Metals- Disclocation Character and Geometry Flashcards
What is a dislocation?
A line defect in a crystal in which the atoms are systematically displaced from their ideal positions. In 2D a dislocation can be described as an extra half row of atoms in an otherwise perfect structure.
Edge dislocation
An extra half plane of atoms in an otherwise perfect crystal
Screw dislocation
A helical spiral defect in the crystal. Moves like a screw. Is like shearing part of the top face of a cylinder down a bit
What is the movement of edge dislocations like?
Like opening a drawer. Shear stress from bottom of drawer and other side top. Slip parallel to shear stress
What is movement of screw dislocations like?
Like tearing paper. Shear stress at top of one side and bottom of other. Slip perpendicular to shear stress
What is the dislocation line vector?
Symbol l underlined. The line that separates the slipped from the unslipped regions of the crystal.
What is the Burgers vector?
Symbol b underlined. Lattice displacement caused by the dislocation
What is true about the slip plane?
A dislocation can only slip on a plane that contains both its line vector and the Burgers vector.
How to find the Burgers vector using bergers circuit
Find dislocation core (where half line ends). Make integer steps between the lattice points around the dislocation core in a right handed way. Note the number of steps in each direction to find the route around the dislocation. Repeat the same route in a perfect crystal. Don’t end up in the same place (have a closure failure). The vector to fix the closure failure defines the Burgers vector b. The Burgers vector must correspond to a vector between lattice points (perfect dislocation)
Geometry of line and Burgers vectors in edge and screw
Edge: Line vector l is perpendicular to the Burgers vector b
Screw: l parallel to b
Mixed dislocation
Where the dislocation shows both screw and edge type dislocation character along its length
Where can dislocation lines end?
Either at the surface or a grain boundary or they can loop or branch.
Dislocation loops
Can go edge to mixed to screw to mixed to edge (and repeat back to original edge). The Burgers vector b does not change in the loop, only the character of the dislocation changes.
