5: Dislocations

Screw Dislocations

• A fundamental type of dislocation relevant to plastic deformation.

Definition and Relation to Burgers Vector

• In screw dislocations, the sense vector is parallel to the Burgers vector unlike in edge dislocations where they are perpendicular.

• The Burgers vector is defined through a Burgers circuit, similar to edge dislocations:

  • Start from a position, move down, over, back up, and back over to complete the circuit.

  • The displacement vector connecting start and finish positions establishes the Burgers vector.

Visual Representation

• Atoms in the crystal exhibit fixed spacing. A slipped region can be visualized where this dislocation occurs.

• Drawing a Burgers circuit involves the following movements:

  • Down: (2 units)

  • Over: (3 units)

  • Up: (2 units)

  • Back Over: (3 units)

• The resultant Burgers vector connects the start and end points of this circuit, indicating the direction of displacement.

Shear Movement

• In a screw dislocation, shear can still occur:

  • Visualize pushing the top surface back while pulling the bottom surface towards you.

  • This action causes relative displacement of the top plane to the ones beneath it.

  • Dislocation can move in different directions depending on the applied shear force.

Types of Screw Dislocations

• Right-Handed Screw Dislocation:

  • Sense vector and Burgers vector are in the same orientation.

  • Example illustrated as both vectors are directed into the board.

• Left-Handed Screw Dislocation:

  • Sense vector is parallel but oriented opposite to the Burgers vector.

  • Despite parallel alignment, they differ in orientation (e.g., sense vector into the board, Burgers vector out of the board).

Summary

• Screw dislocations allow for significant ductility in materials, which is essential for understanding plastic deformation in crystals.

• The right-handed and left-handed designations help clarify the relationship between the sense vector and Burgers vector.