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Micro-scale single crystal Bauschinger effect and reversible plasticity in copper during bending<br />E. Demir*, D. Raabe<...
Overview<br /><ul><li>Introduction
Experiments
ResultsandDiscussion
Conclusions</li></ul>1<br />WWW.MPIE.DE<br />
2<br />Introduction<br /><ul><li>Mechanicalpropertiesofmaterialsaresizedependent (pillarcompression, indentation, beam ben...
Question: Bauschingereffect* also sizedependent ?
Relevant in metal forming and cyclic straining (small parts AND small scale microstructures)
Approach: Bendingandstretchingexperiments on a Cusinglecrystal</li></ul>* Bauschingereffect: flowstrengthchange upon loadp...
3<br />ExplanationsoftheBauschingereffect<br /><ul><li>Polycrystals: polarized dislocations in front of interfaces
Single crystals: polarized cell wall structures
Long range backstresses build up that resist further forward loading but reduce the yield strength under load reversal
Removal of dislocation loops and untangling of dislocations from obstacles upon load reversal releases fresh mobile disloc...
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MRS 2010 Bauschinger Effect Dierk Raabe

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Micro-scale single crystal Bauschinger effect and reversible plasticity in copper during bending

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Transcript of "MRS 2010 Bauschinger Effect Dierk Raabe "

  1. 1. Micro-scale single crystal Bauschinger effect and reversible plasticity in copper during bending<br />E. Demir*, D. Raabe<br />* Cornell Univ. Rhodes Hall, Mechanical & Aerospace Engr. Dept.<br />Düsseldorf, Germany<br />WWW.MPIE.DE<br />d.raabe@mpie.de<br />MRS Fall Conference 1. Dec. 2010 Boston, USA<br />
  2. 2. Overview<br /><ul><li>Introduction
  3. 3. Experiments
  4. 4. ResultsandDiscussion
  5. 5. Conclusions</li></ul>1<br />WWW.MPIE.DE<br />
  6. 6. 2<br />Introduction<br /><ul><li>Mechanicalpropertiesofmaterialsaresizedependent (pillarcompression, indentation, beam bending)
  7. 7. Question: Bauschingereffect* also sizedependent ?
  8. 8. Relevant in metal forming and cyclic straining (small parts AND small scale microstructures)
  9. 9. Approach: Bendingandstretchingexperiments on a Cusinglecrystal</li></ul>* Bauschingereffect: flowstrengthchange upon loadpathchange (reversal)<br />Demir, Raabe, ActaMaterialia 58 (2010) 6055<br />
  10. 10. 3<br />ExplanationsoftheBauschingereffect<br /><ul><li>Polycrystals: polarized dislocations in front of interfaces
  11. 11. Single crystals: polarized cell wall structures
  12. 12. Long range backstresses build up that resist further forward loading but reduce the yield strength under load reversal
  13. 13. Removal of dislocation loops and untangling of dislocations from obstacles upon load reversal releases fresh mobile dislocations. These reduce requirement to activate new dislocation sources. This leads to softer reverse response and a smooth transition between the elastic and elastic-plastic regimes in the reverse stress-strain curves.</li></ul>Demir, Raabe, ActaMaterialia 58 (2010) 6055<br />
  14. 14. Overview<br /><ul><li>Introduction
  15. 15. Experiments
  16. 16. ResultsandDiscussion
  17. 17. Conclusions</li></ul>4<br />WWW.MPIE.DE<br />
  18. 18. 5<br />Experiment<br /><ul><li>Cu single crystal
  19. 19. Cylindrical specimen (10 µm diameter) by wire electro discharge grinding followed by etching in 40% HN03 solution
  20. 20. Cantilever beam cut by FIB (500 pA, 30 keV)
  21. 21. Width and thickness of beam: 8.64 µm and 7.05 µm
  22. 22. 3 deformation cycles (bending and straightening) comprising 6 individual loading tests
  23. 23. Miller indices [5 2 1] in longitudinal beam axis, [4 11 2] in transverse direction, [5 2 21] in normal axis (negative compression direction)
  24. 24. Ex-situ EBSD
  25. 25. Loading in Hysitron indenter
  26. 26. Beam bending to a displacement of 3 µm at a rate of 1 µm/s</li></ul>E. Demir, D. Raabe, ActaMaterialia 58 (2010) 6055<br />
  27. 27. 6<br />Set-upofexperiment<br />Demir et al.: Acta Materialia 58 (2010) 6055<br />
  28. 28. Overview<br /><ul><li>Introduction
  29. 29. Experiments
  30. 30. ResultsandDiscussion
  31. 31. Conclusions</li></ul>7<br />WWW.MPIE.DE<br />
  32. 32. 8<br />Stress-strainresults<br /><ul><li>reverse yield strengths (straightening step) much smaller than those in forward loading (bending step)
  33. 33. for all 3 cyclessimilarflow stress upon loadreversal (depends on flow stress criterion)
  34. 34. Upper bound estimate: load drop of 73% (1st cycle), 76% (2nd cycle), and 83% (3rd cycle) relative to forward yield stress</li></ul>Demir et al.: Acta Materialia 58 (2010) 6055<br />
  35. 35. 9<br />Microstructure and mechanical Bauschinger effect<br />straightening (backward)<br />Kernal average misorientation<br />bending (forward)<br />MechanicalBauschinger effect: yield stress drop upon load path change<br />MicrostructuralBauschinger effect: microstructure reversibility upon load path change<br />orientation map<br />orientation map<br />Kernal average misorientation<br />Demir et al. ActaMater. 57 (2009) 559; M. Calcagnotto et al. Mater. Sc. Engin. A 527 (2010) 2738<br />Demir et al.: Acta Materialia 58 (2010) 6055<br />
  36. 36. 10<br />Microstructure and mechanical Bauschinger effect<br /><ul><li>EBSD reveals dramatic reduction in the bending-induced misorientation gradients upon load reversal (straightening) :
  37. 37. Unexpected form of microstructure reversibility
  38. 38. Two effects: a) internal backstresses that support load reversal. They are created by polarized dislocation arrays that are accumulated during forward bending. b) reduced requirement to activate dislocation sources during reverse loading as the dislocations that were stored during bending did not participate much in crosshardening</li></li></ul><li>Overview<br /><ul><li>Introduction
  39. 39. Experiments
  40. 40. ResultsandDiscussion
  41. 41. Conclusions</li></ul>11<br />WWW.MPIE.DE<br />
  42. 42. 12<br />Conclusions<br /><ul><li>70-80% Bauschinger softening in Cu single crystal
  43. 43. Magnitude of change in Bauschinger effect upon cycling depends on flow stress definition at small scales
  44. 44. Mechanical Bauschinger Effect: yield stress drop upon load path change
  45. 45. Microstructural Bauschinger Effect: Degree of microstructure reversibility upon load path change</li></ul>Roters et al. Acta Mater.58 (2010)<br />
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