Dissemination of IT for the Promotion of Materials Science (DoITPoMS)


Scientific background

In real metals there are a lot of defects such as vacancies, impurities atoms, dislocations, grain boundaries etc. Dislocation is a half-plane in crystalline structure; this is often termed an extra-plane. Dislocations can move by gliding or climbing. If dislocations could only move by gliding on a single slip plane, their motion would be constrained and they would soon become impeded by obstacles, such as other dislocations or fine precipitates.

Dislocations often meet other dislocations, in various configurations. They may amalgamate to form new dislocations, annihilate each other, be repelled or cut through each other.

When two dislocations intersect, jogs form in both. These are short sections with length and direction equal to b of the other dislocation. If a jog lies in a slip plane (when it is sometimes called a kink), it can glide with the rest of the dislocation, but if it lies out of the slip plane, it may be sessile (unable to glide, i.e. not glissile) – see below. Such defects can reduce dislocation mobility after straining.

Formation of jogs by intersection of two edge dislocations. The jog in b2 is undetectable, whereas that in b1 is sessile (unable to glide)

In pure materials the main source of resistance to the motion of dislocations other than the lattice resistance is from other dislocations that intersect the glide plane.

Dislocation Density, ρ ~1010 m-2 (10 μm spacing) when “Annealed”, ~1016 m-2 (10 nm spacing) when “Cold-worked

Forces between Dislocations can promote Annihilation & Alignment, leading to Polygonisation

Dislocation Motion is assisted by Cross-slip and Climb (the latter being Faster at High Temperature)

Intersection of Dislocations during Glide creates Jogs, which may be Sessile (unable to glide)

Dislocation Multiplication can occur, eg at a Frank-Read Source

In general, Plastic Straining Raises the Density of Dislocations, but Reduces their Mobility, leading to “Work Hardening