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CHAPTER 5 REVIEWER IN FUNMATLS BY ALINEA, ROBERT JOHN IMPERFECTIONS IN SOLID – An idealized solid does not exist. – All solids contain large numbers of various defects or imperfections Crystalline Defect refers to a lattice irregularity having one or more of its dimensions on the order of an atomic diameter. CRYSTALLINE DEFECT Point Defects – those associated with one or two atomic positions Linear Defects – one-dimensional defect Interfacial Defects or Boundaries – two-dimensional defect POINTS DEFECTS: Vacancy – Vacant lattice site – Simplest point defect – An atom in a normally occupied lattice site is missing EQUILIBRIUM NUMBER OF VACANCIES Nv = N exp (-Qv/kT) Where Nv = equilibrium number of vacancies N = total number of atomic sites = number of atoms per unit volume Qv = energy required for the formation of a vacancy k = Boltzmann’s Constant = 1.38x10-23 J/atom-K or 8.62x10-5 eV/atom-K T = absolute temperature in Kelvin Self-Interstitial – An atom from the crystal that is crowded into an interstitial site, a small void space that under ordinary circumstances is not occupied IMPURITY POINT DEFECTS – A pure metal consisting of only one type of atom is not possible. – It is very difficult to refine metals to a purity in excess of 99.9999%. Alloy – A metallic substance that is composed of two or more elements. Alloying is ordinarily used in metals to improve mechanical strength and corrosion resistance. Solid Solutions – Formed when the solute atoms are added to the host material, the crystal is maintained and no new structures are formed – Compositionally homogeneous; the impurity atoms are randomly and uniformly dispersed within the solid Solute – Element or compound present in minor concentration Solvent – Element or compound present in the greatest amount; also called host atoms Substitutional – Solute or impurity atoms replace or substitute for the host atoms Interstitial – Impurity atoms fill the voids or interstices among the host atoms Substitutional Factors Affecting Degree of Solubility Atomic size factor o when the difference in atomic radii between the atom types is less than about 15% Crystal structure o the crystal structures for metals of both atom types must be the same Electronegativity o the more electropositive one element and the more electronegative the other, the greater the likelihood that they will form inter-metallic compound Valences o a metal will have more of a tendency to dissolve another metal of higher valency than one of a lower valency Interstitial Factors Atomic Packing Factor Atomic Diameter – impurity must be substantially smaller than that of the host atoms Concentration – normally less than 10% Composition Concentration in terms of constituent elements Weight Percent CONVERSION LINEAR DEFECTS Dislocation – A linear defect one-dimensional defect around which some of the atoms are misaligned Edge Dislocation – A linear defect that centers on the line that is defined along the end of the extra half-plane of atoms Screw Dislocation – Formed by a shear stress that is applied to produce the distortion Mixed Dislocations – Exhibit components of both edge and screw type dislocations Burger Vector – Magnitude and direction of the lattice distortion associated with a dislocation INTERFACIAL DEFECTS Interfacial Defects – Boundaries that have two dimensions and normally separate regions of the materials that have different crystal structures External Surfaces – Surface atoms are not bonded to the maximum number of nearest neighbors, and are therefore in a higher energy state than the atoms at interior positions Grain Boundaries – Boundary separating two small grains or crystals having different crystallographic orientations in polycrystalline materials Phase Boundaries – Exist in multiphase materials, wherein a different phase exists on each side of the boundary Twin Boundaries – A special type of grain boundary across which there is a specific mirror lattice symmetry; that is, atoms on one side of the boundary are located in mirror-image positions of the atoms on the other side MICROSCOPIC EXAMINATION Microscopy – The investigation of micro-structural elements using some type of microscope Microstructure – The structural features of an alloy (e.g. grain and phase structure) that are subject to observation under a microscope MICROSCOPIC TECHNIQUES: Optical Microscopy – The light microscope is used to study the microstructure; optical and illumination systems are its basic elements Electron Microscopy – Employed for much higher magnifications – Image of the structure under investigation is formed using beams of electrons instead of light radiation