Entry by Dongwoo Lee (AP 225, 2010 Fall)
Dislocation means the crystallographic defect in a crystal structure. There are two kinds of dislocations : edge dislocations and screw dislocations. Since dislocation affects the mechanical properties of materials significantly, controlling the density or number of dislocation have been developed. Work hardening and annealing can be used for controlling the density of dislocations.
Dislocation is known as the source of placiticity. Thus, by limiting the dislocation movement, one can induce the brittle property with a ductile materials. Observing dislocations is possible nowadays using the techniques such as Transmission Electron Microscopy (TEM).
 Gyuhyon Lee, Ju-Young Kim, Arief Suriadi Budiman, Nobumichi Tamura, Martin Kunz, Kai Chen, Michael J. Burek, Julia R. Greer, Ting Y. Tsui, "Fabrication, structure and mechanical properties of indium nanopillars", Acta Materialia, Volume 58, Issue 4, Pages 1361-1368, (2010)