Uniaxial and Multiaxial Plastic
Deformation of Large Niobium
Grains
Thomas Gnäupel-Herold1,2,
Adam Creuziger, T.Foecke3
of Maryland
Center for Neutron Research
3NIST Metallurgy Division
2NIST
1University
Formability: strain localization
on grain boundaries
up to 0.5 mm displacement
found between neighboring
grains
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
Tensile Tests
Orientations
Tensile Tests
111
250
S1
S2
S3
S4
S5
S6
S7
S8
S9
S10
S12
S13
S14
S15
S5
S16
S4
S6
true stress [MPa]
200
S3
S2
S1
S9
S14
100
150
S10
S7
S12 S13
S8
S17
110
100
50
0
0
0.2
0.4
0.6
strain
0.8
1
Yield Stress
Polycrystal
Single crystal
 YS between 25
MPa and 40 MPa
 weak anisotropy
 25% YS of polycrystal
R-Values
w
R
t
R-values
1.2
1.0
R
0.8
0.6
0.4
0.2
0.0
g1 g2 g3
g4 g5 g6
g7 g8
g9 g10 g12 g13 g14 g15 P1 P2 P3
Specimen
 Extreme anisotropy
from r=0 (thinning
only) to r>1 (no
thinning)
 Polycrystal r=0.1
 Large r-values for
{210}<-120>
Effect of Annealing
250
120
(1),<100>,1000C
(3)
(2),<111>, 1000C
(3),<110>,1000C
(2)
true stress [MPa]
true stress [MPa]
200
(1)
(4),<211>, 700C
(5),<100>, 700C
150
(2)
100
(4)
50
80
(1)
(5)
(4)
40
(5)
(3)
0
0
0
0.2
0.4
0.6
0.8
1
0
0.01
strain
•Yield stress and yield drop increase with
annealing temperature
0.02
0.03
strain
0.04
0.05
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 smallangle 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 ….
What is needed …

Slip
systems
that
are
 Full strain rate
locally active at a
tensor at every point
given point in time
on the sample and
in time
 orientations
Taylor’s model
 Imposed strain rate
tensor

 ε11

 ε 12
ε  
 2
 ε 13

 2
ε 12
2
ε 22
ε 23
2






- ε 11  ε 22 

ε 13
2
ε 23
2
 Write the strain rate
tensor as a
γ α α
α
α
α



ε

m

n

n

m
α 2
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