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Dislocation. Dr. Richard Chung Department of Chemical and Materials Engineering San Jos é State University. Learning Objectives. Describe the types of line defects (dislocations) and their relationships between the microstructure and the movement of dislocations

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Dislocation

Dislocation

Dr. Richard Chung

Department of Chemical and Materials Engineering

San José State University


Learning objectives
Learning Objectives

  • Describe the types of line defects (dislocations) and their relationships between the microstructure and the movement of dislocations

  • Distinguish the difference between perfect slips and atomic distortions associated with dislocation motion

  • Explain the slip processes in edge and screw dislocations

  • Illustrate the interrelationships among structural irregularities, strain energy and stress field with respect to dislocations

  • Define different terms involved in dislocations such as Frank-Read mechanisms, kinks, jogs, climb, cross-slip, twinning, cell structure, etc.


Dislocations
Dislocations

  • Dislocations are types of line defect.

  • Dislocations usually appear in low- stressed crystalline material.

  • Dislocations are responsible for a plastic flow in a material.

  • The number of dislocation, the deformation mechanisms, the stress field, and strain energies are all associated with the key role dislocations play in a crystal material.


Some useful web pages
Some Useful Web Pages

  • Defects in Crystals

    Prof. Helmut Föll, University of Kiel

    http://www.techfak.uni-kiel.de/matwis/amat/def_en/makeindex.html

  • Dislocation Movement Across Grain Boundaries

    http://www.jwave.vt.edu/crcd/farkas/lectures/dislocations/tsld023.htm

  • Kinks and Jogs

    http://www.tf.uni-kiel.de/matwis/amat/def_en/kap_5/backbone/r5_3_1.html


Inter atomic distance b
Inter-atomic Distance, b

  • The shear stress is responsible for displacing atoms.

  • Assume two rows of atoms are moving against each other in opposite directions. The distance between the center of an atom to the center of another atom is defined as the inter-atomic distance b.

  • At x= b/2, the crystal (lattice) energy is at a maximum, whereas the shear stress is at a minimum.

  • At x= b/4 the shear stress reaches a maximum value, max


Dislocations1
Dislocations

Assume

Take derivative



Large discrepancy between theoretical values and experimental values
Large Discrepancy between Theoretical Values and Experimental Values

  • The theoretical value of maximum shear stress is based on the order of G/2

  • The relationship between the applied force and the atomic separation is not in a symmetry  the sinusoidal curve is not valid

  • Atomic shear is not the driving force for the plastic deformation in a crystal


Motion of edge dislocations
Motion of Edge Dislocations Experimental Values


Edge dislocation
Edge Dislocation Experimental Values


Frictional stress
Frictional stress Experimental Values

  • Peierls and Nabarro developed the equation for calculating the frictional stress:

  • is the Possion’s ratio

    The formula can also be expressed by the width (w)of a dislocation


Climb of a dislocation
Climb of A Dislocation Experimental Values

  • The core of a dislocation can move into an adjacent atomic vacancy  dislocation climbs

  • An adjacent atom can move into the core of a dislocation  a lattice vacancy


  • Kinks and Jogs Experimental Values

    http://www.tf.uni-kiel.de/matwis/amat/def_en/kap_5/backbone/r5_3_1.html


Jogs and slip vector
Jogs and Slip Vector Experimental Values

  • Jogs are continuously created and destroyed in an edge dislocation by randomly exchange its atoms with the surrounding


Shear stress and jogs
Shear Stress and Jogs Experimental Values


Twinning
Twinning Experimental Values


Properties of dislocations
Properties of Dislocations Experimental Values

  • Dislocation stress fields

  • Dislocation energies

  • Dislocation interactions (forces in between)

  • Dislocation kinks and velocities


Dislocation Stress fields Experimental Values


Conclusion
Conclusion Experimental Values

  • Burgers vector indicate the slip direction

  • Burgers vector is normal to an edge dislocation line, but parallel to a screw dislocation line

  • A screw dislocation can cross slip from one slip plane to another

  • An edge dislocation can move out of its slip plane by moving upward, normal to it

  • Dislocation energy is proportional to the square of its Burgers vector


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