Surface Defects
1.External Surface: Surface atoms have unsatisfied atomic bonds, and higher surface energies, than the bulk atoms. To reduce surface free energy, material tends to minimize its surface areas against the surface tension (e.g. liquid drop).
2.Grain boundaries:regions between two adjacent grains slightly disordered low density in grain boundaries high mobility high diffusivity
3.Tilt Boundary: A Tilt Boundary, between two slightlymis-aligned grains appears as an array of edgedislocations. Rotation axis is parallel to the boundary plane
4. Twist Boundary: Rotation axis is perpendicular to the boundary plane Low angle grain boundaries, that appear as an array of Screw dislocations.
5.Twin boundaries: These are the boundaries in the grains at which the atomic arrangement on one side of the boundary is the mirror image of the atoms on the other side . May be produced by shear deformation. The region between the pair of boundaries is called the twinned region.
6.Stacking Faults: Formed by fault in the stacking sequence of atomic planes in crystals.Considering stacking arrangement in FCC: This thin region is a surface imperfection and is called Stacking faults.