Uniaxial and Multiaxial Plastic Deformation of Large Niobium Grains Thomas Gnäupel-Herold 1,2, Adam Creuziger, T.Foecke 3 1 University of Maryland 2 NIST Center for Neutron Research 3 NIST Metallurgy Division Thomas Gnäupel-Herold 1,2, Adam Creuziger, T.Foecke 3 1 University of Maryland 2 NIST Center for Neutron Research 3 NIST Metallurgy Division

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

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

Tensile Tests Orientations

Tensile Tests

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

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

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

Sample prep for multiaxial tests

Multi-axial Testing

A - Uniaxial Test

B - Uniaxial Test, 90 deg rotated

C - Balanced Biaxial Test

D - Plane Strain localization

E - Plane Strain, 90 deg. rotated

E – Plane Strain

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

Analysis of present data What is known ….  Full strain rate tensor at every point on the sample and in time  orientations 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

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

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  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