Improved irradiation resistance of accident-tolerant high-strength FeCrAl alloys with heterogeneous structures
Graphical abstract
Introduction
Over the past decade, iron-chromium-aluminum (FeCrAl) alloys have been proposed as one of the accident-tolerant fuel (ATF) cladding materials for nuclear reactors. Following the Fukushima Daiichi nuclear accident, ATF concepts have focused on the use and application of advanced materials and alloys for nuclear applications. These alloys have superior properties over the conventionally used Zr alloys in the high-temperature environments of the current light-water reactor (LWR) fleet, especially in station blackout scenarios [1], [2], [3], [4], [5]. These ATF cladding materials also show negligible interaction rates with high-temperature water and thus generate little hydrogen, which is a major cause of the hydrogen/steam explosion during a meltdown accident [6], [7], [8]. They are also designed to withstand the extreme conditions in nuclear reactors such as intense neutron irradiation, high temperature, oxidation, and corrosion while showing good high-temperature thermal and mechanical properties [9,10]. FeCrAl alloys have displayed all of the aforementioned properties while maintaining tolerable radiation resistance [11]. In the initial development stage of the FeCrAl alloys, their excellent corrosion resistance is one of the key performance factors that makes them suitable for ATF cladding: because of the formation of passivating chromia and alumina layers formed by the high-temperature steam; further reaction of the fuel cladding and coolant is effectively prevented [12], [13], [14], [15]. The addition of minor elements can improve properties of the ternary FeCrAl system; for example, Y was added to slow the oxidation rate [16], [17], [18], [19], [20], whereas Si, Nb, and Mo were added to enhance the solid solution strengthening and grain refinement for better high-temperature strength [12,[20], [21], [22]].
Despite the advantages of FeCrAl alloys, the existence of Fe-Cr phase separation into α and αʹ phases at modest to low temperatures is a challenging issue. It requires significant effort to balance the alloy composition and reduce the formation of radiation-enhanced Cr-rich clusters: αʹ precipitates have been demonstrated to be a major cause of the irradiation-induced hardening and embrittlement in the FeCr/FeCrAl alloy systems [23]. Several studies performed via pure thermal aging illustrated that the addition of Al and reduction of the Cr content could decrease the propensity for αʹ precipitation and improve oxidation resistance. This trend has also been observed in first-generation (Gen I) FeCrAl alloys under neutron irradiation up to 7 displacements per atom (dpa) and second-generation (Gen II) FeCrAl alloys at 1.8 dpa under neutron irradiation through atom probe tomography (APT) [16,17]. Complementary studies via small-angle neutron scattering (SANS) have shown similar trends. The density of αʹ precipitation was shown to decrease as the Cr content decreases and that of Al increases (including the two commercial FeCrAl alloys Alkrothal 720 and Kanthal APMT) at 0.3–7 dpa [25], [26], [27]. Moreover, a series of ion irradiation experiments demonstrated the dislocation loop evolution up to 16 dpa [28], [29], [30]. Since these studies focused on a higher dose rate and less irradiation time compared to those of the neutron irradiation studies, αʹ precipitates were not observed, and the authors aimed to separate the effect of the irradiation dose and temperature from the presence of the αʹ precipitates. Furthermore, the neutron irradiation data only investigated the dislocation structure up to 1.8 dpa, and the loop formation and structure are still unknown at higher doses.
This work aims to enhance the current knowledge in the microstructural evolution trends in neutron-irradiated FeCrAl alloys and demonstrate the irradiation-induced transition patterns with respect to chemistry, defect sinks, and irradiation conditions. (It is noted that there are also significant ion-beam-irradiation studies available in the literature, but here we focus on neutron-irradiation microstructural evolution [28], [29], [30], [31], [32], [33], [34].) Here, we report the first data of post irradiated microstructures of the second generation (Gen II) wrought FeCrAl alloys is provided via advanced electron microscopy to address the lack of loop evolution data at higher neutron irradiation dose levels. We compare our results on irradiation-induced microstructure in FeCrAl alloys with previous studies. Our work emphasizes the impact of Cr-rich αʹ precipitation on the trend of dislocation evolution under the irradiation conditions examined here. The radiation-enhanced diffusion (RED) perspective is then provided as an alternative way to elucidate the potential mechanisms driving defect nucleation and growth. The irradiation-hardening mechanism is discussed to reflect the barrier strength of each defect, allowing for optimization of such ATF cladding for extreme conditions.
Section snippets
. Alloy and irradiation
The Gen II FeCrAl alloys (wt %) developed at Oak Ridge National Laboratory (ORNL) and designated as C06M (Fe-10Cr-6Al-2Mo), C35M (Fe-13Cr-5Al-2Mo) and C36M (Fe-13Cr-6Al-2Mo) are listed in Table 1. The chemical compositions of these alloys were analyzed using inductively coupled plasma–optical emission spectroscopy (ICP-OES). The combustion analysis measurements were C < 50 weight parts per million (wppm) and S < 30 wppm; the inert gas fusion analysis (IGF) readings were O < 26 wppm and N < 4
Grain structure
The grain size statistics were determined from over 1000 grains for each alloy and are summarized in Table 2. Measured grain sizes for C06M, C35M, and C36M alloys were 0.69 ± 0.57 μm, 0.82 ± 0.63 μm, and 0.70 ± 0.52 μm for the as received unirradiated specimens; and 0.52 ± 0.35 μm, 0.53 ± 0.42 μm, and 0.58 ± 0.27 μm for the irradiated 7 dpa specimens, respectively. The differences among the three alloys are highlighted by the EBSD/TKD-generated inverse pole figure (IPF) maps shown in Fig. 1
Trend of dislocation loop formation
Extensive information on dislocation loop formation can be found in the literature [[16], [17], [18], [19], [20], [21],23,24,46,[53], [54], [55], [56], [57], [58], [59]]. For the HFIR irradiation, the Gen I (F1C5AY, B125Y, B154Y-2, and B183Y-2) and commercial (Alkrothal 720 and Kanthal APMT) FeCrAl alloys have been extensively characterized [16,54] at lower doses up to 1.8 dpa, and previous results indicate that the loop density is not affected by the Cr or Al content up to the total damage
Conclusion
This study performed a systematic and comprehensive investigation of the impact of irradiation-induced microstructure on the high-dose, neutron-irradiated Gen II FeCrAl alloys. The results provide insights on the microstructure-property relationship for the ATF concept. This work also reveals for the first time-the dislocation loop formation under higher-dose (7 dpa) neutron irradiation. The Cr-rich αʹ precipitates in the 7 dpa specimens have a comparable number density but low Cr content than
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
Research was sponsored by the DOE Office of Nuclear Energy, Advanced Fuel Campaign (AFC) of the Nuclear Technology R&D program under contract DE-AC05–00OR22725. Neutron irradiation of FeCrAl alloys at ORNL's HFIR user facility was sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, DOE. A portion of this research was conducted at ORNL's Center for Nanophase Materials Sciences (CNMS), which is a DOE Office of Science User Facility. This research was performed,
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