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Dislocations

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Slip plane is generally taken as the closest packed plane in the system ... Slip Systems ... Polycrystalline slip requires higher values of applied stresses ... – PowerPoint PPT presentation

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Title: Dislocations


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 Burgers 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
  • l001, 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
    brittle.

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
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 grains 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 atoms 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
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
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