Dislocations zBasic concepts yedge dislocation yscrew dislocation zCharacteristics of Dislocations ylattice strains zSlip Systems yslip in single crystals ypolycrystalline deformation zTwinning

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

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

Edge Dislocation

Edge Dislocation Examples zNi-48Al alloy edge dislocation ythe colored areas show the varying values of the strain invariant field around the edge dislocation yShear was applied so that glide will occur to the left. yComputer simulationComputer simulation

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

Screw Dislocation

Screw Dislocation Examples zNi-48Al alloy yl=[001], [001](010) screw dislocation showed significant movement. xAlthough shear was placed so that the dislocation would move along the (010) it moved along the (011) instead. xComputer simulationComputer simulation

Screw Dislocation

Mixed Dislocations zMany dislocations have both screw and edge components to them ycalled mixed dislocations ymakes up most of the dislocations encountered in real life xvery difficult to have pure edge or pure screw dislocations.

Mixed Dislocations

Characteristics of Dislocations zLattice strain yas a dislocation moves through a lattice, it creates regions of compressive, tensile and shear stresses in the lattice. xAtoms 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 xScrew dislocations provide pure shear lattice strain only.

Characteristics of Dislocations

zDuring plastic deformation, the number of dislocations increase dramatically to densities of mm -2. zGrain boundaries, internal defects and surface irregularities serve as formation sites for dislocations during deformation.

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

Slip Systems zFCC 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 zEven if an applied stress is purely tensile, there are shear components to it in directions at all but the parallel and perpendicular directions. yClassified as resolved shear stresses ymagnitude depends on applied stress, as well as its orientation with respect to both the slip plane and slip direction

Slip in Single Crystals

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

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

Polycrystalline Deformation

Dislocation Movement across GBs zAs 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. zAs 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. zLINK TO HELENA2.gif

Twinning zA 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. yDisplacement magnitude in the twin region is proportional to the atom’s distance from the twin plane ytakes place along defined planes and directions depending upon the system. xEx: BCC twinning occurs on the (112)[111] system

Twinning

zProperties of Twinning yoccurs in metals with BCC or HCP crystal structure xoccurs at low temperatures and high rates of shear loading (shock loading) xconditions in which there are few present slip systems (restricting the possibility of slip) ysmall amount of deformation when compared with slip.

Twinning