Dislocations in compression/tension

Re-creation of the Bragg-Nye Bubble-Raft video, from DoITPoMS Teaching and Learning Packages, Cambridge

Deformation in the bulk via dislocations (shear)

Re-creation of the Bragg-Nye Bubble-Raft video, from DoITPoMS Teaching and Learning Packages, Cambridge

Carbide Barriers

Large Precipitates

Stacking Fault Interactions

High Temperature

Motion of dislocations

Incorrect Correct ModeCaterpillar: W. McCallister, Introduction to Materials Science

Video’s: DoITPoMS Teaching and Learning Packages

Brachistochrone Solution, i.e. Path of Least Resistance

Dislocation Movement

Similar to Tomlinson Model1

•Abrupt jump of atoms from one equilibrium position to another• Dissipation completely into bulk (phonons)• Finite friction (even in v = 0 limit)• Uncoupled and instantaneous jumps

Force

1G. A. Tomlinson, Philos. Mag. 7, 905 (1929)

phonon

Motion

• Dislocation (not atom) moves in a quasi-harmonic potential • Radiates phonons as to goes• Small velocity, kinetic energy is radiated away rapidly

F = b P

• Large velocity, viscous drag

Force

TT TTT

phonon

Barriers at the atomic scale

EPE1

T

E2

<E>

EStatic

E

Additional energy E to move dislocation from static configuration

Kinks

Details for motion

Direction of motion

Frank-Rhead Source

Dislocation Source 1

Dislocation Source 2

Dislocations from a grain boundary

CONCEPT OF IMAGE FORCES

A hypothetical negative dislocation is assumed to exist across the free-surface for the calculation of the force (attractive) experienced by the dislocation in

the proximal presence of a free-surface

A dislocation near a free surface (in a semi-infinite body) experiences a force towards the free surface, which is called the image force.

The force is called an ‘image force’ as the force can be calculated assuming an negative hypothetical dislocation on the other side of the surface (figure below).

Image Forces