Deformation by Slip
Slip is defined as sliding of blocks of crystal over one another along definite crystallographic planes called slip planes. So it is the relative displacement along a definite direction. When slip takes place, one part of the lattice moves with respect to the other. Generally slip plane is the plane of highest atomic density and slip direction is the closest packed direction within the slip plane. This is because the bond between these planes is weakest, so when force is applied in proper direction, relative movement takes place very easily.
The shear stress required to produce slip on a crystal plane is called the critical resolved shear stress. A slip plane and a slip direction together make a slip system. There are twelve slip systems in each of FCC and BCC crystals, while there are only three in HCP crystal.
Slip planes are generally fixed for every type of lattice. In FCC Lattice (111) plane, in BCC lattice (110) plane and in HCP lattice (001) planes are the most common slip planes. As the magnitude of the applied stress increases, the number of active slips planes and the distance of slip along these planes increases. The extent of slip is limited to an interatomic distance or an integral multiple of that distance. Slip in all metals of similar crystal structure will occur along the same crystallographic planes and directions. The slip on each plane may be of the order of several microns.
Deformation by Twinning
Twinning is the process by which a portion of the crystal takes up an orientation which makes that portion a mirror image of the parent crystal. In order to form a mirror image across the twin plane, the amount of movement of each plane of atom in the twinned region should be proportional to its distance from the twinning plane.
Twinning planes are mostly fixed like that of slip planes. In FCC (111) plane is the twinning plane, in BCC lattice (112) plane is the twinning plane and in HCP lattice (112) plane is the twinning plane.
If a shear stress is applied, the crystal will turn about the twinning plane. The region to the left of the twinning plane is undeformed. To the right of this plane, the planes of atoms have sheared in such a way so that the lattice makes a mirror image across the twin plane.
Twins may be of the following two types:
(a) Mechanical Twins. These are produced by mechanical deformation. These are produced in BCC or HCP metals under conditions of rapid rate of cooling and decreased temperature.
(b) Annealing Twins. Twins which forms during the process of recrystallisation are called annealing twins. These are produced as a result of annealing. That is why these are called annealing twins.