3. CRYSTAL DEFECTS.
The distortion in the periodic structure of the
individual
grains of crystals are defined as crystal defects. Such as:
• Point Defect
• Line Defect
• Planar Defect
• Volumetric Defect
4. POINT DEFECT.
• “ Point defects are where an atom is missing or at an
irregular place in lattice structure”
In pure metal two types of point defects are possible:
• Intrinsic
• Extrinsic
Intrinsic point defect are of two types:
• Vacancy
• Self Interstitial Atoms
The vacancy has been formed by the removal
of an atom from an atomic site and the interstitials by
introduction of an atom from a non-lattice site
5. Extrinsic point defect is present in the lattice having
impurity atoms. Impurity atom can take up two different
types of sites:
Substitution impurity atom
Interstitial impurity atom
Substitution impurity atom, is an impurity
atom which replace the parent atom in the lattice at lattice
site. Where as, Interstitial impurity atom, is an atom of
impurity at a non-lattice site.
Point defect produce local distortions in
perfect lattice. The amount of distortion depends upon the
amount of space between the atoms and the size of the
atoms.
6.
7. LINE DEFECT.
Defect that is produced due to the misalignment of atoms
in a crystal lattice.
Line defect produces two types of dislocations;
Edge dislocation
Screw dislocation
Edge dislocations are caused by the termination of a
plane of atoms in the middle of a crystal.
The adjacent planes are not straight,
but instead bend around the edge of
the terminating plane so that the crystal structure is
perfectly ordered on either side.
8. If the misalignment shifts a block of ions gradually
downwards or upwards causing the formation of a
screw like deformation, a screw dislocations is
formed.
9. PLANAR DEFECT.
• In planar defect, boundaries or planes are formed,
separate the structure into regions, having the same
crystal structure but different orientation.
• Planar defects include:
Stacking faults
Grain boundaries
Twin boundaries
• Stacking faults
In FCC structure, two types of stacking faults are
present;
.
10. Intrinsic
Extrinsic
The stacking sequence in HCP structure is ABABAB…
So, if the HCP structure is going along as ABABAB
and suddenly switches to ABABABCABAB, there is a
stacking fault present.
• Grain boundaries
The grain boundaries defect may develop when
the arrangement of the atoms in each grain is
identical but with different orientations.
11. Twin boundaries
A twin boundary is a plane, across which there is a
special mirror image miss orientation of the crystal
structure. The twin boundaries can be produced when a
shear force acting along the boundary, causes the atoms
to shift out of position.
12. VOLUMETRIC DEFECTS.
Volumetric defects occur on a much bigger scale than the
rest of the crystal defects.
Voids, porosity and precipitates are the volumetric
defects.
Voids are regions where there are a large number of atoms
missing from the structure. Voids can occur for a number
of reasons.
When voids occur, due to air bubbles which trapped,
when a material solidifies, is commonly called porosity.
Another type of volumetric defect occurs, when impurity
atoms cluster together to form small regions of a different
phase. That is another form of volumetric defect.
