Effects of contents of Al, Zr and Ti on oxide particles in Fe–15Cr–2W–0.35Y2O3 ODS steels

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Highlights

  • Oxides in Fe–15Cr–2W–0.35Y2O3 ODS steels with Al, Zr and Ti were studied by HRTEM.

  • Crystal & interface structures of 175 oxides were identified for 3.7Al–0.1Ti–0.6Zr.

  • Crystal & interface structures of 213 oxides were identified for 3.8Al–0.12Ti–0.5Zr.

  • Proportions of various types of oxides were determined according to HRTEM results.

  • The content ratios of Al/Zr/Ti cause the polymorphic transition of complex oxides.

Abstract

FeCrAl oxide dispersion strengthened (ODS) steel is one of the most promising candidate cladding materials of generation IV nuclear reactors because of its excellent resistance to not only corrosion but also creep and irradiation due to the ultrahigh density nanometer-scale oxides. Effects of the contents of Al, Zr and Ti on the crystal and metal/oxide interface structures of the particles in Fe–15Cr–3.7Al–2W–0.1Ti–0.6Zr–0.35Y2O3 (3.7Al–0.1Ti–0.6Zr) and Fe–15Cr–3.8Al–2W–0.12Ti–0.5Zr–0.35Y2O3 (3.8Al–0.12Ti–0.5Zr) were studied by high resolution transmission electron microscopy (HRTEM). For 3.7Al–0.1Ti–0.6Zr and 3.8Al–0.12Ti–0.5Zr ODS steels, phase identification was accomplished on 175 and 213 particles, respectively, which have peak number fraction and, therefore, represent the oxides contributing most significantly to the macroscopic properties of the ODS steels. The proportions of various types of oxides were determined. For 3.7Al–0.1Ti–0.6Zr and 3.8Al–0.12Ti–0.5Zr ODS steels, the proportions of Y–Zr complex oxides are ∼87.4% and ∼54.4% while the number fractions of Y–Al complex oxides are ∼3.5% and ∼5.2%, respectively, indicating that 0.5–0.6 wt.% Zr inhibits the formation of Y–Al complex oxides remarkably while prompts the significant occurrence of Y–Zr complex oxides. For 3.7Al–0.1Ti–0.6Zr and 3.8Al–0.12Ti–0.5Zr ODS steels, the proportions of Y–Ti complex oxides are ∼5.7% and ∼36.6%, respectively, indicating it is effective for increasing the proportion of Y–Ti complex oxides by increasing the content of Ti from 0.09 wt.% to 0.12 wt.% with the content of Zr decreased from 0.63 wt.% to 0.49 wt.%. The crystallographic orientation correlations of the oxides and matrix were determined. The formation and refinement mechanisms of oxides and, moreover, the reasons of the unusual irradiation tolerance and thermal stability of the ODS steel were discussed based on the results.

Introduction

Access to clean, safe, reliable, sustainable, and economically competitive nuclear energy is important for worldwide economies and environmental issues. The safety, economics, efficiency, reliability and sustainability of Generation IV nuclear fission energy systems such as supercritical pressurized water reactors (SCPWRs) and lead bismuth-cooled fast breeder reactors (LFRs) will ultimately depend on the successful development of new high-performance structural materials with outstanding properties that are sustained under long-term service in ultra-severe environments characterized by high operating temperatures, large time-varying stresses, high level of neutron displacement damage and chemically reactive surroundings [[1], [2], [3], [4], [5], [6], [7], [8], [9], [10]].

FeCrAl oxide dispersion strengthened (ODS) steel has been developed for several decades as one of the most promising candidate cladding materials for SCPWRs and LFRs due to its superior high temperature strength and excellent resistance to creep, irradiation, oxidation and corrosion in supercritical pressurized water (SCPW) [6,11] and lead-bismuth eutectic (LBE) etc. [6,12,13]. Although Al addition is beneficial to the improvement of corrosion resistance due to the occurrence of a dense, stable and protective alumina scale on alloy surface [6,12,13], it leads to weak strengthening due to the formation of Y–Al complex oxides with poor morphology, i.e., large size with sparse dispersion [6,7,10,[14], [15], [16]].

