lecture4-materials-pdf - Chapter 4 IMPERFECTIONS IN IN...

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hapter 4 Chapter PERFECTIONS IMPERFECTIONS SOLIDS Dr. Mubarak ١ IN SOLIDS
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Outline hat are the solidification mechanisms What are the solidification mechanisms? • What types of defects arise in solids? Can the number and type of defects be varied • Can the number and type of defects be varied and controlled? • How do defects affect material properties? • Are defects undesirable? Dr. Mubarak ٢
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Solidification Start with a molten material – all liquid Result of casting of molten material: 1. Nuclei form 2. Nuclei grow to form crystals – grain structure Crystals grow until they meet each other. ٣ Dr. Mubarak
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٤ Dr. Mubarak
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٥ Dr. Mubarak
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٦ Dr. Mubarak
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٧ Dr. Mubarak
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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 f materials are due to the of materials are due to the presence of imperfections. Semiconducting properties eld stress Yield stress Fracture strength ٩ Dr. Mubarak Electrical conductivity
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Imperfections in Solids Many of the important properties of materials are ue to the presence of imperfections due to the presence of imperfections. Semiconducting properties Yield stress acture strength Fracture strength Electrical conductivity ١٠ Dr. Mubarak
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The processing determines the defects Imperfections ١١ Dr. Mubarak
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Types of Imperfections 1. Point defects: atoms missing or in regular places in the lattice irregular places in the lattice vacancies terstitials (space that intervenes between things) interstitials (space that intervenes between things). impurities, weight and atomic composition ne defects: oups of atoms in irregular 2. Line defects: groups of atoms in irregular positions edge dislocations and screw dislocations 3. Area defects: the interfaces between homogeneous regions of the material Grain boundaries ١٢ Dr. Mubarak 4.Bulk or Volume defects: pores, cracks
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Point Defects • Vacancies This figure shows a vacant atomic sites in a structure. • Self Interstitials "extra" atoms positioned between atomic sites. ١٣ Dr. Mubarak
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How many vacancies are there? The equilibrium number of vacancies formed as a result of thermal vibrations may be calculated from thermodynamics: ١٤ Dr. Mubarak
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Dr. Mubarak ١٦
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Other point defects: self-interstitials, impurities Dr. Mubarak ١٧
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interstitials - Self Self interstitials in metals introduce large istortions in the surrounding lattice distortions in the surrounding lattice . The energy of self interstitial formation is ~ 3 mes larger as compared to vacancies (Q ~ times larger as compared to vacancies (Q i 3×Q v ). ilib i tt i f lf t titi l i Equilibrium concentration of self interstitials is very low (less than one self interstitial per cm 3 at room T).
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This note was uploaded on 02/23/2011 for the course CHEMICAL E 0905326 taught by Professor Yousefmobarak during the Spring '11 term at University of Jordan.

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lecture4-materials-pdf - Chapter 4 IMPERFECTIONS IN IN...

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