Which statement best defines a slip system in crystalline materials?

• A. A slip system is a grain boundary orientation
• B. A slip system is the diffusion path for atoms
• C. A slip system is the energy barrier for diffusion
• D. A slip system is a combination of a slip plane and a slip direction required for plastic deformation

Answer: (D)

The concept being tested is how plastic deformation happens in crystals through dislocation motion. A slip system is the specific pair that governs this glide: a slip plane and a slip direction. The slip plane is the crystallographic plane with the highest atomic density, and the slip direction is the closest-packed direction within that plane. A dislocation moves most easily along that plane and direction, so plastic deformation occurs primarily along these defined paths. The onset of slip is governed by the resolved shear stress on that system (Schmid’s law), and deformation proceeds once that stress reaches a critical level. The other ideas—diffusion paths, diffusion energy barriers, or grain boundary orientations—pertain to other mechanisms and do not define how dislocations glide during plastic deformation. In many metals, knowing the slip systems explains why some crystals are highly ductile (many available slip systems) while others are more brittle.