Lecture 8 Dislocations PHYS 430603 material Laszlo Takacs

Lecture 8: 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.

Dislocation on the surface of a real crystal

Sign convention for the Burger’s vector Decide the positive direction for the dislocation line. Go around the line in a complete loop in the positive direction. Draw a loop with the same number of steps in a defect-free lattice. The vector required to complete the loop is the Burger’s vector.

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.

Dislocations are the primary carriers of plastic deformation.

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. There are both an extra Mg 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.