Twinning

—The second important mechanism of plastic deformation is twinning.

—It results when a portion of crystal takes up an orientation that is related to the orientation of the rest of the untwined lattice in a definite, symmetrical way.

—The twinned portion of the crystal is a mirror image of the parent crystal. The plane of symmetry is called twinning plane.

—Each atom in the twinned region moves by a homogeneous shear a distance proportional to its distance from the twin plane.

The lattice strains involved in twinning are small, usually in order of fraction of inter-atomic distance, thus resulting in very small gross plastic deformation.

—The important role of twinning in plastic deformation is that it causes changes in plane orientation so that further slip can occur.

—Twinning generally occurs when slip is restricted, because the stress necessary for twinning is usually higher than that for slip.

—Thus, some HCP metals with limited number of slip systems may preferably twin.

—Also, BCC metals twin at low temperatures because slip is difficult.

—Of course, twinning and slip may occur sequentially or even concurrently in some cases.