E3 – Slip and Twinning in Metal
Introduction
1
Slip deformation refers to the motion of dislocations causing specific crystallographic atomic planes to move over one another in distinct crystallographic directions. This combination of crystallographic plane and direction is called a slip system. Most usually the slip direction will be the close packed direction; this is because the Burgers vector is at a minimum and hence less energy is required for slip to occur. Similarly the close packed planes are generally the crystallographic slip planes due to the wide separation. Slip steps occur from the movement of large numbers of dislocation on the same slip plane.

Twinning most usually occurs if slip is restricted; this can be due to low temperature, high strain or minimal slip systems (i.e. in a HCP system). Twinning results from shearing of sections of the crystal lattice, where the sheared region boundaries are definite crystallographic planes. The result of this is a portion of the material (the twin) that has an orientation that is a mirror image of the matrix, whilst maintaining constant crystal structure.

2 FCC:
Planes: (1 1 1) (1 1 -1) (-1 1 1) (1 -1 1)
Directions: <1 1 0> <1 0 1> <0 1 1>
Slip Systems = Planes x Directions = 4 x 3 = 12 easy slip systems

HCP:
Planes: (0 0 0 1)
Directions: <1 1 2 0> <2 1 1 0> <1 2 1 0>
Slip Systems = 1 x 3 = 3 easy slip systems

Results
1.
Aluminum

Pre Strain Zinc

From the photograph of the microstructure of damaged Aluminum we can see the presence of slip steps (as labeled). These slips steps have arisen from the movement of large numbers of dislocation (from the applied stress) all on the same slip plane. From the photograph of the microstructure of damaged Zinc we can see the presence of twinning (as labeled). This twinning results from shearing of sections of the crystal lattice (from the applied stress), where the sheared region boundaries are definite crystallographic planes. Twinning occurs in Zinc as Zinc has a HCP structure, and hence only 3 slip systems. Thus because there is less than 5 slip systems twinning will occur instead of slip steps as slip is restricted. Grain boundaries can be seen in both the Aluminum and Zinc samples; the grain boundaries in the damaged sample appear to be larger than those in the undamaged samples. The reasoning behind this is as the material is placed under a load the material is ‘stretched’ and hence the grain boundaries will also elongate and appear larger/’stretched’ apart.

***These strain values correspond to the strain applied to the damaged specimens (not the strain applied to cause failure)

2.
The twinning on the surface of the zinc is seen as the sheared lattice causes different light reflection off the surface of the zinc. The twinning regions would still be observed if the zinc were polished back as twinning is not just a surface deformation, but rather affects the whole zinc crystal.

Discussion
3
The slip direction is generally in the close packed direction as the atoms are closer together, that is the burgers vector is smaller. For this reason less energy is required for propagation of the defect/slip over the close packed direction.

4

Figure 3: Actual resolved stress to shift a plane may not be parallel to the direction of the applied stress.

5
If there is less than 5 slip systems the system cannot handle slip deformation and instead twinning occurs.

6
It is important to know whether a metal deforms by slip or twinning as it can tell you whether the material will undergo brittle or ductile failure. From the experiment it was seen that zinc maintain it’s dimensions and underwent brittle fracture and twinning. Aluminum however underwent elongation, ductile failure and slip deformation. Materials which can withstand higher levels of strain, i.e. ductile materials which undergo slip, will be tougher. On the contrary materials in which the dislocations do not