Cyclic and Monotonic Plasticity of Mg Sheet

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Cyclic and Monotonic Plasticity of Mg Sheet
R. H. Wagoner , X. Lou, M. Li, S. R. Agnew,
Dept. Materials Science and Engineering The
Ohio State University University of Virginia
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Acknowledgements
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• Introduction

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Mg Sheet for Forming Applications
Advantages Low density - High strength- High
stiffness
Challenges Room-temp. formability - Asymmetric
flow behavior
Opportunity Forming operation using asymmetric
flow?
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Slip Deformation in Mg
ltagt No strain parallel to basal poles
Pyramidal
Pyramidal
ltcagt Difficult to activate
Prismatic
Basal 0001
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Tensile Twinning in Mg
Tension
ST
TD
RD
original c
Compression
Matrix
Tension
Patridge, Metall. Reviews,1967
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• Experiments

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Tension / Compression Test
Boger et al., Int. J. Plasticity, 2005
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Simple Shear Test
Gracio et al. Adv. Mat. Forum, 2002
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Acoustic Emission
Burst AE (ms) Continuous AE (ms)

• Outputs
• Cumulative Count
• Time Count Rate
• Strain Count Rate

Threshold
One AE Burst Event
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• Results

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Monotonic Tension, Fracture
Mg AZ31B Al 6013
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Monotonic Tension vs. Compression
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Evolution of R-Values
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Single-Cycle Tests C-T-C vs. T-C-T
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Single-Cycle Tests C-T-C vs. T-C-T
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Multiple Cycle Tests
a) Small strain, 0.04 b)
Large strain, 0.07
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Saturation with Cycling
a) Small strain, 0.04 b)
Large strain, 0.07
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Microstructure Evolution
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Evolution of Twinning vs. Slip
6.4 max. twin strain (single xl) 5.9 max. twin
strain (basal texture)
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Texture Evolution
(a) Initial
(b) 0.10 RD tensile strain
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Texture Evolution
(c) 0.07 RD compressive strain
(d) 0.1 reverse tensile strain
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Accoustic Emission, T-C
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Accoustic Emission, C-T
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Acoustic Emission, T-C-T
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Acoustic Emission, C-T-C
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Discussion
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Deformation Mechanism Summary
Tension

• Slip dominated

• Twinning Slip (inflected)

Compression
Reverse Tension

• Detwinning Slip (inflected)

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Roles of Twins on Hardness

• CRSS stwinning- suntwinning st/u
• Obstacles to slip ?sobstacle
• Reorientation of texture, ?stexture sw/twins -
  sw/o twins

? Can these roles be estimated quantitatively?
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Saturated Cycle Segments C, T
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Adjusted Stress-Strain Comparison
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Twinning Effect on Hardness
States 0.01 T 58 twinned, deformation
by untwinning and slip 0.01 C 17
twinned, deformation by twinning and slip
0.05 T 17 twinned, deformation by untwinning
and slip 0.05 C 58 twinned, deformation
by twinning and slip
Equations
Solution
? Obstacle effect of twinning is 1/3-1/4 that of
texture or CRSS
Lebensohn Tome Acta Metall, 1993
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Twinning Effect on Hardness - corrected
States 0.01 T 58 twinned, deformation
by untwinning and slip 0.01 C 17
twinned, deformation by twinning and slip
0.05 T 17 twinned, deformation by untwinning
and slip 0.05 C 58 twinned, deformation
by twinning and slip
Equations
Solution
? Obstacle effect of twinning is 1/3-1/4 that of
texture or CRSS
Lebensohn Tome Acta Metall, 1993
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Conclusions

• Burst AE is correlated with twin nucleation, not
  growth
• Strain from twinning in compression initially
  90, dropping to 50 at a strain of -0.05.
• AE shows a second range of intense twin
  nucleation at strains of -0.06, near the
  exhaustion of twinning.
• Brittle fracture occurs in tension and simple
  shear.
• The relative effects of twinning on flow stress
  have been estimated the obstacle effect is
  1/4-1/3 that of the texture effect or CRSS.

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Conclusions - corrected

• Burst AE is correlated with twin nucleation, not
  growth
• Strain from twinning in compression initially
  90, dropping to 50 at a strain of -0.05.
• AE shows a second range of intense twin
  nucleation at strains of -0.06, near the
  exhaustion of twinning.
• Brittle fracture occurs in tension and simple
  shear.
• The relative effects of twinning on flow stress
  have been estimated the activation stress for
  twinning vs. untwinning is double that of the
  obstacle effect or the texture effect.