4-Atomic+Point+Defects

4-Atomic+Point+Defects - Atomic Point Defects We consider...

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30 Atomic Point Defects We consider first the distortions associated with atomic point defects – impurity or solute atoms in a crystal lattice. Impurities – “not wanted” foreign atoms Solute – intentionally added foreign atoms Substitutional Atoms When solute atoms are comparable is size to the solvent atoms (the host atoms), they typically reside on substitutional sites. That is, they replace the host atoms in the crystal lattice. Very roughly, if the solute radius is 85% of the radius of the solvent atoms or greater, one can expect the solutes to reside on substitutional sites. Rule of thumb: r solute ± 0.85 r solvent then substitutional sites are occupied. Solute atom replaces solvent atom in normal atomic site Interstitial Atoms By comparison if solute atoms are substantially smaller that the host (solvent) atoms then they are likely to occupy interstitial sites Rule of thumb: r solute ± 0.85 r solvent then interstitial sites are occupied. Solute atom fits into interstitial site (between the solvent atoms) if it is sufficiently small.
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31 Small atoms such as: C, N, O, B and He are typically interstitial solutes in metals. Symmetry of distortions about solute atom defects Substitutional defects In lattices of high symmetry (FCC, BCC and HCP) substitutional defects create an essentially spherically symmetric distortion. Consider a substitutional solute atom in a BCC lattice. The defect has eight nearest neighbors that are each
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4-Atomic+Point+Defects - Atomic Point Defects We consider...

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