It is fortunate that the dispersion morphology of the particles in FeCrAl-ODS steel could be considerably improved by the minor addition of Zr due to the significant formation of Y–Zr complex oxides, instead of Y–Al complex oxides [6,7,10,[17], [18], [19]] and, therefore, the Zr-added FeCrAl-ODS steel achieve not only superior high temperature strength and excellent creep properties [6,7,10,17,[20], [21], [22], [23]] but also outstanding resistance to oxidation and corrosion by properly controlling excess oxygen content [6,7,10,24].

It has been demonstrated that the dispersion morphology of the oxide particles in ODS steels can be improved very effectively by the minor addition Ti due to the formation of ultrafine Y–Ti complex oxides with ultrahigh number density [2,7,10,[25], [26], [27]]. To some extent, the beneficial effect of Ti addition in improving the dispersion morphology of the oxide particles in FeCrAl-ODS steel is similar to that of Zr. Therefore, for developing FeCrAl-ODS steel with outstanding macroscopic properties, e.g., superior high temperature strength and excellent resistance to creep, oxidation, corrosion and irradiation, it is essential to optimize the contents of Al, Zr and Ti etc.

As well known that the overall macroscopic properties of ODS alloys can be strongly influenced by not only the dispersion morphology but also the crystal and metal/oxide interface structures of particles [[16], [17], [18], [19],[28], [29], [30], [31]] because oxides pin dislocations, help to self-heal vacancy and self-interstitial damage and trap otherwise highly damaging helium in harmless nanometer-scale interface bubbles [2,32,33]. Moreover, the capability of ODS steels for keeping inherently attractive macroscopic behavior during long term nuclear applications depends on the thermal and radiation stabilities of the oxides, which are also closely related to the phase, morphology and coherency of the particles.

Specifically, some types of oxides with certain crystal structures tend to have much better thermal stability and radiation tolerance than others. For instance, YAH (yttrium-aluminum-hexagonal, YAlO3) has the best thermal stability among all the four types of Y–Al complex oxides, i.e., Y4Al2O9 (yttrium aluminum monoclinic, YAM), Y3Al5O12 (yttrium-aluminum-garnet, YAG), YAlO3 (yttrium-aluminum-perovskite, YAP) and YAH [34]. Moreover, cubic Y2Hf2O7 [35,36], orthorhombic YAP [37,38], Y–Ti–O nanoclusters [39,40] and orthorhombic Y2TiO5 [[36], [41],42,43] also exhibit very good thermal stability. Cubic Y2Hf2O7 having anion-deficient fluorite structure [44], trigonal Y4Zr3O12 being fluorite structural derivative [45,46], cubic Y2Zr2O7 with defective fluorite structure [28,47], orthorhombic YAP [48] and orthorhombic Y2TiO5 [49,50] have excellent resistance to irradiation.

Moreover, the metal/oxide heterogeneous interface structures have significant influence on not only the radiation tolerance and the capability of helium management but also the creep threshold stresses of ODS alloys because the strengthening mechanism based on thermally-activated detachment [51,52] is operative only for incoherent particles and, however, the strengthening mechanisms based on particle shearing [52] and bypass by climb [52,53] with lattice & modulus mismatches between oxides and the matrix playing the key roles in strengthening ODS steels are applicable to coherent [[52], [53], [54]] and semi-coherent particles [55].

Therefore, the knowledge of the phase of the particles and the corresponding proportions of various types of oxides, which is evidently influenced by the contents of minor reactive elements, e.g., Al, Zr and Ti etc., the insights of the morphology of the oxide particles and, especially, the metal/oxide heterogeneous interface structure information concerning the coherency of the particles and their crystallographic orientation correlations with the bcc steel matrix, are crucial to the understanding of the mechanical properties, radiation tolerance and thermal stability of ODS steels, to the understanding and optimization of the chemical and physical processes taking place during the various fabrication steps of ODS steels, and to improving ODS steels by means of targeted nanometer-scale structural tailoring.

