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Understanding Dislocations in Material Selection

Explore the concept of dislocations in materials, their motion, forces acting on them, density, relation to stress, Orowan relation, motion leads to permanent strain, and more. Watch dislocation videos for a better understanding.

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Understanding Dislocations in Material Selection

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  1. 530.352 Materials Selection Lecture #16: More about dislocationsWednesday October 19th, 2005

  2. Dislocation is the boundary between slipped and unslipped regions.

  3. Motion of Edges top moved to left   bottom moved to right

  4.  Motion of Screws

  5. Annihilation of edges:  “Poof” No dislocations !!! 

  6. Annihilation of screws: LHS RHS No dislocations !!!

  7. Fg b Fext = (LW) W L Forces on dislocations: Glide force on an edge dislocation: Principle of Virtual Work: dWext + dWrev = 0 dWext = Fext b =  (LW) b dWrev = - (Fg L) W Fg =  b (same for screws)

  8. Stress and dislocations : Shearstresses push dislocations !!!

  9. Dislocation density : [ cm-2] [cm-2] = [cm / cm3] ??? Density = total length / volume total length = n L volume = L3 Density = n / L2 or # per area

  10. The Orowan Relation: h b nb L  = nb / h =  b L  =  b V .

  11. Dislocation motion and strain : The motion of many many dislocations leads topermanent strain !!!

  12. Plastic strain is permanent : FCC H Easy BCC H Hard

  13. Mixed dislocations : EDGE SCREW  slipped

  14. Dislocation is the boundary between slipped and unslipped regions.

  15. A dislocation loop : + E b +  loop expands - loop shrinks slipped RHS LHS unslipped - E

  16. Dislocation videos : • TEM observations of dislocation motion • Dynamic simulations of dislocation motion • Dislocation activity in composites

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