domingo, 25 de julio de 2010

Tilt and Twist Grain Boundaries

Low angle grain boundary is an array of aligned edge dislocations. This type of grain boundary is called tilt boundary (consider joint of two wedges).
Transmission electron microscope image of a small angle tilt boundary in Si. The red lines mark the edge dislocations, the blue lines indicate the tilt angle.

Twist boundary - the boundary region consisting of arrays of screw dislocations (consider joint of two halves of a cube and twist an angle around the cross section normal).

Low-energy twin boundaries with mirrored atomic positions across boundary may be produced by deformation of materials. This gives rise to shape memory metals, which can recover their original shape if heated to a high temperature. Shape-memory alloys are twinned and when deformed they untwin. At high temperature the alloy returns back to the original twin configuration and restore the original shape.

Electron Microscopy

Dislocations in Nickel (the dark lines and loops), transmission electron microscopy image, Manchester Materials Science Center.

High-resolution Transmission Electron Microscope image of a tilt grain boundary in aluminum, Sandia National Lab.
CI 18694836

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