Study of radiation-induced amorphization of M23C6 in RAFM steels under iron irradiations

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Abstract

The mechanical properties of reduced activation ferritic/martensitic steels (RAFMs) such as creep, thermal aging, and irradiation resistance are improved by the formation of MX and M23C6 particles. However, it is well known that these particles are unstable under irradiation. The purpose of the present study is to clarify the details of the radiation-induced amorphization (RIA) of M23C6 in F82H steels. The instability behavior of M23C6 under a wide range of irradiation temperatures from R.T. to 773 K and various ion beam energies (2.8, 3.7, 6.4 and 10.5 MeV) was systematically investigated. Two types of materials, F82H and its model alloy (F8), were utilized in this study. TEM observation and selected area electron diffraction analysis before and after irradiation were conducted to evaluate the occurrence of RIA, and the chemical composition was analyzed to reveal the possible relation between atom displacement/diffusion and RIA. In the case of the iron irradiation at the temperature below 598 K, a bilayer contrast of the particle, consisting of an amorphous-rim phase and inner crystalline core, was observed. Similar particle survived in the irradiated F82H at the temperature of 623 K, the RIA was not observed in the F8 specimen. Consequently, the critical temperature of RIA was estimated as 623–673 K. Since the melting temperature (Tm) of M23C6 is approximately 2173 K, the critical temperature corresponds to ∼0.29 Tm. From the STEM-EDS analysis, the Fe/Cr ratio of the particle decreased with increasing irradiation temperature in the range of 573–623 K. This behavior was qualitatively explained by the formation of the amorphous phase. It is clarified that the occurrence of the RIA of M23C6 particle can be evaluated by the variation of Fe/Cr ratio.

Introduction

Reduced activation ferritic/martensitic steels (RAFMs) are considered as the main candidate materials for structural applications in the fusion DEMO reactor blanket. Lots of efforts have been made on the development and characterization of these materials, such as F82H in Japan [1] and Eurofer97 in EU [2]. In parallel, extensive studies on the evaluation of these materials such as tensile [3], fatigue [4], creep [5], toughness [6], and irradiation tolerance [7] have been reported. Especially, the degradation of mechanical properties and microstructure due to irradiation have been earnestly investigated [[8], [9], [10]].

In our previous studies, the instability of the fine particles in F82H steel and its model alloys were evaluated via in-situ observations under high voltage electron microscopy (HVEM) and ion irradiation at elevated temperatures [[11], [12], [13], [14]]. The behavior of disappearance and decreasing size of MX particles were observed, and the kinetics for the instability of the particles was further proposed based on the experimental observation. On the other hand, the instability of M23C6 particles in F82H steel under HVEM irradiation exhibited a strong dependence upon irradiation temperature and damage level. M23C6 particles with a relatively lower Cr concentration rim at the interface between M23C6 and the matrix have been observed. The amorphization of M23C6 under neutron and ion irradiations at the temperature of <573 K have been investigated. Tanigawa et al. reported that the radiation induced amorphization (RIA) occurred in both of M23C6 and laves phases [9], where the laves phase is formed by thermal aging of 1.08 × 108 s at 873 K, and the ion irradiation was performed at 573 K up to 10 dpa. This study also pointed out that the RIA of the laves phase is more likely to be relative to M23C6. Sencer et al. reported the phase stability of M23C6 and MX in a modified 9Cr–1Mo ferritic/martensitic steel irradiated by mixed proton and spallation neutron at temperatures below 333 K with a dose of 0.5–9.6 dpa [15], and the results showed that the M23C6 was completely amorphized at the damage level of 0.5 dpa.

Regarding the instability of the M23C6 particle, both the critical dose level and temperature are important indicators. Compared with the irradiation damage of the F82H suffered under a fusion reactor environment, which can hypothetically reach up to ∼100 dpa, the critical dose level required for RIA (Dc) is thus presumed to be relatively low. On the other hand, although the critical temperature for RIA (Tc) is estimated at ~573 K from the previous study, in a strict sense, it requires a systematic survey covering a much wider temperature range, rather than the estimation at the irradiation temperature of 573 K. The systematic investigation on RIA of M23C6 has been scarcely investigated, and the understanding of this behavior is not yet fully clarified. Therefore, the purpose of the present study is to clarify the critical temperature for the amorphization of M23C6 in F82H steel, the morphological change of M23C6 particles under the ion accelerator irradiations at various temperature was investigated.

Section snippets

Experimental procedures

Two types of specimens, RAFMs and its model alloy which will be named as F82H and F8 hereafter, were used as the experimental specimens. The chemical compositions of these specimens are listed in Table 1. F82H was fabricated by elected arc melting with electron slag re-melting processes. The heat treatment condition for normalizing and tempering was 1313 K for 2.4 ks and 1023 K for 3.6 ks, respectively. Detailed information on the thermal history of this specimen can be seen elsewhere [16]. The

Feature M23C6 particle in unirradiated specimens

Fig. 2 shows the lower magnification BF micrograph of unirradiated F82H and F8 specimens. The spherical and orbital shape of the particles was observed in both specimens. The average size of the particles (dAVE,M23C6, dAVE) in F82H and F8 specimens were estimated as 47.2 and 68.8 nm, and the number densities (ρAVE,M23C6, ρAVE) were 2.31 and 4.49 × 1013 cm3, respectively. It is noted that the distinguishment of MX and M23C6 particles in F82H was made based on the STEM-EDS mapping in this study.

Mechanism of variation of Fe/Cr of M23C6 particle under elevating irradiation temperature

The Fe/Cr ratio of M23C6 particle was varied under the irradiation temperature range of 573–623 K in this study. o clarify the change of the chemical composition of the particle due to RIA, the CpCrof the irradiated particle was thus estimated using Eq. (3). This result is indicated in Fig. 13. The Cr concentration is achieved as the average of the whole particle, including both of the crystal inner core and amorphous rim. It is seen that the CpCrincreased with increasing irradiation

Conclusions

As one of the unstable behaviors of M23C6 particle in RAFMs under irradiation, the RIA of M23C6 particle in F82H and its model alloy (F8) was systematically investigated by ion accelerator irradiations under various energies at temperatures from RT to 773 K. After irradiation, detailed TEM observation and chemical composition analyses were conducted. The main results of this study are summarized as follows:

  • (1)

    The spherical and orbital shape of M23C6 particles dispersed into the lath-martensite

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.

Acknowledgment

This study was supported by the National Institutes for Quantum and Radiological Science and Technology (QST) under the joint work as a part of Broader Approach activities. This work has also been supported by the Inter-University Program for the Joint Use of JAEA/QST Facilities (proposal no. 19002). This work was performed under the GIMRT Program of the Institute for Materials Research, Tohoku University (with proposal No. 19M0004) and collaborative research project at Nuclear Professional

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