In this section we will discuss
• Twined Crystals
• Twin Boundaries
• Twin Axis
• Types of Twinning
• Annealing Twins
• Deformation Twins
• Crystal Twinning occurs when two separate crystals share
some of the same crystal lattice points in a symmetrical
• 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 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 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
• 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
• Deformation twins are the result of stress on the
crystal after the crystal has formed. Deformation
twinning is a common result of regional
Micro Structural View of Cu
Figure : Cross-sectional view of the copper microstructure
showing annealing twins, deformation twins, and circular
Figure : Longitudinal view of the copper microstructure
showing annealing twins, deformation twins and
IRON-IRON CARBIDE PHASE
• 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.