MSU Ingot Niobium Investigations

Thomas R. Bieler

D. Kang, D. Baars, S. Chandrasekaran, A. Mapar

G. Ciovati, N.T. Wright, F. Pourboghrat, C. Compton,

J. Murphy, G.R. Myneni

Michigan State University

University of Nevada Reno

Thomas Jefferson National Accelerator Facility

Work funded by DOE/OBES

Overview

• Ingot characterization

• Characterization of a formed half cell from ingot slice

– Internal defect quantification, i.e. dislocations

– Lattice curvature necessarily correlated with dislocations

• Characterization of a large grain tube

– Fabricated from welded polycrystal tube

• Deformation of single crystals

• Crystal plasticity modeling

• Thermal conductivity in single and bi-crystal samples

– Effects of deformation and heat treatment

– Hydrogen

• Summary

BCC dislocations can move in 4 directions

on as many as 48 slip systems*

• Dislocations enable large plastic strains by facilitating shear

on slip systems – there are 48 ways to do this, but most think

that 24 of them are responsible for most of the deformation

[-1 1 1] (1 1 0)[-1 1 1] (1 0 1)[-1 1 1] (0 -1 1)[-1 1 1] (1 -1 2)[-1 1 1] (-1 -2 1)[-1 1 1] (2 1 1)

[-1 1 1] (1 -2 3)[-1 1 1] (2 -1 3)[-1 1 1] (-2 -3 1)[-1 1 1] (-1 -3 2)[-1 1 1] (3 1 2)[-1 1 1] (3 2 1)

4 groups, 12 associated with each <111> direction (connects opposites corners)

One group illustrated here

* FCC: 6 directions in 12 systems

Ingot slice features, dislocation substructure

• Ingots show huge or variable grain size,

• dislocation entanglements in as-received ingot (ECCI image)

• and substantial orientation gradients (less than about 10),

Scan

area of

2-4 mm2

Laue camera used to measure

orientations of several ingots

CBMM Ningxia Heraeus

From measured

orientations, maps are

drawn using orientation

imaging software

• Crystal orientations measured

with a Laue camera

• Indexed using semi-automatic

software

Vertical lines are scars from end milling

~30

cm

~35 cm

Characterization of several ingots show no

trends in crystal orientation or grain shapes

CBMM Ningxia Heraeus

...

{110}

{112}

both

Bi-axial stress Schmid

factor maps for different

slip systems some

ingot slices deform more

uniformly than others

• 5 different ingots from different manufacturers have no common

orientation trends

crystallization during

e-beam refining is a random process

.

..

CBMM

Single/multi crystal cavities fabricated by welding

two half cells – grain boundaries are visible

• Single cell multicrystal cavities made at MSU and J-Lab show effects of grain boundaries, irregular deformation

• MSU single cell cavity grain boundary ridge visible and easily felt with fingers, and cups have ‘ears’

Material in Equator / Weld Experiences

1. Hoop Compression, radial tension

2. Bending + unbending

3. Biaxial stretching (not balanced)

Undeformed single crystals can be welded

together gracefully (sometimes)

• Center piece of sliced ingot

cut in two, flipped, welded,

• the weld was clean between

two crystal orientations, but

new orientations developed

at triple junctions

Weld Direction

AB

B’A’

B

B’

B

B’

A

B’

Weld Direction

Recrystallization features on either

side of equator weld in deformed

heat affected zone of a finished

large-grain cavity

1mm

Recrystallization in HAZ Weld

Baars et al. Transactions Applied Superconductivity 17, June 2007

Not surprisingly, welding deformed single

crystals leads to recrystallization

• Two tensile deformed single crystals were welded together

• Parent orientations in grips have white prism orientation

• Black prisms show crystal orientations after weld

Weld Side F ~(111)[1-10] Side C ~(101)[10-1]

400m

3.1mm 2mm 8.1mm3mmWELD

Some commentary on substructural state

Near

shoulder,

cold end,

some

shear

bands are

evident

Near

shoulder,

cold end,

some

shear

bands are

evident

Recrystallization

front; very blue

grains have few

dislocations

within, but along

grain boundaries,

there is higher

dislocation density

Substantial

dislocation

substructure

present in

recovered grain at

recrystallization

front

Metallurgical characterization of large grain

single cell fabrication study at J-Lab

• Slices H1, H2, H3, H4 examined, with different heat

treatment and etching histories, before / after joining

together, etc.

