Material Science Imperfections In Crystals By Sandip Thorat Mechanical Engg. Department SRES, Sanjivani College of Engineering, Kopargaon. (MS) 423603 1
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Material Science
Imperfections In Crystals
By
Sandip Thorat Mechanical Engg. Department
SRES, Sanjivani College of Engineering, Kopargaon. (MS) 423603
1
Imperfections In Crystal –
1. Lattice Vibrations or Phonons or volume defect: – - Vibration of atoms @ their mean position -Frequency of vibration depends on temp. -Specific heat and electrical resistance of material.
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2. Point Defect or zero dimensional Defect:- -Due to Vibration and Temp braking of bonds -Atoms are free from their lattice sites.
• The simplest of the point defect is a vacancy, or vacant lattice site.
• All crystalline solids contain vacancies.
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A)Vacancy: vacant atomic sites in a structure.
B) Self-Interstitials: -"extra" atoms positioned between atomic sites.
Vacancy distortion of planes
self- interstitial
distortion of planes
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C) Substitutional impurity atoms : Foreign material occupies position of parent atom .
D) Frenkel Defect: Cation (being smaller) get displaced to interstitial voids
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E) Schottky defect- Pair of anion and cation vacancies
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• Frenkel Defect --a cation is out of place.
• Shottky Defect --a paired set of cation and anion vacancies.
Shottky Defect:
Frenkel Defect
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A part of line of atoms will be missing from its regular site this missing row –dislocation. A) Edge dislocation:
- extra half-plane of atoms inserted in a crystal structure b (the berger’s vector) is (perpendicular) to dislocation line
3.Line Defects-
Burger’s vector, b: It indicates amount and direction of shift of the lattice on the slip plane. 8
B) Screw dislocation: – Due to shearing of one part of crystal wrt other parallel to cut. – spiral planar ramp resulting from shear deformation – b is (parallel) to dislocation line
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Edge, Screw, and Mixed Dislocations
Adapted from Fig. 4.5, Callister 7e.
Edge
Screw
Mixed
A) Grain Boundaries- – Observed during Solidification of
polycrystaline materials.
4.Planar Defects in Solids
grain boundary surface groove
polished surface
(a)
• are imperfections, • are more susceptible to etching, • may be revealed as dark lines, • change in crystal orientation across boundary.
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B) Twin Boundaries-
– Essentially a reflection of atom positions across the twinning plane.
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C. Stacking Fault:-
Extrinsic stacking fault Intrinsic stacking fault Perfect stacking
B
A
A
B
A A
A
B
A
B
A
A
B
A A
A
B
A
B
A
A
B
A A
A
B A
C C
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B C
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B C
B
A B
A A
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B C C
C
B
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B C C
A A 13
2. Grain Boundaries
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Elastic and Plastic Deformation
Slip:- A permanent displacement of one part of crystal relative to other.
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Twinning:-
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