Ch4 - ISSUES TO ADDRESS. CHAPTER 4: IMPERFECTIONS IN SOLIDS...

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1 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 4: IMPERFECTIONS IN SOLIDS What are the solidification mechanisms?
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2 Defects in Crystalline Solids So far we considered “ideal” case: unit cells and atoms are in perfect order in crystalline solids. In reality: Deviation from the ideal case: imperfections Properties are significantly altered when imperfections exist In many cases: we introduce imperfections to produce certain properties (e.g alloying)
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3 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 Imperfections in Solids Adapted from Fig.4.14 (b), Callister 7e. Crystals grow until they meet each other nuclei crystals growing grain structure liquid
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4 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. 4.7, Callister 7e.
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5 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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6 Vacancy atoms Interstitial atoms Substitutional atoms Point defects Types of Imperfections Dislocations Line defects Grain Boundaries Area defects
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7 Point Defects: Impurities Pure Metal: consists of only one type of atoms Impurity atom: a foreign atom Alloys: impurity atoms are added intentionally Adding impurity atoms: Solid Solution and/or second new phase Depends on: type of impurity, concentration, temperature E.g: add sugar to water
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8 Point Defects: Vacancies Vacancy: Vacant lattice site that is normally occupied by an atom. All crystalline solids contain vacancies. It is not possible to make a material without vacancies Cprinciples of thermodynamics Equilibrium number of vacancies in any solid, N v mainly depends on temperature
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9 Impurities: Solid Solution Solvent : Host atom (original atom) Solute : Guest atom (added atom) Solvent: greater concentrations In solid solution: crystal structure doesn’t change Impurity atoms are distributed randomly and uniformly Two types of impurity atoms: Substitutional Interstitial
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Vacancies : -vacant atomic sites in a structure. Self-Interstitials
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This note was uploaded on 03/15/2011 for the course CHE 215 taught by Professor Aboyousef during the Spring '11 term at American University of Sharjah.

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Ch4 - ISSUES TO ADDRESS. CHAPTER 4: IMPERFECTIONS IN SOLIDS...

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