High resolution transmission electron microscopy (HRTEM) characterizations on the oxide particles in Zr-added FeCrAl-ODS steels with or without Ti have been extensively performed. For Zr-added FeCrAl-ODS steels without Ti, Ohnuki et al. [56] have found Y6ZrO11 and trigonal δ-phase Y4Zr3O12 in Fe–16Cr–4Al–2W–(0.35–0.5)Y–(0.3–0.45)Zr model alloys; Yu et al. [57] have detected cubic Y2Zr2O7 in Fe–16Cr–4Al–0.6Zr–0.35Y2O3; Xu et al. [58] have found that almost all the small particles of Fe–15Cr–4.5Al–2W–0.3Zr–0.3Y2O3 are consistent with trigonal δ-phase Y4Zr3O12 with YAG occasionally detected; Dong et al. [20] have also found trigonal δ-phase Y4Zr3O12 in Fe–16Cr–3Al–1.5W–0.5Zr–0.35Y2O3.

However, for Zr-added FeCrAl-ODS steels with Ti, Gao et al. [21] have found fine Y–Zr complex oxides in Fe–16Cr–4Al–2W–0.5Ti–1Zr–0.4Y2O3 with a small amount of Y–Al complex oxides existing. OONO et al. [59] have found that almost all the particles of 15Cr–7Al–0.5Ti–0.4Zr–0.47Y2O3 are composed of trigonal δ-phase Y4Zr3O12. Dou et al. [17] have reported that most of the small particles in SOC-14 were found to be consistent with trigonal δ-phase Y4Zr3O12 oxides and coherent with the bcc steel matrix, with semi-coherent orthorhombic Y2TiO5 oxides occasionally detected [16]. Dou et al. [18] have also reported that most of the particles in Fe–15Cr–4Al–2W–0.15Ti–0.3Zr–0.35Y2O3 were found to be consistent with trigonal δ-phase Y4Zr3O12 and coherent with the bcc steel matrix while Y2TiO5 and YTiO3 oxides were also detected.

Moreover, it is worthy to note that for Zr-added FeCrAl-ODS steels with Ti, Dou et al. [19] have performed HRTEM characterization on the crystal and metal/oxide interface structures of a large amount of oxide particles, which have peak number fraction and, therefore, represent the oxides contributing most significantly to the macroscopic properties. Subsequently, the phase identifications of a fairly large number of particles have been accomplished and, moreover, the proportions of various types of oxides were determined by statistical analyses. For example, for Fe–15Cr–7Al–2W–0.5Ti–0.4Zr–0.5Y2O3, Dou et al. [19] have found that ∼60.5% of the particles are consistent with Y–Zr complex oxides with the proportions of trigonal δ-phase Y4Zr3O12 and cubic/orthorhombic Y2Zr2O7 of ∼44.8% and ∼15.7%, respectively, whereas ∼14.9% of the particles are composed of Y–Al complex oxides with Y–Ti complex oxides constituting ∼13.4% of the particles.

For optimizing the contents of Al, Zr and Ti of FeCrAl-ODS steel, the effects of the contents of Al, Zr and Ti on the crystal and metal/oxide interface structures of the particles have been studied in this work. For Fe–15Cr–3.7Al–2W–0.1Ti–0.6Zr–0.35Y2O3 (3.7Al–0.1Ti–0.6Zr) and Fe–15Cr–3.8Al–2W–0.12Ti–0.5Zr–0.35Y2O3 (3.8Al–0.12Ti–0.5Zr), totally 546 and 861 HRTEM lattice images have been taken on the ultrafine oxides and, subsequently, phase identification has been accomplished on 175 and 213 particles, respectively, which have diameter mainly of 3–10 nm and peak number fraction based on the size distribution diagram and, therefore, represent the oxides contributing most significantly to the macroscopic properties of the ODS steels, based on the information of crystal structures obtained from the HRTEM lattice images. The proportions of various types of oxides were determined by statistical analyses. The metal/oxide interface structure information concerning the coherency of the particles and their crystallographic orientation correlations with the matrix has been obtained.

