TEM and HRTEM study of oxide particles in an Al-alloyed high-Cr oxide dispersion strengthened steel with Zr addition

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Abstract

The nanoparticles in an Al-alloyed high-Cr oxide dispersion strengthened (ODS) steel with Zr addition, i.e., SOC-14 (Fe–15Cr–2W–0.1Ti–4Al–0.63Zr–0.35Y2O3), have been examined by transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM). Relative to an Al-alloyed high-Cr oxide ODS steel without Zr addition, i.e., SOC-9 (Fe–15.5Cr–2W–0.1Ti–4Al–0.35Y2O3), the dispersion morphology and coherency of the oxide nanoparticles in SOC-14 were significantly improved. Almost all the small nanoparticles (diameter <10 nm) 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. The large particles were mainly identified as tetragonal or cubic ZrO2 oxide. The results are compared with those of SOC-9 with a brief discussion of the mechanisms of the unusual thermal and irradiation stabilities of the oxides as well as the extraordinary corrosion resistance, excellent irradiation tolerance and superior strength of SOC-14.

Introduction

Generation IV nuclear fission reactors such as supercritical pressurized water reactor (SCPWR) and lead bismuth-cooled fast reactor (LFR) are aimed at making revolutionary improvements in economics, safety and reliability, and sustainability, and will be responding to the increasing worldwide energy demand whilst addressing climate change [1], [2]. Cladding material development for such reactors is very challenging due to the high operating temperatures, large time-varying stresses, high level of neutron displacement damage and chemically reactive surroundings [2], [3], [4], [5], [6], [7], [8], [9].

Oxide dispersion strengthened (ODS) steels have been developed for several decades as a promising structural material for next-generation fission and future fusion energy systems due to their excellent resistance to high temperature creep [7], [10], [11], [12], [13], [14] and irradiation damage [5], [9], [15] as well as extraordinary structural and chemical stability in extremely harsh environments [13], [15], [16], [17]. To meet the stringent requirements of fuel cladding materials for SCPWR and LFR with highly corrosive coolants, an Al-alloyed high-Cr ODS steel with Zr addition, i.e., SOC-14, was successfully developed [13], [18].

SOC-14 has superior high temperature strength [13], [18], [19] and excellent tensile ductility (RA = 59.8% at 973 K), indicating very good fracture toughness at high temperature [20]. SOC-14 achieves optimal creep resistance at 973 K in that its creep strength is comparable to that of 12Cr-ODS steel [10], [13], [18], [19]. SOC-14 has extremely high corrosion resistance to supercritical pressurized water (SCPW) [21] and lead–bismuth eutectic (LBE) [22], [23] at high temperature (e.g., 973 K) as well as extremely low susceptibility to thermal ageing embrittlement [13]. Moreover, Zr addition into Al-alloyed high-Cr ODS steels leads to much better resistance to irradiation damage [24], [25] with the oxides exhibiting excellent irradiation tolerance [26] and thermal stability (no change in dispersion morphology of oxide particles in SOC-14 after annealing for 10 h at 1473 K).

To understand such superior mechanical properties and excellent irradiation resistance owing to the highly stabilized oxide particles, as well as extremely high corrosion resistance to SCPW and LBE at high temperature, a knowledge of the spatial and size distributions (size distribution, number density, volume fraction and inter-spacing), shape, character (crystal structure and chemistry) and metal/oxide interface structure (coherency and orientation relationships between the oxides and the bcc steel matrix) of the nanoparticles is necessary in the research field of ODS steels [27], [28].

Transmission electron microscopy (TEM) characterization on the particles in Al-alloyed high-Cr ODS steels with Zr addition has been performed [25], [29]. Yu et al. [25] detected Y2Zr2O7 oxide in Fe–16Cr–4Al–0.6Zr–0.35Y2O3 ODS steel while Ohnuki et al. [29] found Y6ZrO11 and Y4Zr3O12 oxides in Fe–16Cr–4Al–2W–(0.35–0.5)Y–(0.3–0.45)Zr model alloys. Zr addition leads to forming finer particles with denser dispersion in the matrix [15], [25], [29].

In this work, the dispersion morphology, crystal structure and coherency of oxide particles in SOC-14 were investigated by TEM and high resolution transmission electron microscopy (HRTEM). The results are compared with those of SOC-9 [8], [30]. The underlying mechanisms of the outstanding thermal stability and irradiation tolerance of the oxides as well as the superior strength and the excellent resistance of SOC-14 to creep, irradiation and corrosion in SCPW and LBE at high temperature are briefly discussed.

Section snippets

Experimental

SOC-14 was synthesized by mechanical alloying of the alloy powders, Fe–15Cr–2W–0.1Ti–4Al–0.63Zr (wt.%), with 0.35 wt.% Y2O3 powders in an argon gas atmosphere using a high-performance attrition type ball mill [31], 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. The resultant chemical compositions of SOC-14 is Fe–14.85Cr–1.84W–0.09Ti–3.73Al–0.63Zr (wt.%).

Grain morphologies

The grain morphologies of SOC-14 are shown in Fig. 1. The average grain length, average grain width and grain aspect ratios (GAR) measured in a plane parallel to the extrusion axes, Fig. 1a, are 4.6 μm (uncertainty: +0.5 μm and −0.45 μm), 1.08 μm (uncertainty: +0.3 μm and −0.2 μm) and 4.26 (uncertainty: +0.4 and −0.35), respectively. The mean intercept grain diameter measured in a plane perpendicular to the extrusion axis, Fig. 1b, is 0.99 μm (uncertainty: +0.25 μm and −0.15 μm).

Spatial and size distributions of oxide particles

Fig. 2a and b shows the

Discussion

By comparing the grain size of SOC-14 (Fig. 1 and text in Section 3.1) with that of SOC-9 (Fig. 1 in Ref. [30]), it can be deduced that the average grain size was reduced slightly after Zr addition into Al-alloyed high-Cr ODS steel, although the results of electron backscattered diffraction analyses showed that the grain morphologies of both ODS steels are similar to each other [39].

Weak-beam microscopy is superior for imaging and identifying very small precipitates because it provides better

Conclusions

The dispersion morphology, crystal structure and coherency of oxide particles in SOC-14 were studied by diffraction contrast techniques and HRTEM. The concluding remarks are:

  • (1)

    For SOC-14, the grain size is slightly smaller, the mean size and number density of oxide particles in grains are considerably smaller and much higher, respectively, relative to SOC-9.

  • (2)

    In SOC-14, the formation of Y–Al–O particles is intensively suppressed. Almost all the particles in grains of SOC-14 are consistent with

Acknowledgements

Present study includes the result of “R&D of corrosion resistant super ODS steel for highly efficient nuclear systems” entrusted to Kyoto University by the Ministry of Education, Culture, Sports, Science and Technology of Japan (MEXT). Peng Dou would like to acknowledge Dr. Steven John Zinkle (ORNL) and Professor Gary Was at University of Michigan for comments on the draft manuscript.

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