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530.352 Materials Selection

530.352 Materials Selection. Lecture #16: More about dislocations Wednesday October 19 th , 2005. Dislocation is the boundary between slipped and unslipped regions. . Motion of Edges. top moved to left. . . bottom moved to right. . . Motion of Screws. Annihilation of edges:.

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530.352 Materials 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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