engr 313 - chapter 4 - dislocations

8/9/2019 ENGR 313 - Chapter 4 - Dislocations

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Chapter 4Imperfections in the

Atomic Arrangements

Crystal Structure and Direction

of Planes


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Introduction

All materials contain imperfections in the

arrangement of the atoms in their lattice structure

We classify them by geometric type -- 0-D (point - vacancies interstitials etc!"

#-D (line - dislocations"

$-D (surface - surfaces grain boundaries etc!"

%-D (bul& - crac&s holes etc!"


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Crystalline Imperfections

Defects in metals and ceramics prevent

materials from achieving their ideal strength

Common defects'a"vacancies

b"solute atoms on

interstitial and

substitutional sites

c"dislocations

d"grain boundaries


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Ideal Strength

A bond is bro&en if it is

stretched beyond about

#0 of its original

length ) therefore the

force needed

to brea& a bond isroughly:

Ideally the strength of a

material is the force

necessary to brea&

inter-atomic bonds

Stress-strain curve for a single

atomic bond


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*urther calculations that accountfor the curvature of the

force-distance curve predict

a ratio of #+#,


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Importance of Dislocations

If lattices ere perfect then deformation ouldre.uire an e/tremely large force for all atoms on

one plane to slide over the atoms on the plane

belo it! heoretical force re.uired is 1 2+#0!

he typical force re.uired to cause deformation

is 1 2+#000! Dislocations are the cause for this

much loer deformation force!


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Point Imperfections

Point defects are locali3ed disruptions of the lattice involving one orseveral atoms

Vacancies- an atom missing from a normal site

nv4 5oe/p ( -6+7"

Interstitial - formed hen an e/tra atom is inserted into the latticestructure at a normally unoccupied position

Substitutional - formed hen one atom is replaced by a different type of

atom - the replacement atom can be smaller or larger

Frenkel- a vacancy-interstitial pair

Schottky- a pair of vacancies (anion-cation" in an ionic lattice


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Importance of Point Defects

Point defects disturb the perfect arrangementof the surrounding atoms often for several

hundreds of atom spacings from the actual

point defect! A dislocation moving through thegeneral vicinity of a point defect encounters a

lattice in hich the atoms are not at their

e.uilibrium positions! his disruption re.uiresthat a higher stress be applied to force the

dislocation past the defect therefore

increasing the strength of the material!


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Point Imperfections (8/amples"

(a" vacancy (b" interstitial (c" small substitutional (d" large substitutional (e" *ren&el defect and (f" Schott&y

defect!


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Point Imperfections (8/amples"


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Dislocations (9ine Imperfections"

ypes of dislocations - 8dge Scre :i/ed

he ;urgers (b" vector defines the direction

and distance that the dislocation moves in

each step

8dge Dislocation

Scre Dislocation


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8dge Dislocation

An edge dislocation is formed from an e/tra half plane of atoms

inserted into a perfect lattice! he line along the bottom of the half

plane is the dislocation! 8dge dislocations are called either positiveor negative depending on hether the e/tra half plane is above or

belo the slip plane!

;urgers vector is perpendicular to dislocation and parallel to the slip

direction!


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8dge Dislocations

An edge dislocation is an e/tra half-plane of atoms in the crystal )

in the figure the upper part of the crystal has one more

double-layer of atoms than the loer part ) dislocations distort

the lattice and ma&e metals soft and ductile


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8dge dislocation shoing ;urgers

vector

Movie


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Scre Dislocation

A perfect crystal that is cut and sheared one atom spacing produces a

scre dislocation! he line along hich the shearing occurs is the

scre dislocation! Scres dislocations are either right-hand or left-handdepending on hich direction you follo along the spiral to reach the old

plane!

;urgers vector is parallel to dislocation in order for scre dislocation to

propagate! Dislocation moves in direction perpendicular to the ;urgers

vector!


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Scre dislocation shoing ;urgers

vector

Movie


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Dislocation :ovement

Dislocations move in the closest pac&ed direction

and plane (slip system" available in the lattice

Why

engr 313 - chapter 4 - dislocations

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