TWINNING

In this section we will discuss– Twined Crystals– Twin Boundaries– Twin Axis– Types of Twinning

Annealing Twins Deformation Twins

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CrystalTwinning

Crystal Twinning occurs when two separate crystals share some of the same crystal lattice points in a symmetrical manner.

The result is an intergrowth of two separate crystals in a variety of specific configurations.

So we can say that some crystals have two parts symmetrically related to one another, these crystals are known as Twinned Crystals.

A twin boundary or composition surface separates the two parts of twinned crystals.

Twin Boundaries

Twin boundaries occur when two crystals of the same type intergrow, so that only a slight misorientation exists between them. It is a highly symmetrical interface, often with one crystal the mirror image of the other; also, atoms are shared by the two crystals at regular intervals.

This is also a much lower-energy interface than the grain boundaries that form when crystals of arbitrary orientation grow together.

Two main kinds of twinning are distinguished depending on whether the symmetry operation is 180° rotation about an axis called twin axis or reflection across a plane called twin plane.

Twin boundaries are visible as striations within each crystallite, most prominently in the bottom-left and top-right.

Twin Bands In Zinc

Figure : Twin bands in zinc.Figure : Twin bands

Types of Twinned Crystals

There are generally two types of twinned crystals as follows:– Annealing Twins or Transformation Twins,– Deformation Twins or Mechanical Twins or

Gliding Twins.

Let us discuss them one by one:

Annealing Twins

Annealing or Transformation twins are the result of a change in crystal system during cooling as one form becomes unstable and the crystal structure must re-organize or transform into another more stable form.

Annealing twins, most prevalent in Face Centered Cubic (FCC) metals e.g. aluminum, copper, brass & iron with austenite etc.These metals have been previously worked and heat treated. The twins are formed because of a change in the normal growth mechanism.

Deformation Twins

Deformation or mechanical twins or gliding twins are most prevalent in Close Packed Hexagonal (HCP) metals e.g. Mg, Zn & Be etc. and Body Centered Cubic (BCC) metals e.g. α-Fe &W etc.

Deformation twins are the result of stress on the crystal after the crystal has formed. Deformation twinning is a common result of regional metamorphism.

Micro Structural View of Cu Shows Twinning

Figure : Cross-sectional view of the copper microstructure showing annealing twins, deformation twins, and circular

inclusions.

Figure : Longitudinal view of the copper microstructure showing annealing twins, deformation twins and

elongated inclusions.

IRON-IRON CARBIDE PHASE

DIAGRAM

IRON-IRON CARBIDE PHASE DIAGRAM

A physically distinct & homogeneous portion of a material system is called Phase.

A graphical representation of temperature & composition limits of phase fields in an alloy system as they exist under conditions of complete equilibrium is called Phase Diagram OR Equilibrium Diagram.