Twinning is a prominent mechanism of plastic deformation in some metallic materials. It is a shear force that can produce atomic displacements.
Twinning may be produced by mechanical deformation or as the result of annealing following plastic deformation. High strain rates, low stacking-fault energy and low temperatures facilitate deformation twinning. Twinning is polar (unidirectional), while slip usually is not.
Twinning is also known as mechanical twinning.
Twinning results when a portion of a crystal takes up an orientation that is related to the orientation of the untwinned lattice in a definite, symmetrical way. The mechanical work of deformation twinning is dissipative, resulting from the defect motion associated with shearing. The important role of twinning in plastic deformation is that it causes changes in plane orientation so that further slip can occur.
Mechanical twinning occurs in metals that have body-centered cubic (bcc) and hexagonal close packed (hcp) crystal structures, at low temperatures, and at high rates of loading (shock loading), conditions under which the slip process is restricted. They are most likely to form deformation twins when strained, because they rarely have a sufficient number of slip systems for an arbitrary shape change.
The quantity of bulk plastic deformation from twinning is relatively small compared to slip. However, the real importance of twinning lies with the accompanying crystallographic reorientations; twinning may place new slip systems in orientations that are favorable relative to the stress axis such that the slip process can take place.
Twinning differs from slip in several respects. First, for slip, the crystallographic orientation above and below the slip plane is the same both before and after the deformation; for twinning, there is a reorientation across the twin plane. In addition, slip occurs in distinct atomic spacing multiples, whereas the atomic displacement for twinning is less than the interatomic separation. Slip occurs on relatively widely spread planes, but in the twinned region of a crystal every atomic plane is involved in the deformation.