Proton Irradiated Microstructural and Mechanical Properties Investigation for Alloy 718

Mi Wang1, Miao Song1, Gary. S. Was1, Chinthaka Silva2,

Lawrence Nelson3, Gabriel Ilevbare4

1 University of Michigan2 Oak Ridge National Laboratory

3 JLN Consulting4 Electric Power Research Institute

Project sponsored by EPRI (MA10001593)

International Light Water Reactors Material Reliability Conference and Exhibition, Chicago, IL, USA, August 1 – 4, 2016.

Nickel-base alloy 718• Structural application in Pressurized Water Reactors (PWRs)

• Fuel assembly components

• Hold down springs

• Grid springs

• High strength alloy

• Precipitates strengthening: 𝛾′ (Ni3(Ti, Al)), 𝛾″ (Ni3Nb) phases

• Excellent mechanical properties

• High resistance of corrosion (~18% wt. Cr)

2

Specification Thermal treatment YS (MPa) Tensile Strength (MPa) Elongation (%)

AMS 5596, ASTM B 670,

AMS 5662, 5663

ASTM B, SB 637

Anneal at 927-1010C; Age at 718C(8h);

FC, Age at 621C (10h);

1034 1241, 1276 12

Modified grade 718 Anneal at 1093C; Age at 718C(4h); Age at

621 (16h)

874-884 1112-1126 43-43.4

Project alloy 718

• Objective: Fill in the technical gaps of alloy 718 in fuel assemblies and associated components.

• Non-irradiated microstructure development based on various thermo-mechanical treatments (effect of cold work…)

• Mechanical property variations depending on different thermo-mechanical treatments

• Microstructure changes induced by proton irradiation (phase stablity, radiation hardening,...)

• An understanding for the evolution of mictrosturcture with irradation and its effect on IASCC susceptibility

3

Alloy 718

• Two different heats

• 11 different thermo-mechanical treatments (TMTs)• Group I: SA at 1093∘C for 1h with various CW, two ageing

hardening steps at 718∘C and 621∘C for 8h, each.

• Group II: SA at 945∘C for 1h with various CW, two ageing hardening steps at 718∘C and 621∘C for 8h, each.

• Group III: SA at 1065∘C for 0.5h with various CW, two ageing hardening steps at 760∘C and 649∘C for 10h and 20h, each.

• Group IV: SA at 1093∘C for 1h, two ageing hardening steps at 788∘C, 715∘C and 633∘C for 50h, each.

Alloy Ni Fe Cr Nb Mo Ti Al C Si Ta Cu Co Mn P S

N 52.77 18.95 18.46 5.2 2.9 0.96 0.61 0.036 0.02 0.01 <0.01 <0.01 <0.01 <0.005 <0.0005

Opt. 52.73 19.18 18.49 5.01 2.9 0.76 0.78 0.036 0.02 0.02 <0.01 <0.01 <0.01 <0.005 <0.0005

Group Thermal treatment

I 1093/1+0/10/20%+718/8+621/8

1093/1+0%+718/8+621/8 (O)

II 945/1+0/10/20%+718/8+621/8

945/1+10%+718/8+621/8 (O)

III 1065/0.5+0/10%+760/10+649/20

IV 1093/1+0%+788/16+715/50+663/50

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Cold work effect (Group I)

A:1093C/1h+0%+718C/8h+621C/8h G:1093C/1h+10%+718C/8h+621C/8h H:1093C/1h+20%+718C/8h+621C/8h

Hv=447±16 Hv=448±18 Hv=477±22

0% cold work 10% cold work 20% cold work

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Hardness increases slightly with cold work, between 10% and 20% CW.

Cold work effect (Group II)

E:945C/1h+0%+718C/8h+621C/8h B:945C/1h+10%+718C/8h+621C/8h F:945C/1h+20%+718C/8h+621C/8h

Hv=443±10 Hv=492±10 Hv=495±12

0% cold work 10% cold work 20% cold work

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Hardness increases slightly with cold work, between 0% and 10% CW.

