What is a dislocation in a crystal, and how do edge and screw dislocations move under applied stress?

• A. A line defect representing a misfit at the lattice edge
• B. A dislocation in the crystal that can move by gliding on slip planes under applied stress
• C. An empty lattice site
• D. An extra atom occupying an interstitial site

Answer: (B)

Dislocations are line defects in a crystal lattice that allow plastic, permanent deformation to occur when the material is stressed. They are regions where the regular arrangement of atoms is locally disturbed.

An edge dislocation is like an extra half-plane of atoms that ends inside the crystal. A screw dislocation is a helical twist of the lattice around a line. Under applied stress, the crystal can deform mainly by glide: the dislocation line moves along a specific crystallographic plane, called a slip plane, due to shear stress on that plane. As the dislocation advances, it shifts atoms past each other, giving rise to permanent deformation at a lower overall stress than would be needed to move entire crystal planes.

Both edge and screw dislocations move by this glide mechanism on slip planes when subjected to appropriate shear stress. Climb, another motion path, involves diffusion of atoms and occurs at higher temperatures, but the typical, primary response under applied stress at moderate temperatures is glide on slip planes.