dislocations zbasic concepts yedge dislocation yscrew dislocation zcharacteristics of dislocations...
TRANSCRIPT
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 deformationEdge 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 widealso 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 lifevery 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
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Polycrystalline Deformation
Slip in polycrystalline systems is more complex direction of slip will vary from one crystal to
another in the systemPolycrystalline 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] system
Twinning
Slip Twinning
orientation of atomsremains the same
reorientation of atomicdirection across twin plane
displacements take placein exact atomic spacings
atomic displacement is lessthan 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