For a screw dislocation, the slip direction is perpendicular to the applied shear stress.

• A. Parallel to the applied shear stress
• B. Perpendicular to the applied shear stress
• C. Along the grain boundary
• D. At a 45-degree angle to the applied shear stress

Answer: (B)

In a screw dislocation, glide (slip) occurs by shear motion within the slip plane, and the direction of that glide is tied to the Burgers vector, which for a screw dislocation is parallel to the dislocation line. In the common simple-shear setup used to analyze dislocation motion, the component of the applied shear that effectively drives this glide ends up acting in a direction that lies in the slip plane but is perpendicular to the direction of the externally applied shear. Put simply, the lattice shifts along a direction within the slip plane that is perpendicular to the direction of the applied shear stress.

This is why the slip direction is perpendicular to the applied shear stress in this context. The other possibilities don’t match the geometric relation between the dislocation’s Burgers vector, its slip plane, and the typical orientation of the applied shear in screw dislocation glide.