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Most
technical materials are composed of many differently oriented small crystals
of the same structure joined together on the atomic scale across their
grain boundaries. These boundaries are two-dimensional internal interfaces
which have an atomic packing different from that of the single crystal
grains. If the angle between equivalent planes in adjacent grains is large,
the atomic misfit in the grain boundary is large. Small angle grain
boundaries exhibit less atomic misfit. The top diagram illustrates this
behavior, the photograph is of grain boundaries in iron magnified 500 x.
A
pure tilt boundary is equivalent to an array of edge dislocations and a
pure twist boundary an array of screw dislocations. Twin boundaries are
a special case of a large angle grain boundary for which there is no atomic
misfit. Across the twin boundary crystallites have planes that are the
mirror image of the planes in the other crystalite. The bottom diagram
illustrates this for the (110) plane of a bcc lattice. |
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