Cold work effect (Group III)

Hv=448±12 Hv=463±16

J:1065C/0.5h+0%+760C/10h+649C/20h D:1065C/0.5h+10%+760C/10h+649C/20h

0% cold work 10% cold work

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Hardness increases slightly with cold work.

Grain size and hardness

Group Conditions Grain size (μm) Hardness (Hv)

I A 1093/1+0%+718/8+621/8 110 447 ± 16

K(O) 1093/1+0%+718/8+621/8 (O) 140 378 ± 12

G 1093/1+10%+718/8+621/8 110 448 ± 18

H 1093/1+20%+718/8+621/8 110 477 ± 22

II E 945/1+0%+718/8+621/8 5.9 443 ± 10

B 945/1+10%+718/8+621/8 5.7 492 ± 10

L(O) 945/1+10%+718/8+621/8 (O) 13 460 ± 15

F 945/1+20%+718/8+621/8 5.8 495 ± 12

III J 1065/0.5+0%+760/10+649/20 100 448 ± 12

D 1065/0.5+10%+760/10+649/20 90 463 ± 16

IV C 1093/1+0%+788/16+715/50+663/50 175 423 ± 11

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Tensile tests at room temperature

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Mechanical properties at room temperature

718 Elastic Modulus

(GPa)

Yield Stress

(MPa)

Ultimate Tensile

Stress (MPa)

Fracture Strain

I 718A 195.4 927 1168 0.33

718K(O) 194.7 815 1104 0.321

718G 204.8 1013 1207 0.25

718H 207.9 1106 1271 0.052

II 718E NM NM NM NM

718B 212.2 1234 1409 0.15

718L(O) 219.3 1183 1383 0.179

718 F NM NM NM NM

III 718 J 209.5 1133 1361 0.221

718D 219.9 1030 1310 0.253

IV 718C 205.6 1034 1333 0.165

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Proton irradiated microstructural changes

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Experiment • Focus on 718A (Group I) and 718B (Group II):

• 718-A: 1093C/1h+0%CW+718C/8h+621/8h Modified grid

• 718-B: 945C/1h+10%CW+718C/8h+621/8h Aeronautic heat

• 2 MeV proton irradiation at Michigan Ion Beam Laboratory (MIBL)

• Specially designed stage contains tensile and TEM samples

• Doses: 0.05 dpa, 0.5 dpa, 4 dpa

• Dose rate: ~ 1.3 x 10-5 dpa/s

• Irradiation temperature = 360 ± 5°C

71.8 Ni Fe Cr Nb Mo Ti Al C Si Ta Cu Co Mn P S

N 52.77 18.95 18.46 5.2 2.9 0.96 0.61 0.036 0.02 0.01 <0.01 <0.01 <0.01 <0.005 <0.0005

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Precipitates in alloy 718A

4 dpa0.5 dpa0.05 dpaAs received

50 nm

Dislocation loops in alloy 718A

4 dpa0.5 dpa0.05 dpa

50 nm

Microstructure changes after proton irradiation

Alloy 718 Precipitates Dislocation loops

d (nm) ρ (1022/m3) d (nm) ρ (1021/m3)

718A AR 6.6 ± 1.4 3.99 N/A N/A

718A 0.05 dpa 5.1 ± 1.7 3.42 NM* NM*

718A 0.5 dpa 4.5 ± 1.2 0.87 7.9 ± 2.9 1.86

718A 4 dpa 3.9 ± 1.2 1.01 13.3 ± 7.7 2.99

*NM: There are not enough dislocation loops for statistical characterization.

Irradiation hardening

50g load

No significant hardening or softening was identified.

0.5dpa

Irradiation hardening

50g load

No significant hardening or softening was observed.

Baseline hardness

of as received

materials

IASCC behavior in PW environment

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Experiment - CERT test

Parameter PW

Temperature (∘C) 320

Pressure (psi) 2000

Inlet Conductivity

(µS/cm)

20-30

Outlet Conductivity

(µS/cm)

30-35

O2 Concentration (ppb) < 5

H2 Concentration (cc/kg) 35

[B] (ppm) 900-1100

[Li] (ppm) 2-3

pH at 25∘C 6 – 6.7

Constant Extension Rate Tensile (CERT) tests were conducted at a strain rate of ≤ 1 x 10-7 s-1 in 320°C PWR

primary water with 1000 ppm B as H3BO3, 2 ppm Li as LiOH, and 35 cc/kg H2.. SEM analysis of the

surfaces were conducted on all specimens after testing.

