defects in crystalline materials
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Defects In Crystalline Materials.
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Defects In Crystalline Materials.
Metals, many ceramics and some polymers are crystalline.
Real crystals contain various types of imperfections.
Many of the important properties of materials are due to the presence of these imperfections.
Types Of Defects.
Defects are categorized as:
1) Point defects.
2) Surface defects.
3) Volume defects.
4) Line defects.
Point Defects.
All the atoms in a perfect lattice are at specific atomic site.
In a pure metal two types of point defects are possible.
1) Vacancies.
2) Interstitial point defects.
Vacancies.
Vacant atomic sites in a structure.
Due to plastic deformation & high energy particle irradiation.
Interstitial Point Defects.
“Extra" atoms positioned between atomic sites.
Due to plastic deformation & high energy particle irradiation Interstitial
atom
distortionof planes
Extrinsic Point Defects.
Introduction of outsider atom. They play an important role in physical &
mechanical properties. Impurity atom can take two different sites
in crystal. Substitutional or Interstitial.
vacancy Interstitialimpurity
Substitutionalimpurity
Point Defects
In metallic solids defects are not so important but in ionic solids presence of a vacant site (e.g. in NaCl), changes the electrical properties.
In order to conserve the overall neutral charge vacancy must be occurred either in pairs of opposite sign forming die-vacancies called Schottky defects.
In association of the interstitial of same ion called Frenkel defects.
Schottky & Frenkel Defects
Frenkel defect
Schottky defect
Defects in ionic solidsDefects in ionic solids
Cation vacancy+
cation interstitial
Cation vacancy+
anion vacancy
Stacking Faults.
Stacking fault is a planner defect and formed when regular sequence of stacking is disturbed.
Stacking faults are not may observed In planes where the stacking sequence is ABABAB….because there is no alternative site for “A” layer other than resting on a “B” layer of vice versa.
In ABCABABC….layers it is visible.
AB
C
Types Of Stacking Faults.
1) Intrinsic stacking faults.
2) Extrinsic stacking faults.
Intrinsic Stacking Faults.
Removal of a layer from an arranged sequence layer.
E.g. CBACBA…..becomes CBABAC….
Extrinsic Stacking Faults.
Addition of layer to the arranged sequence layer.
E.g. BACBAC….becomes BACABAC…
Grain Boundaries.
A Grain Boundary is a general planar defect that separates regions of different crystalline orientation (i.e. grains) within a polycrystalline solid.
The atoms in the grain boundary will not be in perfect crystalline arrangement
Grain Boundaries Cont...
Grain boundaries are usually the result of uneven growth when the solid is crystallizing.