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Material Science Imperfections In Crystals By Sandip Thorat Mechanical Engg. Department SRES, Sanjivani College of Engineering, Kopargaon. (MS) 423603 1

Crystal imperfections

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Material Science

Imperfections In Crystals

By

Sandip Thorat Mechanical Engg. Department

SRES, Sanjivani College of Engineering, Kopargaon. (MS) 423603

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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

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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

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A B

A A

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B C C

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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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