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For each of edge, screw, and mixed dislocations, cite the...

Key Concepts

Peach-Koehler Force

The Peach-Koehler force describes the force experienced by a dislocation when an external stress field is applied. This force is given by the cross product of the Burgers vector and the applied stress, indicating that only the resolved component of the shear stress contributes to the motion of the dislocation. It provides a framework for predicting the glide direction and velocity of dislocations based on the applied stress orientation relative to the dislocation line and Burgers vector.

Burgers Vector

The Burgers vector is a fundamental concept in dislocation theory, representing both the magnitude and the direction of the lattice distortion caused by the dislocation. It plays a crucial role in determining the slip direction and the nature of the dislocation. In the context of applied shear stress, the orientation of the Burgers vector relative to the dislocation line dictates how the dislocation will respond, with different geometries for edge, screw, and mixed dislocations influencing their mobility and the direction of motion.

Screw Dislocation

A screw dislocation results from a shear distortion in the crystal, with atoms displaced in a spiral manner around the dislocation line. Its Burgers vector is parallel to the dislocation line, so the movement of a screw dislocation is directly along the direction of the Burgers vector when a shear stress is applied. This directs the dislocation’s motion along the slip plane, and in many cases, allows the dislocation to glide without the constraints imposed by an extra half-plane.

Edge Dislocation

An edge dislocation is characterized by an extra half-plane of atoms inserted in the crystal structure. Its Burgers vector is perpendicular to the dislocation line, meaning that the dislocation moves when a shear stress is applied in a direction parallel to this vector within the slip plane. The magnitude and direction of the applied shear stress determine the glide motion along the plane in which the extra half-plane exists.

Mixed Dislocation

A mixed dislocation contains both edge and screw character, meaning that its Burgers vector forms an angle with the dislocation line. The movement of a mixed dislocation under applied shear stress is governed by the resolved shear components acting on each segment of the dislocation. As a result, its glide behavior is a composite of the responses of pure edge and pure screw dislocations, with the dislocation moving in the direction where the net shear force, derived from the vector decomposition of the applied stress, is most effective.

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