Lecture05 - VK250 Lecture 05 DEFECTS AND MICROSTRUCTURE...

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VK250 Lecture 05 1 D EFECTS AND M ICROSTRUCTURE Point, interfacial & line defects in crystals Any region where order of crystal is broken Control structure-sensitive properties Point defects Defects limited to isolated lattice sites Vacancies: Missing atoms from crystal lattice (regions of tension) Interstitials: Extra atoms squeezed into crystal lattice (compression) Lattice may locally distort around defect, resulting in a displacement of atoms from equilibrium position local regions of stress Vacancies tensile stresses Interstitials compressive stresses Interfacial defects Defects in a plane Grain boundaries: Disordered region between grains
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VK250 Lecture 05 2 Twin boundaries: Portions of lattice related to each other by simple rotation Net result of twinning is shear deformation Note that plastic deformation is a permanent shear deformation (occurs in response to shear stresses change of shape not volume) Twinning is a possible mechanism of plastic deformation Twinning can occur in h.c.p. metals (Zn, Sn and Mg), not in f.c.c. metals However, a much more important mechanism of plastic deformation is slip Slip occurs by motion of dislocations Line defects Dislocations: Two types of dislocation: Edge and Screw Can be visualized in terms of atoms or as a continuum model
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VK250 Lecture 05 3
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Lecture05 - VK250 Lecture 05 DEFECTS AND MICROSTRUCTURE...

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