Study of Irradiation Damage in Domestically Fabricated Nuclear Grade Stainless Steel

doi:10.11900/0412.1961.2017.00117

Acta Metall Sin

2017, Vol. 53

Issue (12): 1588-1602 DOI: 10.11900/0412.1961.2017.00117

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Study of Irradiation Damage in Domestically Fabricated Nuclear Grade Stainless Steel

Ping DENG^1,², Qunjia PENG^1,³(

), En-Hou HAN¹, Wei KE¹, Chen SUN³, Haihong XIA³, Zhijie JIAO⁴

1 CAS Key Laboratory of Nuclear Materials and Safety Assessment, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China
2 School of Materials Science and Engineering, University of Science and Technology of China, Shenyang 110016, China
3 State Power Investment Corporation Research Institute, Beijing 102209, China
4 Department of Nuclear Engineering and Radiological Sciences, University of Michigan, Ann Arbor, MI 48109, U.S.A.

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Ping DENG, Qunjia PENG, En-Hou HAN, Wei KE, Chen SUN, Haihong XIA, Zhijie JIAO. Study of Irradiation Damage in Domestically Fabricated Nuclear Grade Stainless Steel. Acta Metall Sin, 2017, 53(12): 1588-1602.

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Abstract

The radiation-induced segregation (RIS) and microstructure evolution such as dislocation loops and cavities are major microstructural causes for the irradiation-assisted stress corrosion cracking (IASCC) of austenitic stainless steel (SS) core components. While a couple of studies have been reported on the irradiation induced damage in nuclear grade (NG) austenitic SS, the evolution of dislocation loop density and size and its correlation with the mechanical properties have still remained incompletely understood. In addition, the correlation between the segregation at the grain boundary and that at the dislocation loop has received limited attentions. In particular, there is still a lack of a systematic study of the irradiation damage in domestically fabricated NG austenitic SS. In this work, the proton-irradiation induced microstructural damage in domestically fabricated 304NG SS was characterized, in an effort to correlate the RIS and the dislocation loop density and size with the irradiation dose, as well as the dislocation loop density and size with the radiation-induced hardening. The results revealed that the radiation-induced microstructure damage was mainly dislocation loops with a few micro-voids. The loop density was in the order of 10²² m^-3 with an average size of <10 nm. The square root of the product of loop density and size (Nd)^0.5, scaled linearly with the square root of irradiation dose with a factor of 6.8×10³ dpa^-0.5mm^-1. The loops were believed to be mainly responsible for the hardening in 304NG SS, which also scaled linearly with (Nd)^0.5 with a factor of 1.16×10^-2 HV_0.025mm. A comparative analysis about the segregation at the grain boundary and at the dislocation loop was conducted. While the depletion of Cr and enrichment of Ni at the dislocation loop and grain boundary showed no difference, the enrichment of Si at the dislocation loop could be of about 6 times of that at the grain boundary. In addition, the loop density and loop size, as well as RIS and radiation-induced hardening were all increased by a higher dose and tended to saturate by a dose of 3.0~5.0 dpa.

Key words: nuclear grade stainless steel proton irradiation dislocation loop radiation-induced segregation radiation-induced hardening

Received: 06 April 2017

ZTFLH:

TG139.4

Fund: Supported by International Science & Technology Cooperation Program of China (No.2014DFA50800) and National Natural Science Foundation of China (No.51571204)

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	Ping DENG
	Qunjia PENG
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	Wei KE
	Chen SUN
	Haihong XIA
	Zhijie JIAO

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https://www.ams.org.cn/EN/10.11900/0412.1961.2017.00117 OR https://www.ams.org.cn/EN/Y2017/V53/I12/1588

Fig.1 Schematic of irradiation specimen bars geometry (The irradiated region, hardness measurement positions and 3 mm TEM disks are highlighted by shaded area, red lines and circles, respectively)

Fig.2 Procedure for making 3DAP tips(a) the region of interest protected by Pt deposition(b) trenches cut around the Pt deposition(c) a microtip coupon with a couple of microtip posts(d) top view of the sample wedge sitting on the top of the post(e) Pt weld used to connect part of the sample wedge and the microtip post(f) the sample attached to the microtip post(g) side view of the sample post(h) view of the sample after cutting all round off(i) the final 3DAP tip

