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Uniaxial and Multiaxial Plastic Deformation of Large Niobium Grains

Uniaxial and Multiaxial Plastic Deformation of Large Niobium Grains

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Uniaxial and Multiaxial Plastic Deformation of Large Niobium Grains

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  1. 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

  2. Formability: strain localization on grain boundaries up to 0.5 mm displacement found between neighboring grains

  3. Plastic Properties of Niobium BCC Crystal Structure < 111 > slip direction (close-packed direction) Any plane containing < 111 > is a potential slip plane Experimentally observed in (110), (112) & (123) planes

  4. Tensile TestsOrientations

  5. Tensile Tests

  6. Yield Stress • YS between 25 MPa and 40 MPa • weak anisotropy • 25% YS of poly-crystal Polycrystal Single crystal

  7. R-Values • Extreme anisotropy from r=0 (thinning only) to r>1 (no thinning) • Polycrystal r=0.1 • Large r-values for {210}<-120>

  8. Effect of Annealing • Yield stress and yield drop increase with annealing temperature

  9. Sample prep for multiaxial tests

  10. Multi-axial Testing

  11. A - Uniaxial Test

  12. B - Uniaxial Test, 90 deg rotated

  13. C - Balanced Biaxial Test

  14. D - Plane Strain localization

  15. E - Plane Strain, 90 deg. rotated

  16. E – Plane Strain

  17. 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

  18. 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 Analysis of present data

  19. Taylor’s model • Imposed strain rate tensor • Write the strain rate tensor as a combination of all the slip systems

  20. 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