Slip Systems
Figure 12.7.1 and Figure 12.7.2 show in 2D and 3D that dislocations slip in particular directions. In crystalline materials, there are certain crystallographic planes (Section 5.8) on which dislocations move most easily, known as slip planes. Within these planes, there are certain favored crystallographic directions (Section 5.7) the dislocation can slip, known as slip directions. A slip system is the the combination of a slip plane and direction.
Figure 12.7.1 A GIF created with NetLogo model 12.4.1 of dislocation motion.
Figure 12.7.2 Animation of edge dislocation moving in a 3D crystal.
Dislocations move on slip systems because the atoms don't have to move as far. This means that as bonds are broken to allow the dislocation to move, the moving atoms start feeling attraction to the atoms they are moving towards sooner. Another looking at it is: slip systems are the combination of planes and directions on which dislocations can move while distorting the overall lattice the least. Because of this:
- slip planes are those with the highest planar density (Exercise 5.8.2) of atoms, i.e., those where atoms are closest together
- slip directions are directions within those planes with the highest linear atomic density.
The slip system will therefore depend on the crystal structure of a metal.
Take 2-3 minutes to solve this problem.
-
The image below shows three crystallographic planes of an FCC unit cell. Which will be the slip plane?
-
The image below shows the close-packed plane, $\{110\}$, for a BCC unit cell. Which direction within this plane will be the slip direction?
Three crystallographic directions in the close-packed plane $\{110\}$ of a BCC unit cell
Take 2-3 minutes to solve this problem.
-
Look at the simple cubic structure below.
From the following options, which slip system do you think would be the preferred slip system - that is, the one of lowest energy?
- $\text{{100}}\langle100\rangle$
- $\text{{100}}\langle110\rangle$
- $\text{{100}}\langle111\rangle$
- $\text{{110}}\langle100\rangle$
- $\text{{110}}\langle110\rangle$
- $\text{{110}}\langle111\rangle$
A simple cubic crystal structure.
Slip systems and ductility
Different crystal structures have different numbers of slip systems since they have different numbers of close-packed planes and directions. The table below summarizes the major slip systems for common metal crystal structures. The number of slip systems equals the number of planes in the slip plane family times the number of slip directions in the slip direction family.
If there are more slip systems, there are more ways for dislocations to move, leading to more ductile materials. As a result, BCC and FCC metals with more slip systems tend to be more ductile while HCP metals tend to be more brittle.
| Crystal Structure | Slip plane | Slip direction | # of slip systems | Examples |
|---|---|---|---|---|
| FCC | $\{111\}$ | $\langle110\rangle$ | $4 \times 3=12$ | Al, Cu, γ-Fe, Ni |
| BCC | $\{110\}$ | $\langle111\rangle$ | $6 \times 2=12$ | α-Fe, Mo, W |
| HCP | $\{0001\}$ | $\langle11\bar{2}0\rangle$ | $1 \times 3=3$ | Cd, Mg, α-Ti, Zn |