Dislocation

A dislocation is a crystallographic defect or irregularity, within a crystal structure. The presence of dislocations strongly influences many of the properties of materials. The theory was originally developed by Vito Volterra in 1905. Some types of dislocations can be visualized as being caused by the termination of a plane of atoms in the middle of a crystal. In such a case, the surrounding planes are not straight, but instead bend around the edge of the terminating plane so that the crystal structure is perfectly ordered on either side. The analogy with a stack of paper is apt: if a half a piece of paper is inserted in a stack of paper, the defect in the stack is only noticeable at the edge of the half sheet. There are two primary types: edge dislocations and screw dislocations. Mixed dislocations are intermediate between these. Two main types of dislocation exist: edge and screw. Dislocations found in real materials typically are mixed, meaning that they have characteristics of both. A crystalline material consists of a regular array of atoms, arranged into lattice planes (imagine stacking oranges in a grocers, each of the trays of oranges are the lattice planes). One approach is to begin by considering a 3-d representation of a perfect crystal lattice, with the atoms represented by spheres. The viewer may then start to simplify the representation by visualising planes of atoms instead of the atoms themselves. An edge dislocation is a defect where an extra half-plane of atoms is introduced mid way through the crystal, distorting nearby planes of atoms. When enough force is applied from one side of the crystal structure, this extra plane passes through planes of atoms breaking and joining bonds with them until it reaches the grain boundary. A simple schematic diagram of such atomic planes can be used to illustrate lattice defects such as dislocations. (Figure B represents the "extra half-plane" concept of an edge type dislocation). The dislocation has two properties, a line direction, which is the direction running along the bottom of the extra half plane, and the Burgers vector which describes the magnitude and direction of distortion to the lattice. In an edge dislocation, the Burgers vector is perpendicular to the line direction. A screw dislocation is much harder to visualize. Imagine cutting a crystal along a plane and slipping one half across the other by a lattice vector, the halves will fit back together without leaving a defect. If the cut only goes part way through the crystal, and then slipped, the boundary of the cut is a screw dislocation. It comprises a structure in which a helical path is traced around the linear defect (dislocation line) by the atomic planes in the crystal lattice (Figure C). Perhaps the closest analogy is a spiral-sliced ham. In pure screw dislocations, the Burgers vector is parallel to the line direction.

Transmission Electron Micrograph of Dislocation

Further information

1. Физическая энциклопедия: В 5 т.: т.1: Ааронова-Длинные линни/Ред-кол.: Прохоров А.М.(гл. ред.) и др.-М.: Сов. энцикл.,1988.-704с.
2. Гуртов В.А., Осауленко Р. Н. Физика твердого тела для инженеров/учебное пособие.-М.:Техносфера, 2007-520 с.
3. А.Коттрел Теория дислокаций М:Мир, 1969
4. Дж.Хирт, И.Лоте Теория дислокаций. М:Атомиздат. 1972
5. Reed-Hill, R. E. (1994) "Physical Metallurgy Principles" ISBN 0-534-92173-6
6. Meyers and Chawla. (1999) Mechanical Behaviors of Materials. Prentice Hall, Inc. 228-231.
7. Article Dislocation from Wikipedia, the Free Enciclopedia. Available under the license Creative Commons Attribution-Share Alike.

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