TWINNING

                   Twinning is the movement of atoms in the lattice, resulting in a division of lattice into two parts which are symmetrical but differently oriented. The amount of movement of each plane in the twinned region is proportional to its distance from the twinning plane and therefore a mirror image is formed across the twin plane.

                 Deformation by twinning is most common in HCP metals and its effect on others is to move parts of the grains such that they acquire favorable orientation for the slip to occur. Twinning is produced suddenly and is accompanied with sound; the well known cry of Tin is the result of twinning.

Twins may be of following two types;

(1) Mechanical twins: These are produced by mechanical deformation. These are produced in BCC or HCP metals under conditions of rapid rate of loading (i.e., shock loading ) and decreased temperature. However, FCC metals are not ordinarily considered to deform by mechanical twinning although gold-silver alloys twin readily at low temperature. Mechanical twins have been produced at 4K.

(2) Annealing twins: These are produced as a result of annealing following plastic deformation.