• Instead of machining off excess, rings were EDM cut at iris

and equator, to provide material that could be examined

microstructurally.

• These pieces were characterized with OIM to identify local

orientation gradients.

• Thickness was measured in various locations to identify

effects of crystal orientation and grain boundaries.

• Cavity performance was assessed – mediocre until barrel

polishing was done ~ 150-200 microns removed, then it

performed well.

2.2

2.4

2.6

2.8

3

3.2

3.4

3.6

0 1 2 3 4 5 6 7 8 9 10 11 12

Th

ickn

ess

, m

m

Radial A Radial B Radial C

Radial D Radial E Radial F

-- Iris --- Position --------- Equator --

2.2

2.4

2.6

2.8

3

3.2

3.4

3.6

0 1 2 3 4 5 6 7 8 9 10 11 12

Th

ickn

ess

, m

m

Radial A Radial B Radial C

Radial D Radial E Radial F

-- Iris --- Position --------- Equator --

Forming a half cells leads to irregular ears in edge –

influenced by grain boundaries, thickness issues

No obvious trends between crystal orientation and thickness

H1 was significantly thicker, had a thick spot at GB that grabbed die Biggest Ear

Sharp Ear

B

A

C

D

E

F

Sharper

Ear

• Note surface damage extending into surface by ~100 µm

• Slow heating prevents recrystallization, leaves geometrically

necessary dislocations in place

• Local Average Orientation identifies regions with more/less defects

Grain Orientation Local Orientation Gradient

Unlike a weld, a large grain piece from equator/iris region before

and after heat treatment shows no recrystallization after 800C

furnace anneal (or a 1000C anneal either)

Local Average Misorientation (orientation gradient),

measures geometrically necessary dislocation

density

4mm

+ 800

anneal

As-

deformed

GB Equator GB

GB Iris GB

+ 800

anneal

As-

deformed

Grain Orientation Local Orientation Gradient

Quantification of LAM measurements

Results after heat treatment are ~ same

Average of

6 depth

traces in iris

and equator

rings in 6

different

locations

indicates

effects of

surface-die

interactions,

i.e. the

damage

layer is

quantified,

60-200 µm

deep

Hot spot correlated with thick GB spot on

thick half cell side (no issue on other side!)

After H1-H2 cavity

was fabricated,

etched, the

performance was

not so good.

1400ºC anneal did

not help.

Optical inspection

of hot spot location

was at location

where etch pits at

grain boundary

were located.

Barrel polishing 100

µm led to excellent

performance.

After

1400ºC

anneal

The possibility of hydroforming a large grain

tube was explored

• Large grain tube made from welded polycrystalline tube after

heat treatment made by Jim Murphy at U. Nevada Reno

• Grain were grown to very large size,

but not a single crystal

– See unwrapped grain map

• Center region has only one grain

orientation

• Tube was slightly warped, difficult to

achieve seal for pressurizing

• Deformed heterogeneously, cracked

within the large center crystal

• Still under analysis, will be simulated

using crystal plasticity model

Single Crystals taken from (Ningxia) ingot,

have highly varying mechanical propertiesCrystal

orientations

chosen

strategically

to favor {110}

vs. {112} slip,

single vs.

duplex slip.

Results

support

preference

for {112} slip;

two lower

stressed

{112} is more

favorable

than favored

{110}

{112} slip almost accounts for all the initial

hardening behavior of the 9 specimens

Specimen

Difference between highest

resolved shear stress on

primary and secondary {112}

slip systems (MPa), and ratio

of resolved shear on two

highest {112} systems

Initial

hardening

rate

X3 4 1.316 Barely

Q2 4 1.263 Barely

S3 3.5 1.237 Barely

R2 3.5 1.200 Slight

W3 1.4 1.091Moderate-

Low

T3 1.3 1.073 High

U3 0.6 1.027 Very High

V3 0.4 1.022Moderate-

High

P3 0.1 1.042 HighBaars, Investigation of active slip systems in high

purity single crystal niobium, PhD dissertation

Annealing the samples leads to significant decrease in

strength, and more regular deformation characteristics

As extracted from ingot Deformed in-situ after 800C 2hr anneal

Tensile axis orientationsWhite {110} Gray {112}Schmid Factor Contours 0.5, 0.499, 0.49, 0.47, 0.44, 0.40, 0.36, 0.32

P

U

V

W

XS

T

R

Q P

U

V W XS T

RQ

(a) (b)

• As-extracted curves show some softening in softer orientations

with single slip conditions, suggesting unlocking of dislocation

tangles.

