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

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

2. Overview • Introduction • Experiments • ResultsandDiscussion • Conclusions WWW.MPIE.DE

3. Introduction • Mechanicalpropertiesofmaterialsaresizedependent (pillarcompression, indentation, beam bending) • Question: Bauschingereffect* also sizedependent ? • Relevant in metal forming and cyclic straining (small parts AND small scale microstructures) • Approach: Bendingandstretchingexperiments on a Cusinglecrystal * Bauschingereffect: flowstrengthchange upon loadpathchange (reversal) Demir, Raabe, ActaMaterialia 58 (2010) 6055

4. ExplanationsoftheBauschingereffect • 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 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

5. Overview • Introduction • Experiments • ResultsandDiscussion • Conclusions WWW.MPIE.DE

6. Experiment • Cu single crystal • Cylindrical specimen (10 µm diameter) by wire electro discharge grinding followed by etching in 40% HN03 solution • Cantilever beam cut by FIB (500 pA, 30 keV) • Width and thickness of beam: 8.64 µm and 7.05 µm • 3 deformation cycles (bending and straightening) comprising 6 individual loading tests • Miller indices [5 2 1] in longitudinal beam axis, [4 11 2] in transverse direction, [5 2 21] in normal axis (negative compression direction) • Ex-situ EBSD • Loading in Hysitron indenter • Beam bending to a displacement of 3 µm at a rate of 1 µm/s E. Demir, D. Raabe, ActaMaterialia 58 (2010) 6055

7. Set-upofexperiment E. Demir, D. Raabe, ActaMaterialia 58 (2010) 6055

8. Overview • Introduction • Experiments • ResultsandDiscussion • Conclusions WWW.MPIE.DE

9. Stress-strainresults • reverse yield strengths (straightening step) much smaller than those in forward loading (bending step) • for all 3 cyclessimilarflow stress upon loadreversal (depends on flow stress criterion) • Upper bound estimate: load drop of 73% (1st cycle), 76% (2nd cycle), and 83% (3rd cycle) relative to forward yield stress

10. 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

11. Microstructure and mechanical Bauschinger effect • EBSD reveals dramatic reduction in the bending-induced misorientation gradients upon load reversal (straightening) : • Unexpected form of microstructure reversibility • 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

12. Overview • Introduction • Experiments • ResultsandDiscussion • Conclusions WWW.MPIE.DE

13. Conclusions • 70-80% Bauschinger softening in Cu single crystal • Magnitude of change in Bauschinger effect upon cycling depends on flow stress definition at small scales • Mechanical Bauschinger Effect: yield stress drop upon load path change • Microstructural Bauschinger Effect: Degree of microstructure reversibility upon load path change Roters et al. Acta Mater.58 (2010)