Question

What are dislocations? 

a) What is a Burgers vector? 

b) What are the orientations of Burgers vectors relative to the dislocation line for a screw and an edge dislocation? 

c) Sketch an edge dislocation (ideally in 2D).

Answer 1

Dislocations are a type of crystallographic defect found in crystalline systems. In a dislocated crystal, the atoms are out of position in the crystalline lattice. Dislocation affects the properties of the crystal. There are two major type of dislocations namely edge dislocation and screw dislocation. Edge dislocations are mainly due to the presence of an extra half plane atoms which distorts the nearby atomic planes. Edge dislocation moves in the same direction of applied stress. Whereas screw dislocation moves in the opposite direction of applied stress. In screw dislocation the atoms are arranged in the helical plane normal to the direction of stress.

a) Burgers vectors: Burgers vector is the representation of magnitude and direction of distortion in a crystal due to crystallographic defect. It is usually represented by the letter ‘b’. For BCC and FCC crystals the magnitude of ‘b’ is usually represented by the following equation.

b) the following figure shows the direction of Burger vector direction along edge and screw dislocations.

c) Schematic representation of edge dislocation