Transcript Dislocations - Jwave.vt.edu

Dislocations
Basic concepts
edge dislocation
screw dislocation
Characteristics of Dislocations
lattice strains
Slip Systems
slip in single crystals
polycrystalline deformation
Twinning
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.
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
Edge Dislocation
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
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.
Screw Dislocation
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
Screw Dislocation
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.
Mixed Dislocations
Mixed Dislocations
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.
Characteristics of
Dislocations
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.
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.
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 brittle.
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
Slip in Single Crystals
 R   cos cos
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.
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
Polycrystalline
Deformation
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
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]
Twinning
Slip
Twinning
orientation of atoms
remains the same
reorientation of atomic
direction across twin plane
displacements take place atomic displacement is less
in exact atomic spacings than interatomic spacing
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.
Twinning