Question

Explain the significance of, and the mechanisms behind, slip in metals. What is a slip system? In a pure material, what governs the ease of slip? Why are some crystal structures easier to slip than others? Why are ceramic materials normally very resistant to slip?

Answer 1

Slip is the prominent mechanism of plastic deformation in metals. It involvea sliding of crystals over one another in a definite crystallographic planes called slip planes. During slip each atom move almost same integral atomic distance along the plane producing a step but the orientation of the crystal remains the same. Shear stress is responsible for this phenomenon.

Slip system refers to the plane along which slip and dislocation motion occurs in the crystal. It consists of slip plane and slip direction. There are three types of slip systems :- 1. FCC system.....no of slip is 12. 2. BCC system :- no of slip is as large as 48. 3. HCP slip is generally between FCC and BCC.

In metals slip plane is generally plane of greatest atomic density and slip direction is the closed pack direction in the plane. So planes of highest atomic density is most widely spaced plane while closed pack direction has the least translation distance. Hence, slip system occurs along a specific plane for a given crytal strcture. That plane is the plane which satisfies the above two criteria. Extent of slip depends on external load, shear stress produced, geometry of the system and active crystallographic plane.

Due to very rigid structure, there are very few slip systems available in the ceramics, thereby, they deform very slowly.