Planar defects and Boundaries

A Planar Defect is a discontinuity of the perfect crystal structure across a plane.

Grain Boundaries:

A Grain Boundary is a general planar defect that separates regions of different crystalline orientation (i.e. grains) within a polycrystalline solid.

The atoms in the grain boundary will not be in perfect crystalline arrangement.

Grain boundaries are usually the result of uneven growth when the solid is crystallising.

Grain sizes vary from 1 µm to 1 mm.

Tilt Boundaries:

A Tilt Boundary, between two slightly mis-aligned grains appears as an array of edge dislocations.

Twin Boundaries:

A Twin Boundary happens when the crystals on either side of a plane are mirror images of each other.

The boundary between the twinned crystals will be a single plane of atoms.

There is no region of disorder and the boundary atoms can be viewed as belonging to the crystal structures of both twins

Twins are either grown-in during crystallisation, or the result of mechanical or thermal work.