This document discusses various types of dislocations and deformation mechanisms in crystalline materials. It describes edge dislocations, which involve an extra half-plane of atoms, and screw dislocations, where motion is perpendicular to the stress direction rather than parallel. Most dislocations exhibit both edge and screw characteristics. Dislocations create lattice strains and increase dramatically in number during plastic deformation. Materials deform through slip systems, which depend on crystal structure, involving the motion of dislocations on preferred crystallographic planes in certain directions. Polycrystalline materials require higher stresses than single crystals to deform due to constraints between grains. Twinning is another deformation mechanism that occurs in BCC and HCP crystals under high shear stresses.

1. Dislocations
Basic concepts
edge dislocation
screw dislocation
Characteristics of Dislocations
lattice strains
Slip Systems
slip in single crystals
polycrystalline deformation
Twinning

2. Edge Dislocation
In edge dislocations, distortion exists
along an extra half-plane of atoms. These
atoms also define the dislocation line.
Motion of many of these dislocations will
result in plastic deformation
Edge dislocations move in response to
shear stress applied perpendicular to the
dislocation line.

3. Edge Dislocation
As the dislocation moves, the extra half
plane will break its existing bonds and
form new bonds with its neighbor opposite
of the dislocation motion.
This step is repeated in many discreet steps
until the dislocation has moved entirely
through the lattice.
After all deformation, the extra half plane
forms an edge that is one unit step wide
also called a Burger’s Vector

4. Edge Dislocation

5. Edge Dislocation Examples
Ni-48Al alloy edge dislocation
the colored areas show the varying values of
the strain invariant field around the edge
dislocation
Shear was applied so that glide will occur to
the left.
Computer simulation

6. Screw Dislocation
The motion of a screw dislocation is also
a result of shear stress.
Motion is perpendicular to direction of stress,
rather than parallel (edge).
However, the net plastic deformation of both
edge and screw dislocations is the same.
Most dislocations can exhibit both edge
and screw characteristics. These are
called mixed dislocations.

7. Screw Dislocation

8. Screw Dislocation
Examples
Ni-48Al alloy
l=[001], [001](010) screw dislocation showed
significant movement.
Although shear was placed so that the dislocation
would move along the (010) it moved along the
(011) instead.
Computer simulation

9. Screw Dislocation

10. Mixed Dislocations
Many dislocations have both screw and
edge components to them
called mixed dislocations
makes up most of the dislocations
encountered in real life
very difficult to have pure edge or pure screw
dislocations.

11. Mixed Dislocations

12. Mixed Dislocations

13. Characteristics of
Dislocations
Lattice strain
as a dislocation moves through a lattice, it
creates regions of compressive, tensile and
shear stresses in the lattice.
Atoms above an edge dislocation are squeezed
together and experience compression while atoms
below the dislocation are spread apart abnormally
and experience tension. Shear may also occur
near the dislocation
Screw dislocations provide pure shear lattice
strain only.

14. Characteristics of
Dislocations

15. Characteristics of
Dislocations
During plastic deformation, the number of
dislocations increase dramatically to
densities of 1010
mm-2
.
Grain boundaries, internal defects and
surface irregularities serve as formation
sites for dislocations during deformation.

16. Slip Systems
Usually there are preferred slip planes
and directions in certain crystal systems.
The combination of both the slip plane
and direction form the slip system.
Slip plane is generally taken as the closest
packed plane in the system
Slip direction is taken as the direction on the
slip plane with the highest linear density.

17. Slip Systems
FCC and BCC materials have large
numbers of slip systems (at least 12) and
are considered ductile. HCP systems
have few slip systems and are quite

18. Slip in Single Crystals
Even if an applied stress is purely tensile,
there are shear components to it in
directions at all but the parallel and
perpendicular directions.
Classified as resolved shear stresses
magnitude depends on applied stress, as well
as its orientation with respect to both the slip
plane and slip direction

19. Slip in Single Crystals
λφστ coscos=R

20. Polycrystalline
Deformation
Slip in polycrystalline systems is more
complex
direction of slip will vary from one crystal to
another in the system
Polycrystalline slip requires higher values
of applied stresses than single crystal
systems.
Because even favorably oriented grains
cannot slip until the less favorably oriented
grains are capable of deformation.

21. Polycrystalline
Deformation
During deformation, coherency is
maintained at grain boundaries
grain boundaries do not rip apart, rather they
remain together during deformation.
This causes a level of constraint in the
grains, as each grain’s shape is formed by
the shape of its adjacent neighbors.
Most prevalent is the fact that grains will
elongate along the direction of deformation

22. Polycrystalline
Deformation

23. Dislocation Movement
across GBs
As dislocations move through polycrystalline materials,
they have to move through grains of different
orientations, which requires higher amounts of energy, if
the grains are not in the preferred orientation.
As they travel between grains they must be emitted
across the grain boundary, usually by one half of a
partial dislocation, and then annihilated by the second
half at a time slightly after the first one.
LINK TO HELENA2.gif

24. Twinning
A shear force which causes atomic
displacements such that the atoms on one
side of a plane (twin boundary) mirror the
atoms on the other side.
Displacement magnitude in the twin region is
proportional to the atom’s distance from the
twin plane
takes place along defined planes and
directions depending upon the system.
Ex: BCC twinning occurs on the (112)[111] system

25. Twinning
Slip Twinning
orientation of atoms
remains the same
reorientation of atomic
direction across twin plane
displacements take place
in exact atomic spacings
atomic displacement is less
than interatomic spacing

26. Twinning
Properties of Twinning
occurs in metals with BCC or HCP crystal
structure
occurs at low temperatures and high rates of
shear loading (shock loading)
conditions in which there are few present slip
systems (restricting the possibility of slip)
small amount of deformation when compared
with slip.

27. Twinning