Note that the ODS steel used in the research of Ref. [17], i.e., SOC-14, is exactly same with Fe–15Cr–4Al–2W–0.1Ti–0.6Zr–0.35Y2O3 (3.7Al–0.1Ti–0.6Zr), which has been studied in this work. In the work of Ref. [17], although a very high amount of HRTEM micrographs have also been taken on a large number of particles in the ODS steel, not every HRTEM lattice image has been analyzed for phase identification based on the crystal structure information because there was no research objective to get the detailed knowledge on various types of oxides in the ODS steel, not mention to the statistical analyses on the proportions of various kinds of oxides.

Therefore, to the best knowledge of the authors, for both 3.7Al–0.1Ti–0.6Zr and 3.8Al–0.12Ti–0.5Zr ODS steels, there has been no comprehensive report on the experimental results of phase identifications of a large amount of particles based on the information of crystal structure derived from HRTEM lattice images, not to mention the insights of various kinds of oxides and the statistical analysis results of the corresponding proportions of various types of oxides yet.

Section snippets

Material and methods

Both 3.7Al–0.1Ti–0.6Zr and 3.8Al–0.12Ti–0.5Zr ODS steels were synthesized by mechanical alloying of the alloy powders, Fe–15Cr–3.7Al–2W–0.1Ti–0.6Zr and Fe–15Cr–3.8Al–2W–0.12Ti–0.5Zr, with 0.35 wt.% Y2O3 powders in an argon gas atmosphere using a high-performance attrition type ball mill, followed by degassing at 673 K in 0.1 Pa vacuum for 3 h and hot extrusion at 1423 K. After hot extrusion, a homogenization heat treatment was performed at 1323 K for 1 h followed by air cooling. Actually, the

Results

Based on the statistic results of HRTEM analyses, the proportions of various types of oxides in 3.7Al–0.1Ti–0.6Zr and 3.8Al–0.12Ti–0.5Zr ODS steels are present in Table 1 and, moreover, the number fractions of Y–Zr, Y–Ti and Y–Al complex oxides and ZrO2 are shown in Fig. 1. In Table 1, YAP and YAH represent orthorhombic YAlO3 (yttrium-aluminum-perovskite, YAP) and hexagonal YAlO3 (yttrium-aluminum-hexagonal, YAH), respectively.

In 3.7Al–0.1Ti–0.6Zr ODS steels, about 87.4% of the particles are

Discussion

To some extent, it is difficult to characterize ultrafine precipitates with diameter less than 10 nm by HRTEM because they are likely buried in matrix due to the fact that most of the thin regions of TEM specimens where HRTEM lattice images can be taken are thicker than 10 nm even the twin-jet electro-polishing method is used to prepare them. In conventional HRTEM instruments with no aberration corrector equipped, the image delocalization due to microscopic aberrations causes the mixing of the

Conclusions

The effects of the contents of Al, Zr and Ti on the crystal and metal/oxide interface structures of the particles in Fe–15Cr–3.7Al–2W–0.1Ti–0.6Zr–0.35Y2O3 (3.7Al–0.1Ti–0.6Zr) and Fe–15Cr–3.8Al–2W–0.12Ti–0.5Zr–0.35Y2O3 (3.8Al–0.12Ti–0.5Zr) have been studied by high resolution transmission electron microscopy (HRTEM). For 3.7Al–0.1Ti–0.6Zr and 3.8Al–0.12Ti–0.5Zr ODS steels, phase identification has been accomplished on 175 and 213 particles, respectively, which have peak number fraction and,

Data availability

The raw data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study. The processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.

CRediT authorship contribution statement

Peng Dou: Conceptualization, Methodology, Software, Visualization, Investigation, Validation, Writing - review & editing, Supervision. Shaomin Jiang: Data curation, Writing - original draft, Investigation, Visualization, Software. Lanlan Qiu: Data curation, Writing - original draft, Investigation, Visualization, Software. Akihiko Kimura: Validation.

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.

Acknowledgements

This work was sponsored by National Natural Science Foundation of China with grant No. 51571042 and 51871034. The present research was also supported by the Fundamental Research Funds for the Central Universities with grant No. 2019CDXYCL0031.

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