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 Mode Caterpillar: 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 Model 1 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 1 G. 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 T T T T T phonon

Barriers at the atomic scale EPEP E1E1 T E2E2 E Static E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