a
b
prism
hcp
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Dislocation Density & Characteristics
200 nm
Dislocation density (
)
: Length of dislocation line per unit volume
(
cm/cm
3
) ;
Avg. spacing ≈1/
Example
:
Dislocation density in high strength steel
10
12
cm/cm
3
Dislocation density in Si used in IC
1-10
cm/cm
3
TEM photomicrograph of
dislocations in Ni-base superalloy
Rules
:
Dislocations do not end inside the
crystal; hence, dislocation loops inside
An
edge
dislocation is perpendicular to
b
(Burgers vector)
Edge
dislocation moves in slip plane in
the direction of
b
A
screw
dislocation is parallel to
b
Screw
dislocation moves in slip plane
perpendicular to
b
Edge
dislocation movement
is
confined
in a unique plane
Screw
dislocation can glide in any
direction, as long as it moves parallel to
its original direction, hence,
cross slip
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