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

MRS Fall Conference 1. Dec. 2010 Boston, USA

Düsseldorf, GermanyWWW.MPIE.DEd.raabe@mpie.de

E. Demir*, D. Raabe

* Cornell Univ. Rhodes Hall, Mechanical & Aerospace Engr. Dept.

Overview

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Introduction

Experiments

Results and Discussion

Conclusions

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Mechanical properties of materials are size dependent (pillar

compression, indentation, beam bending)

Introduction

* Bauschinger effect: flow strength change upon load path change (reversal)

Demir, Raabe, Acta Materialia 58 (2010) 6055

Question: Bauschinger effect* also size dependent ?

Relevant in metal forming and cyclic straining

(small parts AND small scale microstructures)

Approach: Bending and stretching

experiments on a Cu single crystal

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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.

Explanations of the Bauschinger effect

Demir, Raabe, Acta Materialia 58 (2010) 6055

Overview

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Introduction

Experiments

Results and Discussion

Conclusions

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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

Experiment

E. Demir, D. Raabe, Acta Materialia 58 (2010) 6055

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Set-up of experiment

Demir et al.: Acta Materialia 58 (2010) 6055

Overview

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Introduction

Experiments

Results and Discussion

Conclusions

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Stress-strain results

reverse yield strengths (straightening step) much smaller than those in forward loading (bending step)

for all 3 cycles similar flow stress upon load reversal (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

Demir et al.: Acta Materialia 58 (2010) 6055

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Microstructure and mechanical Bauschinger effect

Kernal average misorientation orientation map

orientation map

Kernal average misorientation

bending (forward)

straightening (backward)

Demir et al.: Acta Materialia 58 (2010) 6055

Mechanical Bauschinger effect: yield stress drop upon load path change

Microstructural Bauschinger effect: microstructure reversibility upon load path change

Demir et al. Acta Mater. 57 (2009) 559 ; M. Calcagnotto et al. Mater. Sc. Engin. A 527 (2010) 2738

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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 cross hardening

Overview

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Introduction

Experiments

Results and Discussion

Conclusions

13Roters et al. Acta Mater.58 (2010)

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