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MeVArc Summary: DislocationsNick Shipman - Wednesday, 03 August 2011

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Marc Fivel 3D Discrete Dislocation Dynamics SimulationsTEM video showing a Frank Read Source in actionPersistent Slip Band formation Model/Simulation/Experimental Evidence

Aarne Pohjonen Dislocation mechanisms on a near surface voidSteve Fitzgerald Dislocations

Model for protrusion formation under E field supported by simulationsDetails of the experimental evidence for the required voidsMay explain conditioning and BD resistance ranking by crystal structure{

TRDIS simulation of Frank Read sourceTaken using Transverse-Electron-MicroscopyA Frank-Read Source is a mechanism explaining the generation of multiple dislocations in specific well spaced slip planes in crystals when they are deformed. Wikipedia1. TEM video showing a Frank-Read source in action2

2. Persistent Slip Band Formation

The diagrams on the right show a way in which slip bands can be formed by a dislocation moving sideways across and interacting with other dislocations.

A schematic view of persistent slip bands and a composite TEM image in which the channels are clearly visible.

2. Persistent Slip Band Formation~10umOne frame from the new nodal 3D discrete dislocation simulation code Numodis which was used to simulate low strain fatigue in AISI 316L Steel.An SEM image courtesy of Markus Aicheler showing the damage due to pulse surface heating on copper.3. Model for protrusion formation under externally applied E field The concept is as follows:-Protrusions can be created by prismatic loops which are drawn to the surface due to an externally applied E filed.But how are the prismatic loops formed?By the interaction of dislocations with near surface voids.

DefinitionsPrismatic loop platelet of extra atoms

Void3. Model for protrusion formation under externally applied E field

4. Details of experimental evidence of required voids5. Things this theory can help explain

Possible reason for conditioning: voids are used up

Possible reason for rank by crystal structure: dislocations travel at different speeds in different structures

Steel has highest breakdown resistance: full of C which significantly impedes dislocation motion