Editing Dislocation (section)

== History ==

The theory describing the elastic fields of the defects was originally developed by [[Vito Volterra]] in 1907.<ref>[[Vito Volterra]] (1907) [https://eudml.org/doc/81250 "Sur l'équilibre des corps élastiques multiplement connexes"], ''Annales Scientifiques de l'École Normale Supérieure'', Vol. 24, pp. 401–517</ref> The term 'dislocation' referring to a defect on the atomic scale was coined by [[G. I. Taylor]] in 1934.<ref>
{{cite journal|author=G. I. Taylor|title=The Mechanism of Plastic Deformation of Crystals. Part I. Theoretical|journal= Proceedings of the Royal Society of London. Series A|volume=145|issue=855|year=1934|pages=362–87|doi=10.1098/rspa.1934.0106|jstor=2935509|bibcode = 1934RSPSA.145..362T |author-link=G. I. Taylor|doi-access=free}}</ref>

Prior to the 1930s, one of the enduring challenges of materials science was to explain [[plasticity (physics)|plasticity]] in microscopic terms. A simplistic attempt to calculate the [[shear stress]] at which neighbouring atomic planes ''slip'' over each other in a perfect crystal suggests that, for a material with [[shear modulus]] <math>G</math>, shear strength <math>\tau_m</math> is given approximately by:

::<math> \tau_m = \frac {G} {2 \pi}.</math>

The shear modulus in [[metal]]s is typically within the range 20 000 to 150 000 [[MPa]] indicating a predicted shear stress of 3 000 to 24 000 MPa. This was difficult to reconcile with measured shear stresses in the range of 0.5 to 10 MPa.

In 1934, [[Egon Orowan]], [[Michael Polanyi]] and G. I. Taylor, independently proposed that plastic deformation could be explained in terms of the theory of dislocations. Dislocations can move if the atoms from one of the surrounding planes break their bonds and rebond with the atoms at the terminating edge. In effect, a half plane of atoms is moved in response to shear stress by breaking and reforming a line of bonds, one (or a few) at a time. The energy required to break a row of bonds is far less than that required to break all the bonds on an entire plane of atoms at once. Even this simple model of the force required to move a dislocation shows that plasticity is possible at much lower stresses than in a perfect crystal. In many materials, particularly ductile materials, dislocations are the "carrier" of plastic deformation, and the energy required to move them is less than the energy required to fracture the material.