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.

DIFFERENCE BETWEEN SLIP AND TWINNING

DIFFERENCE BETWEEN SLIP AND TWINNING

	Slip (1)	Twinning (2)
(i) Orientation	The orientation of the crystal above and below  the  slip  plane remains same after deformation.	Twin causes orientation  difference in the crystal across  the twin plane.
(ii) Movement  during the process	Slip usually occurs in discrete  multiples of atomic spacing.	The movement of atoms are  much smaller in comparison  to atomic spacing.
(iii) deformation  planes and directions	Slip occurs on relatively wide planes.Usually, the slip plane is the plane of greatest atomic  density,and the slip direction  is the closest packed direction  within the slip plane.	In twinning, every atomic plane is involved in the deformation in the   twinned   region   of   the crystal. For each crystal twin occurs in a  definite direction on a specific  crystallographic plane
(iv) time	Slip takes several million seconds to occur, i.e. there is a delay time  of several million seconds prior to forming of one slip band.	Twins can form in a few micro-seconds only.
(v) stress	Slip takes place when the shearing  stress on the slip plane in the slip  direction reaches a threshold value  called the critical resolved  shear stress.	Resolved critical shear stress in  twinning almost has no role.
(vi) Number	Slip lines may be present in even or  odd numbers.	Twin lines always occur in pairs.