Uniaxial and Multiaxial Plastic Deformation of Large Niobium Grains

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Uniaxial and Multiaxial Plastic Deformation of
Large Niobium Grains

• Thomas Gnäupel-Herold1,2,
• Adam Creuziger, T.Foecke3
• 1University of Maryland
• 2NIST Center for Neutron Research
• 3NIST Metallurgy Division

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Formability strain localization on grain
boundaries
up to 0.5 mm displacement found between
neighboring grains
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Plastic Properties of Niobium
BCC Crystal Structure
lt 111 gt slip direction (close-packed
direction) Any plane containing lt 111 gt is a
potential slip plane Experimentally observed in
(110), (112) (123) planes
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Tensile TestsOrientations
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Tensile Tests
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Yield Stress

• YS between 25 MPa and 40 MPa
• weak anisotropy
• 25 YS of poly-crystal

Polycrystal
Single crystal
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R-Values

• Extreme anisotropy from r0 (thinning only) to
  rgt1 (no thinning)
• Polycrystal r0.1
• Large r-values for 210lt-120gt

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Effect of Annealing

• Yield stress and yield drop increase with
  annealing temperature

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Sample prep for multiaxial tests
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Multi-axial Testing
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A - Uniaxial Test
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B - Uniaxial Test, 90 deg rotated
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C - Balanced Biaxial Test
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D - Plane Strain
localization
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E - Plane Strain, 90 deg. rotated
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E Plane Strain
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EBSD Misorientations at the tri-junction

• Slip lines and small-angle grain boundaries
• Diffuse slip, most likely from rapidly changing
  strain gradients leading to succession of
  activation/deactivation of localized slip systems

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Analysis of present data

• What is known .
• Full strain rate tensor at every point on the
  sample and in time
• orientations

• What is needed
• Slip systems that are locally active at a given
  point in time

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Taylors model

• Imposed strain rate tensor
• Write the strain rate tensor as a combination of
  all the slip systems

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Conclusions

• 5 Multi-axial straining tests of tri-crystal
  plates with identical orientation performed
• Local strain rate data collected
• Orientation analysis with EBSD
• GOAL determination of locally active slip
  systems for any given moment