Twinning

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Twinning

  1. 1. TWINNING In this section we will discuss • Twined Crystals • Twin Boundaries • Twin Axis • Types of Twinning • Annealing Twins • Deformation Twins
  2. 2. Muhammad Umair Bukhari Engr.umair.bukhari@gmail.com http://onlinemetallurgy.com
  3. 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. 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. 5. Twin Bands In Zinc Figure : Twin bands in zinc.Figure : Twin bands
  6. 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. 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. 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. 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. 10. IRON-IRON CARBIDE PHASE DIAGRAM
  11. 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.

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