Week5 Defacts In Crystalline Structure @ Www.07 Met.TkPresentation Transcript
Twinning in Crystals Sometimes during the growth of a crystal, or if the crystal is subjected to stress or temperature/pressure conditions different from those under which it originally formed, two or more intergrown crystals are formed in a symmetrical fashion. These symmetrical intergrowths of crystals are called twinned crystals. Twinning is important to recognize, because when it occurs, it is often one of the most diagnostic features enabling identification of the mineral. What happens is that lattice points in one crystal are shared as lattice points in another crystal adding apparent symmetry to the crystal pairs. Twinning, because it adds symmetry, never occurs in relation to the existing symmetry of the crystal.
Symmetry Operations that Define Twinning
Because symmetry is added to a crystal by twinning, twining can be defined by the symmetry operations that are involved. These include:
Reflection across a mirror plane. The added mirror plane would then be called a twin plane .
Rotation about an axis or line in the crystal. The added rotation axis would then be called a twin axis .
Inversion through a point. The added center of symmetry would then be called a twin center .
Most crystalline metals and alloys are produced by solidification from the melt. On the one hand, this is because in the molten state alloying components can be best mixed. On the other hand, casting is an easy way to give the workpiece the desired shape. The grain structure and texture of the solid are mainly determined by the conditions of solidification. Generally, cast metals exhibit three distinct zones of grain structures chill zone (I), a columnar zone (II) and an equiaxed zone (III). In the columnar zone the crystallites are elongated in the direction of heat flow. The equiaxed zones I and III are found in the rim and central region of the cast ingots, respectively. While the texture of zones I and III is random it is a fibre texture in zone II. In cubic crystals <100> has been reported as direction of rapid growth along the direction of heat flow. The grain structure, texture and size of the columnar zone determines the anisotropy of the material properties, such as the mechanical and magnetic properties. Therefore, in order to optimize the material properties for certain applications it is necessary to study the cast structures and textures.
Formation of Grains
from a molten state:
The growth starts from the nuclei of crystallization, and the crystals grow toward each other ( A-E ).
When two or more crystals collide, their growth is stopped.
Finally, the entire space is filled with crystals ( F ).
Each growth crystal is called a “ grain ”. Grains contact each other at “ grain boundaries ”.
In general, the smaller the grain size of the metal, the better its physical properties.
Control of Grain Size
Number of nuclei of crystallization
The more rapidly the liquid state can be changed to the solid state, the smaller or finer the grains will be.
Rate of crystallization
If the crystals form faster than do the nuclei of crystallization, the grains will be larger.
Slow cooling results in large grains.
The shape of the grains may be influenced by the shape of the mold in which the metal solidifies.
Structure of Cast Metals/Alloys
Cast Structures of Metals Schematic illustration of three cast structures of metals solidified in a square mold: (a) pure metals; (b) solid-solution alloys; and (c) structure obtained by using nucleating agents. Source : G. W. Form, J. F. Wallace, J. L. Walker, and A. Cibula.