ENGR 240 - 9.14.07

ENGR 240 - 9.14.07 - ENGR 240 Dislocations can dramatically...

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ENGR 240 – September 14, 2007 Dislocations can dramatically affect mechanical properties Surface Defects o Boundaries between different crystal structures and crystallographic orientations o Free Surface Boundary where crystal structure terminates Part of atom exposed at surface not bonded Atoms prefer all bonds to be filled Difference in energy for atoms partially bonded (surface) and atoms with filled bonds Surface energy increases as density of atoms at surface increases Surface energy decreases as surface area decreases All materials want to minimize surface area For metals, at room temperature, the driving force to minimize surface energy is not large enough for change o Grain Boundaries Boundaries between crystals in polycrystalline material Crystal orientations are not uniform (they don’t line up) Atoms have all bonds filled, but bonds are strained Small angle grain boundary, less mismatch than large angle grain boundary
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This note was uploaded on 04/07/2008 for the course ENGR 240 taught by Professor Gross during the Fall '07 term at Bucknell.

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ENGR 240 - 9.14.07 - ENGR 240 Dislocations can dramatically...

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