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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