

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ch05-Imperfections-in-Solids - Chapter 5 Imperfections in...

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