Fig.3 Bright-field TEM images of 304 stainless steel before (a) and after proton irradiation to a dose of 0.5 dpa (b), 1.5 dpa (c), 3.0 dpa (d) and 5.0 dpa (e) at 360 ℃, and the amplified image of a dislocation loop in Fig.3e (f)

Fig.4 Dark-field TEM images using g=200 (ZA [110]) showing dislocation components in 304 stainless steel before (a) and after proton irradiation to a dose of 0.5 dpa (b), 1.5 dpa (c), 3.0 dpa (d) and 5.0 dpa (e) at 360 ℃, and SAED pattern used for the dark-field TEM observation (f)

Fig.5 Loop size distributions in 304 stainless steel following proton irradiation at 360 ℃ to a dose of 0.5 dpa (a), 1.5 dpa (b), 3.0 dpa (c) and 5.0 dpa (d)

Fig.6 Measured changes in density and size of dislocation loops as a function of irradiation dose

Table 1 Summary of microstructure and Vickers hardness measurements for 304 stainless steel following proton irradiation at 360 ℃

Fig.7 Bright field TEM images of voids in 304 stainless steel following proton irradiation at 360 ℃ to a dose of 0.5 dpa (a), 1.5 dpa (b), 3.0 dpa (c) and 5.0 dpa (d) (The voids are observed in slightly under-focused condition and highlighted by black arrows)

Fig.8 Radiation-induced segregation at the grain boundaries in 304 stainless steel

(a) bright field TEM image of a grain boundary in the unirradiated steel showing locations of EDS point- and line-scan analyses

(b~f) variations in the composition at grain boundaries in the steel following proton irradiation at 360 ℃ to a dose of unirradiated (b), 0.5 dpa (c), 1.5 dpa (d), 3.0 dpa (e) and 5.0 dpa (f)

(g) grain boundary composition variations with irradiation doses

Fig.9 3DAP atom maps showing radiation-induced segregation in 304 stainless steel following protons irradiation at 360 ℃ to a dose of 3.0 dpa (Enrichment of Si and depletion of Mn is evident. The dimension of the box is 69 nm×70 nm×220 nm)

Fig.10 3DAP atom isoconcentration surface plot of 304 stainless steel following irradiation to a dose of 3.0 dpa at 360 ℃

(a) dislocation loops and Si-rich clusters are more "visible" using Si isoconcentration surface plot at 2.5%

(b) overlap of isoconcentration surface plot of Si, Ni, Cr and Mn

(c) isoconcentration surface plot of Ni at 16% (green) and Si 2.5% (gray) showing enrichment of Ni at dislocation loops and Si-rich clusters

(d) isoconcentration surface plot of Cr at 12% (gold) and Si 2.5% (gray) showing depletion of Cr at dislocation loops and Si-rich clusters

(e) isoconcentration surface plot of Mn at 0.6% (blue) and Si 2.5% (gray) showing depletion of Mn at dislocation loops and Si-rich clusters

Fig.11 3DAP atom maps showing radiation-induced segregation at a dislocation loop highlighted by shaded area in Fig.10a in 304 stainless steel following irradiation to a dose of 3.0 dpa at 360 ℃ (a) and composition profile across the dislocation loop along the rectangle with dimensions of 4 nm×4 nm×40 nm in Fig.11a (b) (The arrows in Fig.11b show the approximate positions of the dislocation core)

Fig.12 Variations of microhardness as a function of irradiation dose (a) and variations of radiation-induced hardening (ΔH) as a function of the square root of irradiation dose (b)

Fig.13 Variations of the square root of the product of dislocation loop density and size (Nd)^0.5 as a function of the square root of irradiation dose

Fig.14 Schematic showing vacancy and solute atom flows and composition changes for mechanisms of radiation-induced segregation

Fig.15 Radiation induced segregation at the grain boundary and at the dislocation loop in 304 stainless steel following irradiation to 3.0 dpa at 360 ℃ (a) and ratio of the segregation at dislocation loop to that at grain boundary (b)

Fig.16 Variations of irradiation-induced hardening (ΔH) as a function of the square root of the product of dislocation loop density and size

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