Deformation twinning

When a stress is applied to a crystalline material deformation occurs. The deformation can be absorbed by the material in several ways:

Slip
Crystal planes slide past each other by means of dislocations gliding through the material. Slip is an irreversible deformation. (In this animation, the actual dislocations are not shown, and all that is depicted is the fact that the strain is localised to specific planes, on which "sliding" occurs.)

Deformation twinning
Each atom moves only a small distance relative to its neighbour but in a well defined way. (In this animation, the motion of atoms is shown taking place relatively slowly, whereas in practice it is very fast - ie at approximately the speed of sound in the material)

Click on either diagram to see more

 

Slip

The animation shows how planes in a crystal structure are moved by a dislocation passing through the material. The dislocation itself is not shown. Many dislocations passing through a material leads to irreversible plastic deformation. More information on dislocations can be found here: Dislocation TLP

Slip in an fcc crystal structure

Slip Plane
(111)

Slip direction

110 planes

Twinning

Deformation twinning is described by the twinning plane and a twinning direction. Atoms move in the twinning direction by a displacement, generating twinning shear. The twin's structure is a mirror image of the parent structure about the twinning plane. The distance moved is such that a fully coherent boundary is formed, that is each atom is in the correct position related to the original system and the twin lattice.

Twinning in an fcc crystal structure

Twinning Plane
(111)

Twinning direction
(112)

110 plane

     Lattice points
Arrows

The shape of the twin is governed by the elastic strain which developes as it grows within the parent grain. The twin grows in a thin, sharply pointed plate as this minimises the elastic strain energy producing a lenticular shaped twin. This lenticular shape differs from annealing twins which form parallel sided twins to minimise interfacial energy.

Deformation twins

Annealing twins

Sample of cold worked Zn showing deformation twinning as an alternative to dislocation motion. Twinning is commonly observed in structures of low crystallographic symmetry (eg. insufficient number of independant slip systems) such as hcp metals like Zn. (Micrograph Library sample 41)

Sample of Brass (Cu-30wt% Zn) showing annealing twins produced as a result of recrystalisation and grain growth. Recrystallisation (at high temperatures) allows sufficiant time for a diffusional rearrangement of the atoms to form twins with parallel sides (Micrograph Library sample 430)