Stress-strain curves

Surface characterization of 718A

4.15%

Irradiated area

500 μm

• General observations:

• Covered with heterogeneous oxide over the surface

• No obvious cracks were observed at low magnification

Irradiated area

500 μm

4.25%

Stress

718A 0.05dpa

718A 4dpa

Crack observation in 718A

0.05 dpa

4.15%

10 μm10 μm

Unirradiated region Irradiated region

5 μm 5 μm

4 dpa

4.25%

Surface characterization of 718B

718B 0.05dpa

718B 4dpa

500 μm

500 μm

Irradiated area

3.4%

3.4%

Irradiated area Stress

• General observations:

• 718B 4dpa: tensile bar was necking and failed at the unirradiated area

• 718B 0.05dpa: no obvious necking were observed, sample failed

Crack observation in 718BUnirradiated region Irradiated region

No cracks observed

in 718B 4dpa

unirradiated area.

Long and large cracks

were observed in

718B 4dpa irradiated

area.10 μm 10 μm

10 μm

10 μm

4 dpa

0.05 dpa

3.4%

3.4%

Cracking susceptibility summary of 718

Group Alloy (dpa)

(plastic strain)

Area Crack

count

Grain Size

(μm)

Yield Stress

(MPa) (@320°C)

Average Crack

Length* (μm)

Crack density

(#Cracks/mm2)

Crack Length/unit area (Avg

length x density) (μm/mm2)

I 718 A

(4dpa)

(4.15%)

UIA 12 110 790 ± 10.2 15 ± 3 13 ± 0.1 192 ± 42

IA 12 11 ± 2 8 ± 0.1 92 ± 20

718 A

(0.05dpa)

(4.25%)

UIA 17 110 791 ± 20.2 17 ± 3 18 ± 0.1 320 ± 62

IA 13 18 ± 5 14 ± 0.1 248 ± 69

II 718 B

(4dpa)

(~3.4%)

UIA 0* 5.7 1130 ± 25.5 / / /

IA 15 35 ± 12 16 ± 0.1 578 ± 198

718 B

(0.05dpa)

(~3.4%)

UIA 67 5.7 1142 ± 43.9 3.7 ± 0.2 216 ±1.3 818 ± 43

IA 50 6 ± 0.6 161 ± 1 961 ± 101

*: abnormal result

IASCC behavior of alloy 718 in PW

Group I:

The difference in

value is statistically

insignificant for

comparison.

3.4% 3.4%4.15%4.25%

Group I Group II

*

*: abnormal result

IASCC behavior of nickel-base alloys in PW

Alloy 718 A is the least susceptible to IASCC among these nickel-base alloys.

*

*: abnormal result

Crack growth rate of alloy 718 in PW

• Constant displacement test in PW at 360°C on alloy 718 with a wide variety of heat treatments at different stress intensities.

• The heat (same with 718 A) shows the lowest crack growth rate.

Crack Growth Rate

(in/sec x 10-9)

Stress Intensity (ksi 𝑖𝑛)

Koch et al., CC Technologies - TR-

103290, Miglin, B&W;

Conclusion

• A variety of alloy 718 (two heats, 11 TMTs) were studied for the microstructure and mechanical properties.

• Alloy 718A (1093C/1h+0%CW+718C/8h+621C/8h):

• No significant changes in precipitates after proton irradiation while dislocation loops formed significantly after a dose of 0.5 dpa.

• No dose effect (0.05 dpa and 4 dpa) shown on IASCC behavior.

• Alloy 718B (945C/1h+10%CW+718C/8h+621C/8h) has a higher IASCC susceptibility in PW environment, further characterization is needed to interpret the IASCC behavior.