• After annealing all curves show lower yield stress and always

positive work hardening, and indicating fewer defects present

Effect of heat treatment favors operation of

{110} slip system (crystal rotations)

• Slip trace features

suggest bursts of

dislocations occurring

at the micron scale

• Rotations that

develop during

deformation will

evolve in complex

ways as dislocations

accumulate

• From analysis of crystal rotations during the tension test, slip on

{110} planes dominates, rather that {112} slip in as-received

• This information about conditions that affect slip

system activity is needed for codes than can predict

heterogeneous strain in large grain forming

Crystal plasticity models are able to capture

overall trends, imperfectly…

• Different crystal plasticity modeling approaches that monitor

relative amounts of slip in different slip directions do a better

job than classical hardening models to predict flow behavior

– calibration based upon two samples has some capability

to predict deformation in other orientations

0

10

20

30

40

50

60

70

80

90

0 10 20 30 40

Eng.

Str

ess

, MP

a

Eng. Strain, %

P - Experiment P - Classical P - DynamicT - Experiment T - Classical T - DynamicU - Experiment U - Classical U - DynamicW - Experiment W - Classical W - Dynamic

0

10

20

30

40

50

60

70

80

90

0 10 20 30 40

Eng.

Str

ess

, MP

a

Eng. Strain, %

P - Experiment P - Diff-ExponentialR - Experiment R - Diff-ExponentialU - Experiment U - Diff-ExponentialW - Experiment W - Diff-Exponential

High thermal conductivity minimizes

degrading effect of ‘hot spots’

• Phonon peak appears and disappears

but dislocation content is a key factor

for thermal conductivity…

• How does dislocation substructure

affect RRR?

Different

phenomena

above and

below ~ 3K

Dislocation substructure

& thermal properties

• Prior work (Cotts, Northrup, Anderson 1981-83) examined phonon transport

(dissipation) in LiF crystals with known dislocation substructures

– When phonon transport direction was parallel to a mobile dislocation

segment, phonon was dissipated (phonon converted to random vibration)

• Phonons travel according to (anisotropic) elastic properties

– Grain boundaries also dissipate phonons

• Is the low T phonon-peak killed when there are mobile dislocations that can

couple with phonon and disperse its energy?

If so, then dislocation substructure

may need to be managed with

respect to crystal orientation to

maximize phonon transport

So, Recovery or Recrystallization?

Subgrain Bdy

Dislocation walls

in Fe

Subgrain

Boundaries

in Fe

Hull & Bacon Introduction to Dislocations, 1984

High thermal conductivity minimizes

degrading effect of ‘hot spots’

• Phonon peak appears and disappears

but dislocation content is a key factor

for thermal conductivity…

• How does dislocation substructure

affect RRR?

Different

phenomena

above and

below ~ 3K

Dislocations clearly kill the phonon peak

• Carefully analyzed thermal conductivity shows that the phonon

peak component (kpp2) plateaus at about 1000ºC. The

decrease in kpp2 is greatest in orientations where multiple slip

occurs nearer edge of triangle, which causes greater increases

in dislocation content.

Introduction of Hydrogen also affects

phonon peak, sometimes not reversible

Phonon peak was restored Phonon peak not restored

on side 2

q q

Observations and Speculations

• From making an ingot to final function, dislocations are an

omnipresent enabler and suspect,

– Additional suspects: H, O, impurities, interfaces, magnetic

fields, surface energy

– Dislocations can be removed most effectively by

recrystallization;

– Recovery leaves substructure that is oriented in

crystallographic directions

• Is the perfect cavity a recrystallized single crystal with

dislocation segments not lined up in a radial direction?