This document discusses different types of crystal twinning, including twined crystals, twin boundaries, twin axis, and types of twinning such as annealing twins and deformation twins. It provides details on how annealing twins form as a result of changes during cooling when one crystal structure becomes unstable and transforms into a more stable structure. Deformation twins form as a result of stress on crystals after formation and are common in HCP and BCC metals. Examples of twinned crystal microstructures in copper are also shown.

1. TWINNING
In this section we will discuss
• Twined Crystals
• Twin Boundaries
• Twin Axis
• Types of Twinning
• Annealing Twins
• Deformation Twins

2. Muhammad Umair
Bukhari
Engr.umair.bukhari@gmail.com
http://onlinemetallurgy.com

3. Crystal Twinning
• 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.

4. 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.

5. Twin Bands In Zinc
Figure : Twin bands in zinc.Figure : Twin bands

6. 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:

7. 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.

8. 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.

9. 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.

10. IRON-IRON CARBIDE
PHASE
DIAGRAM

11. 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.