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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)
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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
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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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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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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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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 observed.
Baseline hardness
of as received
materials
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.
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.