NEET-3 FY 2012 Award

Accelerated Development of Zr-Bearing Ferritic Steels for Advanced Nuclear Reactors

Lizhen Tan, Ying Yang Oak Ridge National Laboratory

Beata Thburska-Püschel, Kumar Sridharan University of Wisconsin-Madison

DOE-NE Materials Crosscut Coordination Meeting | Sept. 16, 2015

Research Plan

Alloy Design (ORNL)   Thermodynamic database development

  Alloy design & fabrication

Recommendation

Yes

Yes

Testing & Characterization (ORNL)   Mechanical testing

  Thermal aging experiment   Microstructural characterization

  Microstructural simulation

Radiation Resistance (UW)   Proton and heavy ion irradiations   Microstructure and hardness

No

No

The Need for Advanced Material Development

n  The escalating global clean-energy need drives higher operating temperatures of power plants for improved thermal efficiency.

n  Advanced materials with superior high-temperature strength can effectively improve plant economics (reduced commodities, increased thermal efficiency, longer lifetimes), safety margins, and design flexibility.

200 mm Dia.593.3°C32 MPa~320°C

~50%

Ferritic Steels Have Outstanding Properties for Engineering Design

n  Ferritic steels are important structural materials for nuclear reactors –  Advantages of FM steels over austenitic stainless steels

  High resistance to radiation-induced void swelling (e.g., ~10 times better at temperatures above 300°C)   High thermal conductivity and

low thermal expansion   Low-cost

0 200 400 600 8000

5

10

15

20

25

30

35

40

10

15

20

25

30

NF616X20CrMoV121T91

P22

TP316

Ther

mal

Con

duct

ivity

(W/K

m)

Temperature (oC)

P22

T91

NF616

X20CrMoV121

TP316

Mea

n C

oeffi

cien

t of L

inea

r Exp

ansi

on (1

0-6/K

)

500 700 800 600

100

10

1

Ni-base

FM Steels

ODS Ferritic Steels

New Steels

Cur

rent

Allo

y C

ost (

$/lb

)

Use Temperature (°C)

Austenitic Steels

[Source: adapted from B.A. Pint] [L. T

an, e

t. al

., JN

M 4

22 (2

012)

45.

]

n  Higher Cr23C6 amount results in greater creep rate.

Concerns of The Current FM Steels

High Cr23C6

Low Cr23C6

[F. Abe, Nature 2003]

Tested at 650°C/140MPa

Bet

ter P

erfo

rman

ce

n  Coarse Z-phase forms by consuming fine MX.

n  Laves phase coarsening deteriorates strength.

/70MPa

[K. Sawada, MST 2013]

[Q. Li, Metall Mater Trans A 2006]

637°C Early stage (Fine Laves)

Long-term (Coarse Laves)

Fe2(Mo,W)

Stress accelerates the replacement of MX by Z-phase.

Alloy Design Routes

n  Route I: Advanced FM steels –  Adjust alloy composition to reduce M23C6, increase MX, and prevent Z-phase.

n  Route II: Fully ferritic steels –  Prevent softening caused by the α à γ phase transformation

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n&

Temperature&(oC)&

γ" δ"α"

P91

0"

0.01"

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400" 600" 800" 1000" 1200" 1400"Ph

ase&Mole&Frac-o

n&Temperature&(oC)&

MX"

M23C6"

Laves"

Z"

P91

Approach to Accelerate The Development of Zr-Containing Ferritic Steels

Now/Future: Materials-by-Design; High efficient and low-cost

Past: Trial and Error Method; Time-consuming and expensive

Experiment Microstructure Property

Computational Tools (Software + Database)

Computational Microstructural Modeling

Microstructural  Simula�on  &  Valida�on  

Computational Thermodynamics

Kinetics Simulation

Phase  stability,  frac�on,  composi�on  

Precipitate size, distribution

CALPHAD (CALculation of PHAse Diagram)

Nucleation, Growth and Coarsening Theory

NEW thermodynamic property (G) and Mobility (D) databases of

the Zr-containing Fe-base system  

Science-based approaches

Computational tools used in this study

Thermodynamic property   OCTANT (in-house)

Mobility   MCFe (Non-encrypt)

