Slip is the process by which plastic deformation is produced by a dislocation motion. By an external force, parts of the crystal lattice glide along each other, resulting in a changed geometry of the material. Depending on the type of lattice, different slip systems are present in the material. More specifically, slip occurs between planes containing the smallest Burgers vector. The picture on the right shows a schematic view of the slip mechanism.
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Slip systems
FCC
Slip in FCC (face centered cubic) crystals occurs along the close packed plane. Specifically, the slip plane is of type {111}, and the direction is of type <110>. In the diagram, the specific plane and direction are (111) and [-110], respectively. Given the permutations of the slip plane types and direction types, FCC crystals have 12 slip systems. In the FCC lattice, the Burgers vector, b, can be calculated using the following equation:[1]
Where a is the lattice constant of the unit cell.
BCC
Slip in BCC (body centered cubic) crystals occurs along the plane of shortest Burgers vector as well; however, unlike FCC, there are no truly close-packed planes in the BCC crystal structure. Thus, a slip system in BCC requires heat in order to activate. Some BCC materials (e.g. α-Fe) can contain up to 48 slip systems. There are six slip planes of type {110}, each with two <111> directions (12 systems). There are 24 {123} and 12 {112} planes each with one <111> direction (36 systems, for a total of 48). While the {123} and {112} planes are not exactly identical in activation energy to {110}, they are so close in energy that for all intents and purposes they can be treated as identical. In the diagram on the right the specific slip plane and direction are (110) and [-111], respectively.[1]
Elemental metals which are found in the BCC crystal structure include lithium, sodium, potassium, vanadium, chromium, manganese, iron, rubidium, niobium, molybdenum, cesium, barium, tantalum, tungsten, radium, and europium. Some compound materials with the BCC crystal structure include the cesium halides (other than CsF).
HCP
Slip in HCP (Hexagonal Close Packed) metals is much more limited than in BCC and FCC crystal structures. This is because few active slip systems exist in HCP metals. The result of the small number of slip systems is the metal is generally brittle.
Cadmium, Zinc, Magnesium, Titanium, and Beryllium have a slip plane at {0001} and a slip direction of <11(-2)0>. This creates a total of three slip systems, depending on orientation. (Remember that a slip system is a combination of a slip plane and a slip direction!) Other combinations are also possible.[2]
See also
References
External links
- An online tutorial on slip, explained on a promotional site for materials science at the University of Cambridge
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