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Chapter 4: Imperfections in Solids

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Imperfections in Solids 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

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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

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Solidification Grains can be - equiaxed (roughly same size in all directions) - columnar (elongated grains)

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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 of materials are due to the presence of imperfections.

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Types of Imperfections

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Point Defects in Metals

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Equilibrium Concentration: Point Defects

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Measuring Activation Energy

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Estimating Vacancy Concentration

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Observing Equilibrium Vacancy Conc.

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Imperfections in Metals (i)

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Imperfections in Metals (ii) Conditions for substitutional solid solution (S.S.) W. Hume – Rothery rule 1. r (atomic radius) < 15% 2. Proximity in periodic table i.e., similar electronegativities 3. Same crystal structure for pure metals 4. Valency All else being equal, a metal will have a greater tendency to dissolve a metal of higher valency than one of lower valency

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Imperfections in Metals (iii) Application of Hume–Rothery rules – Solid Solutions 1. Would you predict more Al or Ag to dissolve in Zn? 2. More Zn or Al in Cu?

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Impurities in Solids Specification of composition weight percent

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Line Defects

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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

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Imperfections in Solids Edge Dislocation

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Motion of Edge Dislocation

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Imperfections in Solids Screw Dislocation

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VMSE: Screw Dislocation In VMSE: a region of crystal containing a dislocation can be rotated in 3D dislocation motion may be animated

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Edge, Screw, and Mixed Dislocations

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Imperfections in Solids Dislocations are visible in electron micrographs

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Dislocations & Crystal Structures

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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

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Catalysts and Surface Defects A catalyst increases the rate of a chemical reaction without being consumed Active sites on catalysts are normally surface defects

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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.

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Optical Microscopy

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Optical Microscopy

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Optical Microscopy Polarized light metallographic scopes often use polarized light to increase contrast Also used for transparent samples such as polymers

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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.