Database

Computational thermodynamics   Matcalc 5.51   Pandat 8.0

Precipitation kinetics   Matcalc 5.51

Software

OCTANT: ORNL Computational Thermodynamics for Applied Nuclear Technology

Thermodynamic database Fe-C-Cr-Mo-Nb-Ti-W-Zr

Binaries C Cr Mo Nb Ti W Zr

Fe Fe-C Fe-Cr Fe-Mo Fe-Nb Fe-Ti Fe-W Fe-Zr

C C-Cr C-Mo C-Nb C-Ti C-W C-Zr

Cr Cr-Mo Cr-Nb Cr-Ti Cr-Ti Cr-Zr

Mo Mo-Nb Mo-Ti Mo-W Mo-Zr

Nb Nb-Ti Nb-W Nb-Zr

Ti Ti-W Ti-Zr

W W-Zr

Fe-X-Y Ternaries Cr Mo Nb Ti W Zr

Fe-C Fe-C-Cr Fe-C-Mo Fe-C-Nb Fe-C-Ti Fe-C-W Fe-C-Zr

Fe-Cr Fe-Cr-Mo Fe-Cr-Nb Fe-Cr-Ti Fe-Cr-W Fe-Cr-Zr

Fe-Mo Fe-Mo-Nb Fe-Mo-Ti Fe-Mo-W Fe-Mo-Zr

Fe-Nb Fe-Nb-Ti Fe-Nb-W Fe-Nb-Zr

Fe-Ti Fe-Ti-W Fe-Ti-Zr

Fe-W Fe-W-Zr

X-Y-C Ternaries Mo Nb Ti W Zr

Cr-C Cr-Mo-C Cr-Nb-C Cr-Ti-C Cr-W-C Cr-Zr-C

Mo-C Mo-Nb-C Mo-Ti-C Mo-W-C Mo-Zr-C

Nb-C Nb-Ti-C Nb-W-C Nb-Zr-C

Ti-C W-Ti-C Ti-Zr-C

W-C W-Zr-C

From literature From this work

[Y. Y

ang,

et.

al.,

JNM

441

(201

3) 1

90.]

Design of Zr-Bearing Alloys

n  Route I: Advanced FM steels –  9Cr ferritic-martensitic steels (T alloys): Better phase stability and lower

radiation-induced DBTT shift than 12Cr FM steels.

n  Router II: Fully ferritic steels –  15Cr ferritic stainless steels (L alloys): Better corrosion resistance than lower

Cr steels, negligible SCC issue, and without temperature-induced α – γ phase transformation in FM steels.

–  Intermetallics-strengthened ferritic alloys (Z alloys): Brand-new ferritic alloys without temperature-induced α – γ phase transformation in FM steels.

n  Reference alloy: Grade 91

9Cr Ferritic-Martensitic Steels T-Alloys

n  Aims: –  Increase MX; –  Reduce M23C6; –  Eliminate Z-phase; –  Not much change to Laves phase.

n  Advantages: The experience on steelmaking and welding of conventional FM steels can be directly employed.

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Phase&Mole&Frac-o

n&

Temperature&(oC)&

MX"

M23C6"

Laves"

T-alloy

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Phase&Mole&Frac-o

n&

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M23C6"

Laves"

Z"

P91

n  T alloys showed noticeable increases in yield strength (100-300 MPa) compensated with reductions in total elongation as compared to P91.

–  The miniature type SS-3 specimens have less material for deformation than regular specimens, partly resulting in the reduced elongation.

9Cr Ferritic-Martensitic Steels T-Alloys

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Yield&Stress&(M

Pa)&

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T(alloys"

P91"

T-alloy SS-3

P91 NIMS

Gauge Cross-Section

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45"

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Yield&Stress&(M

Pa)&

Temperature&(oC)&

T+alloys"

P91"

T-alloy SS-3

P91 NIMS

Gauge Cross-Section

9Cr Ferritic-Martensitic Steels T-Alloys

n  Precipitate-strengthening

n  Dislocation-strengthening

–  MX precipitates exhibited greater contribution to strength than free dislocations.

–  Δσcalc. = σT-alloy – σP91 = 258 MPa, comparable to the room-temperature tensile results.

P91

T-alloy

σ i == 0.8MGb / λi == 6.98××10−−5 rini

σ d == 0.5MGb ρd == 3.17××10−−5 ρd

T-alloy P91 Size of MX (r, nm) 5 20 Density of MX (n, m-3) 1022 1021

Density of dislocations (ρd, m-2) 1014 1013

σMX, MPa 493 312 σd, MPa 317 100 , MPa 586 328

with M = 3.06, G = 83 GPa, b = 0.25 nm

σMX2 ++σ d

2

n  Aims: –  Develop in-situ composites composed of hard intermetallics and soft matrix; –  Discover a balanced intermetallics-matrix microstructure for superior properties.

n  Advantages: –  Simpler steelmaking processes than FM steels; –  Without α – γ phase transformation during heating and cooling.

