Lecture 9/2: Dislocations PHYS 430/603 material Laszlo Takacs UMBC Department of Physics

Edge dislocation The crystal is partially cut, a new half-plane of atoms inserted coherently with the rest of the structure and the bonds between layers re-established. The Burger’s vector is perpendicular to the dislocation line.

Screw dislocation The crystal is partially cut, one side shifted by a lattice parameter, then the bonds are re-established between the two sides. The Burger’s vector is parallel to the line of the dislocation.

Deformation can take place by both edge and screw dislocations

A mixed dislocation. The dislocation line can be bent, the Burger’s vector remains the same along the dislocation.

Dislocation in a compound Dislocation in a compound. There are both an extra Mg and and extra O plane ending at the edge of the dislocation.

Dislocation loop Dislocations can begin and end at the surface of the crystal or at lattice defects. They can also form closed loops. Notice that the Burger’s vector is the same along the loop and its nature is edge or screw depending on the direction of the dislocation line.

Prismatic loop The Burger’s vector is perpendicular to the plane of the loop, thus it is edge dislocation all around. Equivalent to a partial extra lattice plane or a finite area of a lattice plane missing.

Twinning: The crystal continues as its own mirror image beginning with Twinning: The crystal continues as its own mirror image beginning with a lattice plane as the “mirror.” After a few layers of atoms the crystal is may return to the original orientation via another twin boundary, but shifted. Twinning can result in plastic deformation, just like dislocation movement does.

Plastic deformation produced by twinning and by slip

Twinning in an fcc lattice The rectangles represent {1 1 0} planes.

The (-1 1 0) plane in the cubic fcc unit cell and a view of this plane showing the edge of the close-packed planes.

Twinning in brass (optical micrograph)

Twinning in La aluminate (oblique illumination optical image)

Stacking faults and twinning in Si (HRTEM)