Line Defects Dislocations are line defects slip between crystal planes result when dislocations move produce permanent plastic deformation Schematic of Zinc HCP before deformation after tensile elongation slip steps 1 Imperfections in Solids Linear Defects Dislocations Are one dimensional defects around which atoms are misaligned Edge dislocation extra half plane of atoms inserted in a crystal structure b perpendicular to dislocation line Screw dislocation spiral planar ramp resulting from shear deformation b parallel to dislocation line Burger s vector b measure of lattice distortion 2 Imperfections in Solids Edge Dislocation 3 Dislocation Motion Dislocations plastic deformation Cubic hexagonal metals plastic deformation is by plastic shear or slip where one plane of atoms slides over adjacent plane by defect motion dislocations If dislocations don t move deformation doesn t occur Imperfections in Solids Screw Dislocation Screw Dislocation b Dislocation line Burgers vector b b a 5 Edge Screw and Mixed Dislocations Mixed Edge Screw 6 Imperfections in Solids Dislocations are visible in electron micrographs 7 Dislocations Crystal Structures Structure close packed planes directions are preferred view onto two close packed planes close packed plane bottom close packed directions close packed plane top Comparison among crystal structures FCC many close packed planes directions HCP only one plane 3 directions BCC none Specimens that were tensile tested Mg HCP tensile direction Al FCC 8 Planar Defects in Solids One case is a twin boundary plane Essentially a reflection of atom positions across the twin plane Stacking faults For FCC metals an error in ABCABC packing sequence Ex ABCABABC 9 Catalysts and Surface Defects A catalyst increases the rate of a chemical reaction without being consumed Active sites on catalysts are normally surface defects Fig 4 10 Callister Rethwisch 8e Single crystals of Ce0 5Zr0 5 O2 used in an automotive catalytic converter 10 Microscopic Examination Crystallites grains and grain boundaries Vary considerably in size Can be quite large ex Large single crystal of quartz or diamond or Si ex Aluminum light post or garbage can see the individual grains Crystallites grains can be quite small mm or less necessary to observe with a microscope 11 Optical Microscopy Useful up to 2000X magnification Polishing removes surface features e g scratches Etching changes reflectance depending on crystal orientation crystallographic planes Micrograph of brass a Cu Zn alloy 0 75mm 12 Optical Microscopy Grain boundaries are imperfections are more susceptible to etching may be revealed as dark lines change in crystal orientation across boundary polished surface surface groove grain boundary a ASTM grain size number N 2n 1 number of grains in2 at 100x magnification Fe Cr alloy b 13 Optical Microscopy Polarized light metallographic scopes often use polarized light to increase contrast Also used for transparent samples such as polymers 14 Microscopy Optical resolution ca 10 7 m 0 1 m 100 nm For higher resolution need higher frequency X Rays Difficult to focus Electrons wavelengths ca 3 pm 0 003 nm Magnification 1 000 000X Atomic resolution possible Electron beam focused by magnetic lenses 15
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