Micro-scale single crystal Bauschinger effect and reversible plasticity in copper during bending The study examined the micro-scale Bauschinger effect and reversible plasticity in copper single crystals during bending experiments. Bending and straightening experiments were performed on a copper cantilever beam at the micro-scale. Results showed 70-80% reductions in yield strength upon load reversal, indicating a strong Bauschinger effect. Microstructure analysis also revealed reductions in bending-induced misorientation gradients upon load reversal, demonstrating microstructural reversibility. The findings provide evidence that internal backstresses and reduced dislocation storage contribute to size-dependent reversible plasticity at the micro-scale.

1. Micro-scale single crystal Bauschinger effect and reversible plasticity in copper during bending E. Demir*, D. Raabe * Cornell Univ. Rhodes Hall, Mechanical & Aerospace Engr. Dept. Düsseldorf, Germany WWW.MPIE.DE d.raabe@mpie.de MRS Fall Conference 1. Dec. 2010 Boston, USA

3. Experiments

4. ResultsandDiscussion

5. Conclusions1 WWW.MPIE.DE

7. Question: Bauschingereffect* also sizedependent ?

8. Relevant in metal forming and cyclic straining (small parts AND small scale microstructures)

9. Approach: Bendingandstretchingexperiments on a Cusinglecrystal* Bauschingereffect: flowstrengthchange upon loadpathchange (reversal) Demir, Raabe, ActaMaterialia 58 (2010) 6055

11. Single crystals: polarized cell wall structures

12. Long range backstresses build up that resist further forward loading but reduce the yield strength under load reversal

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.Demir, Raabe, ActaMaterialia 58 (2010) 6055

15. Experiments

16. ResultsandDiscussion

17. Conclusions4 WWW.MPIE.DE

19. Cylindrical specimen (10 µm diameter) by wire electro discharge grinding followed by etching in 40% HN03 solution

20. Cantilever beam cut by FIB (500 pA, 30 keV)

21. Width and thickness of beam: 8.64 µm and 7.05 µm

22. 3 deformation cycles (bending and straightening) comprising 6 individual loading tests

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. Ex-situ EBSD

25. Loading in Hysitron indenter

26. Beam bending to a displacement of 3 µm at a rate of 1 µm/sE. Demir, D. Raabe, ActaMaterialia 58 (2010) 6055

27. 6 Set-upofexperiment Demir et al.: Acta Materialia 58 (2010) 6055

29. Experiments

30. ResultsandDiscussion

31. Conclusions7 WWW.MPIE.DE

33. for all 3 cyclessimilarflow stress upon loadreversal (depends on flow stress criterion)

34. Upper bound estimate: load drop of 73% (1st cycle), 76% (2nd cycle), and 83% (3rd cycle) relative to forward yield stressDemir et al.: Acta Materialia 58 (2010) 6055

35. 9 Microstructure and mechanical Bauschinger effect straightening (backward) Kernal average misorientation bending (forward) MechanicalBauschinger effect: yield stress drop upon load path change MicrostructuralBauschinger effect: microstructure reversibility upon load path change orientation map orientation map Kernal average misorientation Demir et al. ActaMater. 57 (2009) 559; M. Calcagnotto et al. Mater. Sc. Engin. A 527 (2010) 2738 Demir et al.: Acta Materialia 58 (2010) 6055

37. Unexpected form of microstructure reversibility

39. Experiments

40. ResultsandDiscussion

41. Conclusions11 WWW.MPIE.DE

43. Magnitude of change in Bauschinger effect upon cycling depends on flow stress definition at small scales

44. Mechanical Bauschinger Effect: yield stress drop upon load path change

45. Microstructural Bauschinger Effect: Degree of microstructure reversibility upon load path changeRoters et al. Acta Mater.58 (2010)