Intermetallics-Strengthened Ferritic Alloys Z-Alloys

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Phase&Mole&Frac-o

n&

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Laves"

Ferrite"

Z7

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n&

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γ" δ"α"

P91

Intermetallics-Strengthened Ferritic Alloys Z-Alloys

n  Z-alloys showed comparable or greater yield strength than P91, especially at temperatures above ~600oC.

–  Ductility (total elongation) of the Z-alloys can be adjusted by microstructural (composition) control of the alloys.

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Yield&Strength&(M

Pa)&

Temperature&(oC)&

Open"circles:"P91"

Z-alloy SS-3

P91 NIMS

Gauge Cross-Section

Z7alloys"

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Total&Elonga*

on&(%

)&

Temperature&(oC)&

Z-alloy SS-3

P91 NIMS

Gauge Cross-Section

Z+alloys"

P91"

Intermetallics-Strengthened Ferritic Alloys Z-Alloys

n  Different from P91, Z-alloys are composed of eutectic network in a ferritic matrix, which are strongly dependent on alloy composition.

n  Primary phase need to be eliminated in the Z-alloys, which had brittle fracture, in contrast to the intermetallic particles favored ductile facture.

20 µm

Z7

20 µm

Z9

Primary phase

[L. Tan, et al., MMTA 46 (2015) 1188.]

20 µm

P91

Aging Effect on Strength and Ductility

n  Aging resulted in softening of FM steels (T-alloys) but strengthening of ferritic steels (Z-alloys).

n  Composition adjustment can noticeably mitigate aging-induced softening in FM steels.

–  As compared to alloy TT1, Zr-alloying (alloy TTZ1) mitigated the aging-induced softening.

!60$

!40$

!20$

0$

20$

40$

60$

80$

100$

120$

140$

YS,$TS$an

d$EL$Cha

nges$(%

)$

YS$

TS$

EL$

Aged$at$600oC$for$5800$h$

TTZ1$$$$$$$$$$$TT1$$$$$$$$$$$$$TT2$$$$$$$$$$$$T3t$$$$$$$$$$$$$$Z6$

!60$

!40$

!20$

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YS,$TS$an

d$EL$Cha

nges$(%

)$

YS$

TS$

EL$

Aged$at$700oC$for$5800$h$

TTZ1$$$$$$$$$$$TT1$$$$$$$$$$$$$TT2$$$$$$$$$$$$T3t$$$$$$$$$$$$$$Z6$

Creep Resistance of The New Alloys

n  T-alloys and Z-alloys showed comparable or greater creep rupture life than P91 at 650oC.

–  Generally, Z-alloys have greater creep life and strain than T-alloys.

0"

10"

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30"

40"

0" 200" 400" 600" 800" 1000"

Strain'(%

)'

Creep'Time'(h)'

650oC&110"MPa"

Z1alloy"

T1alloy"

P/T91"

T-alloy SS-3

P/T91 NIMS

Gauge Cross-section

70#

80#

90#

100#

110#

120#

130#

140#

150#

100# 1000# 10000#

Creep%Stress%(M

Pa)%

Creep%Life%(h)%

650oC#

P91#

SS304#

T1alloy#

Z1alloy#

Intermetallics-Strengthened Ferritic Alloys Z-Alloys

n  High densities of precipitates (1019 to 1022 m-3) formed in the Z-alloys during creep testing at 650oC, which enhanced creep resistance of the alloys but did not impair creep ductility.

9Cr Ferritic-Martensitic Steels T-Alloys

n  Creep at 650oC resulted in significant amount of dislocations and recovery of lath boundaries, but not much effect on precipitates.

n  Recovery of lath boundaries is the primary mechanism resulting in softening of T-alloys, similar to general 9-12% Cr FM steels.

g211$

T&alloy@Tab$

g200$

T&alloy@Gauge$

g211$

T&alloy@Gauge$

Ion-Irradiation Experiments

n  Radiation resistance of the alloys has being evaluated using proton ion irradiation. Heavy ion (Fe2+) irradiation experiments will be conducted.

–  Radiation-hardening, radiation-induced phase stability, swelling and segregation will be studied.

1.7 MV Tandem Accelerator Ion Beam @ UW-Madison

Proton Irradiation Experiments

n  Twelve (12) different alloys were irradiated using protons to 0.1 and 1 dpa at 420oC and a dose rate of 3x10-6 dpa/s.

Fig. 1: Damage profile in Fe-10Cr irradiated with 2 MeV proton to 1 dpa.

Fig. 2: Picture of L-alloys before and after proton implantation.

