TWINNINGIn this section we will discuss – Twined Crystals – Twin Boundaries – Twin Axis – Types of Twinning Annealing Twins Deformation Twins
Muhammad Umair Bukhari Engr.email@example.com www.bzuiam.webs.com 03136050151
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 Twin boundaries are visible as striations arbitrary orientation grow together. within each crystallite, most prominently Two main kinds of twinning are distinguished in the bottom-left and top-right. depending on whether the symmetry operation is 180 rotation about an axis called twin axis or reflection across a plane called twin plane.
Twin Bands In ZincFigure : Twin bands Figure : Twin bands in zinc.
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 TwinningFigure : Cross-sectional view of the copper microstructure Figure : Longitudinal view of the copper microstructureshowing annealing twins, deformation twins, and circular showing annealing twins, deformation twins and inclusions. elongated inclusions.
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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