PROGRESS ON LOW ACTIVATION STRUCTURAL STEELS

RECENT RESULTS ON NANOSTRUCTURED FERRITIC ALLOYS

A.F.ROWCLIFFE (ORNL)

ARIES PROJECT MEETINGU.C.S.D.

Jan 22nd. – Jan 23rd 2013

Nanograin structure & high density of nanoclusters. High energy mechanical alloying plus high power consolidation Very large interfacial area (NC-matrix and grain boundaries) enhances

recombination of point defects as well as accommodation of helium atoms or bubbles in irradiation.

In ferritic phase, the nanograins & NCs are exceptionally stable up to very high temperatures.

Nanostructured Ferritic Alloys (NFAs)

<Grain size> = 136 (+/- 14) nm

BF-TEM of 14YWT SM10 HeatNv = 1-7 x 1023 m-3 <r> = 1.8 (+/- 0.4) nm

HR-TEM

2

NFAs have outstanding strength and creep properties

M.A.Sokolov, D.T.Hoelzer, L.Tan (ORNL)

12YWT: First discovery of NC by 3DAP in 1999 (ORNL)

MA957: Patented by INCO (1978); NC discovered by 3-DAP in 2003 (ORNL)

14YWT: Developed at ORNL; initial project started in 2001

• MA957 and 12YWT are not commercially available- 12YWT was produced only

once as a small heat by Kobe Steel, Ltd.

- Production of MA957 was discontinued by INCO

Temperature, oC

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Yie

ld S

tren

gth

, M

Pa

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12YWT9Cr-2WVTa14YWT-SM614YWT-SM10PM2000MA95714YWT-SM7F82H

FRACTURE TOUGHNESS-STRENGTH PROPERTIES

• UNIQUE NANO-SCALE MICROSTRUCTURE OF THE LOW ACTIVATION NFA 14YWT CHALLENGES THE HIGH STRENGTH-LOW TOUGHNESS PARADIGM FOR STRUCTURAL ALLOYS

D.T.Hoelzer ORNL, unpublished work

0

0.2

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0.8

1

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1.4

0 400 800 1200 1600 2000 2400

14YWT

7000 Series Al Alloy

Multi-phase Nb Alloy

Zr BMG

Super Bainite Steel

AF1410

Fra

ctu

re T

ou

ghn

ess /

T

(MP

am

1/2/K

)

max

(MPa)

Neutron irradiation of 14YWT and 14WT at 300oC, 580oC, and 670oC to ~1.5 dpa (HFIR Rabbit capsules)

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14YWT (unirradiated) 14WT (unirradiated)

14YWT (Tirr

=300oC) 14WT (Tirr

=300oC)

14YWT (Tirr

=580oC) 14WT (Tirr

=580oC)

14YWT (Tirr

=670oC) 14WT (Tirr

=670oC)

Str

ess

[MP

a]

Temperature [C]

Irradiated Yield Stress

D.A. McClintock et al., JNM 386-388 (2009)

No DBTT shift was observed in 14YWT after irradiation at 300oC to ~ 1.5 dpa compared to 85oC

shift in Eurofer-ODS

-250 -200 -150 -100 -50 0 50 100 150 200 250 30025

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14YWT [KJc(1T)

] (unirradiated)

14YWT [KJIc

] (unirradiated)

14YWT [KJc(1T)

] (Tirr

= 300oC)

14YWT [KJIc

] (Tirr

= 300oC)Fra

ctur

e T

ough

ness

[M

Pa

m]

Temperature [C]

Irradiated 14YWT Fracture Toughness

D.A. McClintock, et al., JNM 392 (2009)

Eurofer-ODS

HELIUM MANAGEMENT IN ADVANCED STRUCTURAL ALLOYS

• TEM images of Eurofer 97 (TMS), (a, b), and MA957 (NFA) (c,d)• Neutron irradiated to 25dpa at 500C and simultaneously implanted with

~1450 appm He• G.R.Odette et al., Fusion Materials Semi-Annual Report, DOE/ER-0313/51

HELIUM MANAGEMENT IN ADVANCED STRUCTURAL ALLOYS

• High helium content (1450appm) promotes the formation of a large population of voids in neutron irradiated Eurofer 97

• Same quantity of helium is trapped by ~2nm clusters/particles in MA957 with no void formation

• Swelling rate theory extrapolations indicate that NFA will remain damage-tolerant to much higher levels of dpa and helium

• Extrapolations for the TMS indicate continuing growth of voids with eventual transition to a linear (with dose) swelling regime

NFAs WITH IMPROVED FRACTURE RESISTANCE

• NFAs based on 12-14% Cr tend to exhibit reduced grain boundary fracture resistance with increasing temperature > 400C

• New approaches to NFA fabrication involve controlled phase transformation during high temperature rolling to strengthen grain boundaries while retaining the nano-scale microstructure for high strength and radiation damage resistance

• New compositions are based on Fe-9Cr-2W-0.4Ti-0.2V- 0.3 Y2O3

• Initial results from a joint ORNL-KAERI project funded by Office of Nuclear Science and Technology show promise.

MICROSTRUCTURE OF NEW NFAs

T.S.Byun,D.T.Hoelzer (ORNL), J.H.Koon (KAERI); unpublished work10

9YWTV-PM2Fe-9Cr-2W-0.4Ti-0.2V-0.05C+0.3Y2O3

9YWTV-PM1Fe-9Cr-2W-0.4Ti-0.2V-0.12C-0.3Y2O3

PM1PM2

Strength & Ductility of 9Cr NFAs

• New base NFAs retain YS higher than 500 MPa at 700°C.• Improved ductility for both new NFAs compared to 14YWT

T.S.Byun, D.T.Hoelzer (ORNL), J.H.Koon (KAERI); unpublished work 11

12

Effect of Controlled Rolling 9YWTV-PM on High Temperature Fracture Resistance

T.S.Byun, D.T.Hoelzer ( ORNL ), J.H.Koon (KAERI); unpublished work

• Improvement of fracture toughness in controlled rolled 9YWTV-PM2 is significant.

• The 9YWTV-PM2 controlled rolled at 900°C for 50% reduction resulted in the best fracture toughness among NFAs, which is as high as those of non-ODS F/M steels.

• Potential avenue for NFAs based on 9Cr with improved high temperature fracture resistance

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22000 24000 26000 28000 30000 32000 34000 36000

NFA 12YWTNFA MA9579Cr-WMoVNb SteelNFA 14YWT

LMP T(K)[25 + log10t(h)]

Str

ess

(MP

a)

900°C, 1104h

800°C, 14235h

800°C, 817h

650°C, 13000h

600°C, 17000h

650°C, 1080h650°C

16h

650°C>72h

825°C, 57600h(in test)

800°C, 38555h

800°C, 16584h(in test)

Creep performance of NFAs

• Creep test on ruptured MA957 started in Oct., 2003 (INE

• Creep test on 14YWT-SM10 started in April, 2008

• Klueh et al, J. Nucl. Mat., (2005)• I-NERI FY01-04

-1.000

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CR

EE

P S

TR

AIN

(%

)

TIME (HOURS)

Ruptured after ~38,555 h