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CSUN MSE 227 - Imperfections in Solids

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Chapter 5: Imperfections in SolidsImperfections in SolidsAtom Purity and Crystal PerfectionTypes of ImperfectionsPoint Defects in Metalsc05f01Point Defects in CeramicsPoint Defects: Frenkel and SchottkyEquilibrium Concentration: Point DefectsMeasuring Activation EnergyEstimating Vacancy ConcentrationPoint Defects in PolymersAlloyingSolid SolutionSolid Solution - continuedImperfections in MetalsHume - Rothery RulesSubstitutional Solid Solution Example: Copper and NickelInterstitial Solid SolutionImperfections in Ceramicsc05f06Slide 22Line DefectsSlide 24Motion of Edge DislocationDislocationsSlide 27Dislocation MotionCharacteristics of DislocationsDislocations During Cold WorkingSlide 31Slide 32Polycrystalline MaterialsPlanar Defects in Solids - TwinningSlide 35Properties of TwinningComparisonSlip SystemsSlip systemsSlide 40Slide 41Slide 42Microscopic ExaminationOptical MicroscopySlide 45Change in Microstructure due to Cold WorkPolycrystalline DeformationMicroscopyc05cof01c05f20abChapter 5:Imperfections in SolidsImperfections in Solids•The properties of some materials are profoundly influenced by the presence of imperfections. •It is important to have knowledge about the types of imperfections that exist and the roles they play in affecting the behavior of materials. 23Atom Purity and Crystal Perfection•If we assume a perfect crystal structure containing pure elements, then anything that deviated from this concept or intruded in this uniform homogeneity would be an imperfection.1. There are no perfect crystals.2. Many material properties are improved by the presence of imperfections and deliberately modified (alloying and doping).4• Vacancy atoms• Interstitial atoms• Substitutional atomsPoint defects1-2 atomsTypes of Imperfections• DislocationsLine defects1-dimensional• Grain BoundariesArea defects2-dimensional5• Vacancies:-vacant atomic sites in a structure.• Self-Interstitials:-"extra" atoms positioned between atomic sites.Point Defects in MetalsVacancydistortion of planesself-interstitialdistortion of planes•In metals, a self interstitial introduces relatively large distortions (strain) in the surrounding lattice since the atom is substantially larger than the interstitial site. Self Interstitials7• Vacancies -- vacancies exist in ceramics for both cations and anions • Interstitials -- interstitials exist for cations -- interstitials are not normally observed for anions because anions are large relative to the interstitial sitesAdapted from Fig. 5.2, Callister & Rethwisch 3e. (Fig. 5.2 is from W.G. Moffatt, G.W. Pearsall, and J. Wulff, The Structure and Properties of Materials, Vol. 1, Structure, John Wiley and Sons, Inc., p. 78.)Point Defects in CeramicsCation Interstitial Cation Vacancy Anion Vacancy8• Frenkel DefectTo maintain the charge neutrality, a cation vacancy-cation interstitial pair occur together. The cation leaves its normal position and moves to the interstitial site.• Schottky DefectTo maintain the charge neutrality, remove 1 cation and 1 anion; this creates 2 vacancies.Adapted from Fig. 5.3, Callister & Rethwisch 3e. (Fig. 5.3 is from W.G. Moffatt, G.W. Pearsall, and J. Wulff, The Structure and Properties of Materials, Vol. 1, Structure, John Wiley and Sons, Inc., p. 78.)Point Defects: Frenkel and SchottkySchottky DefectFrenkel Defect9Boltzmann's constant (1.38 x 10-23 J/atom-K) (8.62 x 10-5 eV/atom-K) NvNexpQvkTNo. of defectsNo. of potential defect sitesActivation energy – energy required for formation of vacancy TemperatureEach lattice site is a potential vacancy site• Equilibrium concentration varies with temperature.Equilibrium Concentration:Point Defects10• We can get Qv from an experiment. NvN=expQvkTMeasuring Activation Energy• Measure this...NvNTexponential dependence!defect concentration• Replot it...1/TNNvln-Qv/kslope11• Find the equil. # of vacancies in 1 m3 of Cu at 1000C.• Ex 5.1 - Given:ACu = 63.5 g/mol = 8.4 g/cm3Qv = 0.9 eV/atomNA = 6.02 x 1023atoms/molEstimating Vacancy ConcentrationFor 1 m3, N =NAACu xx 1 m3= 8.0 x 1028 sites8.62 x 10-5eV/atom-K0.9 eV/atom1273 K NvNexpQvkT= 2.7 x 10-4 • Answer:Nv = (2.7 x 10-4)(8.0 x 1028) sites = 2.2 x 1025 vacancies12Point Defects in Polymers•Defects due in part to chain packing errors and impurities such as chain ends and side chainsAdapted from Fig. 5.7, Callister & Rethwisch 3e.Alloying•Given a metal (with only 1 type of atom) refined to 99.9999% purity, there would still exist 1022 to 1023 impurity atoms in 1 cubic meter of material.•Most metals are alloys. Alloying is done to improve strength, corrosion resistance, ductility, lower melting T.•For example, sterling silver is an alloy of 92.5% silver, 7.5% copper. At room temperature, “pure” silver is highly corrosion resistant, but also very soft. The addition of copper improves the strength and maintains good corrosion behavior.13Solid Solution•The addition of impurity atoms to a metal results in the formation of a solid solution.•The solvent represents the element that is present in the greatest amount (the host atoms). For example, in Lab 8 (MSE 227) Precipitation Hardening of Aluminum, aluminum is the solvent and copper is the solute (present in minor concentration ).•Solid solutions form when the solute atoms (Cu) are added to the solvent (Al), assuming the crystal structure is maintained and no new structures are formed.14Solid Solution - continued•A solid solution is a homogenous composition throughout.•The impurity atoms (Cu) are randomly and uniformly dispersed within the solid.•The impurity defects in the solid solution are either substitutional or interstitial.1516What are the outcomes if impurity (B) is added to host (A)?• Solid solution of B in A (random distribution of point defects)• Solid solution of B in A plus particles of a new phase (usually for a larger amount of B)ORSubstitutional solid solution.(e.g., Cu in Ni)Interstitial solid solution.(e.g., C in Fe)Second phase particle-- different composition-- often different structure.Imperfections in Metals17Hume - Rothery RulesThe Hume-Rothery rules are basic conditions for an element to dissolve in a metal, forming a substitutional solid solution. 1. The atomic radius of the solute and solvent atoms must


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