Radiation Hardening Vickers Microhardness

n  Vickers micro-indentation with 25 gf (~1.9–2.6 µm = ~10% of Rp): –  L-alloys exhibited the greatest hardening (120%) after 1 dpa proton irradiation. –  Z-alloys had a large variation in hardening (~30-90%) after ~1 dpa, indicating a strong effect of

solute elements on radiation hardening. –  T-alloys showed a small level of hardening (20-40%) after up to 2 dpa. –  As compared with Grade 91 (open circle) with ~40% hardening, the T-alloys and selective Z-alloys

have lower hardening.

-­‐50

0

50

100

150

200

1  dpa0.1  dpa

P roton,  420°CL -­‐a lloy:  black  s quareT -­‐a lloy:  red  c irc leZ -­‐a lloy:  blue  triang les

Cha

nge  in  hardn

ess  [%

]

Radiation Hardening Nanoindentation

Hardness calculated by Olive-Pharr method:

𝐻𝐻= 𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃

𝑃𝑃𝑃𝑃𝑃𝑃=24.5ℎ𝑃𝑃𝑃𝑃𝑃𝑐2𝑃

Ac - projected contact area; hc - actual indentation depth; Pmax – maximum load

Loading curve for irradiated T22 alloy (1 dpa)

Nano-hardness -- results

Hardness calculated by Olive-Pharr method:

Hardness vs contact depth for all irradiated steel samples (red) as compared to un-irradiated base material (blue).

𝐻𝐻= 𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃 𝑃𝑃𝑃𝑃𝑃𝑃=24.5ℎ𝑃𝑃𝑃𝑃𝑃𝑐2𝑃

Ac - projected contact area; hc - actual indentation depth; Pmax – maximum load

-  Surface roughness causes increase in standard deviation a the 2um mark.

-  Roughness correction can be applied using the advanced nonlinear Olive-Pharr method.

-  Irradiated T22 alloy shows steel softening possible due to presence of voids.

XRD analysis

  T-alloys are most resistant to precipitate formation/phase changes due to proton irradiation, which results in the best hardness performance

  Z-alloys underwent some phase change during proton irradiation at various damage levels. Fe2Zr hexagonal phase could be clearly identified, especially in the samples with a higher Zr content.

[110

]

[200

]

[211

]

[110

]

[103

]

[114

][202

][008

]

[300

]

[216

]

[001

2]

[201

0]

[220

]

20 30 40 50 60 70 80

 0  dpa  0.1  dpa  1  dpa

 

 

 

2  MeV  protons  into  Z 3,  420°C  

2θ  [°]

Intens

ity  [a

.u.]

 04-­‐003-­‐4099  F eC r  00-­‐026-­‐0809  F e

2Z r

[110

]

[200

] [211

]

20 30 40 50 60 70 80

 

 

 0  dpa  0.1  dpa  1  dpa

2  MeV  protons  into  T 12,  420°C  

2θ  [°]

Intens

ity  [a

.u.]

 04-­‐003-­‐4099  C rF e

Microstructure of Z-Alloy Ferritic Matrix

  BCC single phase (d(110) = 1.97 Å) accords with α-Fe.   Dislocation lines with Burgers vectors of <111>/2 or

<100>   Long dislocation lines are mostly nucleated from

matrix grain boundaries or phase interfaces   Short dislocation threads (10~100 nm) are uniformly

dispersed throughout the grain. They are identified as edge dislocations with Burgers vector of <100>

Diffraction patterns in [101] zone axis of α-Fe. (d) Bright-field TEM image of the grain boundary. Defects and/or strain field are noticed near the boundary. (Inset) diffraction pattern showing a small misorientation angle.

WBDF TEM image of the same grain at different g vectors. All images were recorded in (g, 3.1g) condition

Microstructure of Zr/W-rich Phase

  Aberration-corrected HRSTEM revealed high density of planar defects in the β-Fe2Zr, a C36 type (P63/mmc) Laves phase (b-circle), and β’-Fe2Zr, a C14 type (P63/mmc) Laves phase (c-circle).

  The stacking order of structural units agrees perfectly with structure models of faulted C36 and pristine C14 Laves phases

  Atomic ratio between Fe and Zr in these Laves phases is about 2.6 – chemically imperfect

Crystal structure of Zr/W-rich phase. (a) Bright-field TEM image shows severely faulted bands divided by defect-free bands. (b, c) Selected area diffraction patterns from circle b and c in (a), respectively.

Summary

  Two classes of alloys with Zr-alloying were developed, i.e., T-alloys (advanced FM steels) and Z-alloys (intermetallics-strengthened alloys), which showed promising results for superior high-temperature performance as compared with Grade 91.   Increased yield/tensile strength (by ~100–300 MPa from ~700 to 25°C)

compensated with some decreases in ductility.   Improved creep resistance with significantly greater creep lives.

  Alloy composition exhibited noticeable effect on radiation hardening/softening for both Z-alloys and T-alloys.