Uniaxial and Multiaxial Plastic Deformation of Large Niobium Grains

Thomas Gnäupel-Herold1,2,Adam Creuziger, T.Foecke3

1University of Maryland2NIST Center for Neutron Research

3NIST Metallurgy Division

Formability: strain localization on grain

boundaries

up to 0.5 mm displacement found between neighboring grains

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

Plastic Properties of Niobium

Tensile TestsOrientations

Tensile Tests

0

50

100

150

200

250

0 0.2 0.4 0.6 0.8 1strain

true

str

ess

[MPa

]

S1S2S3S4S5S6S7S8S9S10S12S13S14S15

1 0 0 1 1 0

1 1 1

S1

S2

S3

S4

S5

S6

S7

S8

S9S10

S12 S13S14

S16

S17

Yield StressYield Stress

0

20

40

60

80

100

120

140

160

g1 g2 g3 g4 g5 g6 g7 g8 g9 g10

g12

g13

g14

g15

P1

P2

P3

Specimen

s0.

2 [M

Pa]

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}<-120>

R-values

0.0

0.2

0.4

0.6

0.8

1.0

1.2

g1 g2 g3 g4 g5 g6 g7 g8 g9 g10 g12 g13 g14 g15 P1 P2 P3

Specimen

R

t

wR

Effect of Annealing

0

50

1 00

1 50

2 00

2 50

0 0 .2 0 .4 0 .6 0 .8 1

strain

true

stre

ss[M

Pa]

(1),<100> ,1000C(2),<111> , 1000C(3),<110> ,1000C(4),<211> , 700C(5),<100> , 700C

0

4 0

8 0

1 2 0

0 0 .0 1 0 .02 0 .0 3 0 .04 0 .05

strain

true

stre

ss[M

Pa]

(1)

(1)

(2)

(2)

(3)

(3)

(4)

(5)

(4)

(5)

•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 dataWhat 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

22112313

2322

12

131211

εε- 2

ε 2

ε2

ε ε 2

ε2

ε 2

ε ε

ε

αααα mnnmε α

α

2γ

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