Nanosized oxide phases in oxide-dispersion-strengthened steel PM2000

ABSTRACT

Oxide-dispersion-strengthened (ODS) steels are promising material candidates for nuclear fission and fusion applications. Nanosized oxide particles in as-rolled ODS steel PM2000 were investigated by using transmission electron microscopy. Uniformly distributed nanosized particles and heterogeneously distributed large dispersoids were observed in the steel. Three nanosized oxide phase types, Al₂Y₄O₉, Y₃Al₅O₁₂ and YAlO₃, were present in the steel. Nanosized aluminum yttrium oxide Al₂Y₄O₉, which has an orthorhombic P Bravais lattice rather than a monoclinic P Bravais lattice, was identified. Nanosized yttrium aluminum oxides that consisted of Y₃Al₅O₁₂ and YAlO₃ were also identified. The formation mechanism of Al₂Y₄O₉/Y₃Al₅O₁₂/YAlO₃ phases in the steel resulted from the processing (hot-extrusion/hot-rolling) procedures. Al diffusion from the steel matrix into the Y₂O₃ particle fragments induced the nucleation and growth of the Al₂Y₄O₉/Y₃Al₅O₁₂/YAlO₃ phases.

KEYWORDS:

• ODS steel PM2000
• microstructure analysis
• Al₂Y₄O₉ oxide
• Y₃Al₅O₁₂ oxide
• YAlO₃ oxide
• formation mechanism

1. Introduction

Oxide-dispersion-strengthened (ODS) alloys are of considerable potential interest for structural applications in nuclear fission and fusion power plants because of their good high-temperature strength, creep rupture strength, corrosion resistance and resistance to radiation damage. ODS steels are manufactured by using a mixture of metallic powders with yttrium-oxide powder by mechanical alloying after consolidation and thermomechanical processing. The ferrite matrix yttria (Y₂O₃) is thermodynamically stable and was found to improve the high-temperature strength and maintain the resistance to superior ODS steel radiation [1–3]. Titanium addition improves the creep rupture strength and produces finer oxide particles [4]. An increasing proportion of chromium and aluminum improves the resistance to corrosion and oxidation [5,6].

The nature of the oxide dispersoids determines the ODS steel properties. Because nanoscale oxide particles in ODS steels can serve as a strong block for mobile dislocations and as a sink for radiation defects at the particle-matrix interfaces [7], which enhances the high-temperature creep properties and radiation resistance, a study on the nature of dispersoids would be valuable in terms of establishing how the ODS steel compatibility would change in nuclear applications.

When added to the starting powder to prepare ODS Fe–Cr–Al alloys, Y₂O₃ dispersoids were not retained in the final product [8]. Rather, mixed oxides formed because Y₂O₃ and metallic elements react during the high-temperature processing of PM2000. The change in oxide type during ODS fabrication requires further investigation.

Limited investigations and discussions exist on oxide types, size distributions and the small oxide-phase formation mechanism in PM2000 [9–13]. Although several nanosized oxide phases have been identified in ODS steel PM2000, the oxide dispersoids in PM2000 remain unclear. The purpose of this work is to report oxide phases with different types, as Al₂Y₄O₉ an orthorhombic P Bravais lattice and reveal smaller size distribution of these dispersoids. In this work, we focused on three oxides: Al₂Y₄O₉, Y₃Al₅O₁₂ and YAlO₃. The crystal structure of Y₃Al₅O₁₂ was determined to be tetragonal P Bravais, which was not clarified in previous research. The formation mechanism and smaller size distributions of these three oxides were also revealed in this work.

2. Experimental procedures

The material that was used was a commercial ODS steel PM2000, which is an iron–chromium-based ferritic steel that was produced by Plansee GmbH. The steel was supplied as an as-rolled 60-mm-diameter rod. The nominal composition of the steel was 20Cr, 5.5Al, 0.5Ti and 0.5Y₂O₃ (weight percent). Metallographic specimens were cut from the steel rod by spark erosion and prepared by using standard procedures of grinding, polishing and etching [14]. The etchant that was used to reveal the steel microstructure was a solution of 15 mL HNO₃–30 mL HCl–45 mL glycerol. The use of extraction carbon replica is an effective method to identify the nature of nanosized precipitates in steels [15]. Extraction carbon replicas were prepared from the etched metallographic specimens by the method described elsewhere [16]. Replicas were examined by a FEI Tecnai F20 and FEI Tecnai F30 transmission electron microscopy (TEM) at 200 and 300 kV. Energy dispersive X-ray (EDX) spectrometers on the TEM were used to analyze the chemical composition of the extracted oxide particles on the replicas. The oxide phases were identified by a combination of EDX analysis and electron diffraction along with the Powder Diffraction File database of the Joint Committee on Powder Diffraction Standards (JCPDS). The oxide particle size was measured from TEM micrographs of the replicas.

3. Overview of yttrium aluminum oxides

Current well-studied Y–Al–O oxides include Y₃Al₅O₁₂, YAlO₃ and Y₄Al₂O₉. These oxides form from two stable crystals as Al₂O₃ and Y₂O₃ with different combination ratios. In this work, we focused only on the crystal structures of these Y–Al–O oxides.

As a single crystal, Y₃Al₅O₁₂ has a cubic P Bravais lattice (a = 12.009 Å, b = 12.009 Å, c = 12.009 Å, JCPDS file 033-0040) or tetragonal P Bravais lattice (a = 7.510 Å, b = 7.510 Å, c = 4.240 Å, JCPDS file 009-0310). The melting point is 1970°C [17].

YAlO₃ has a perovskite-like orthorhombic P Bravais lattice (a = 5.330 Å, b = 7.375 Å, c = 5.180 Å) with space group Pnma [18]. According to the orthorhombic structure’s mechanical stability criteria, YAlO₃ has a mechanical stability from 0–20 GPa and 0–1300 K [19–21]. YAlO₃ also has another crystal structure as hexagonal P Bravais lattice (a = 3.678 Å, b = 3.678 Å, c = 10.483 Å) [22].

In most cases, Y₄Al₂O₉ is determined to be a monoclinic crystal by X-ray and neutron diffraction on powder and single-crystal samples [23]. Al₂Y₄O₉ has another crystal structure as an orthorhombic P Bravais lattice (a = 10.460 Å, b = 10.546 Å, c = 3.684 Å, JCPDS file 014-0475).

These three oxide crystal structures were determined by researchers in the past. However, for ODS steel, especially for as-rolled PM2000, the crystal structures and formation mechanism of nanosized dispersoids remain in discussion. In this work, we analyzed every Al–O–Y crystal structure and tried to explain the formation mechanism.

4. Results and discussion

Figures 1 and 2 show examples of TEM images that were taken from replica samples of the PM2000 steel. Figure 1 shows the dispersoids distribution in the steel. The large dispersoids clustered heterogeneously in different areas with 25–100 nm diameters. Figure 2 shows that the nanosized oxide particles were distributed uniformly in the steel with an average diameter of ∼20 nm.

Figure 1. TEM micrograph of carbon replica prepared from as-rolled PM2000 steel, showing dispersoids distributed heterogeneously in steel.

Figure 2. TEM micrograph of carbon replica prepared from as-rolled PM2000 steel, showing nanosized oxide particles distributed uniformly in steel.

To study the uniformly distributed nanosized particles, more than 50 TEM images and 500 particles were investigated. In Figure 3, dispersoids of interest of 10–50 nm comprised 70% of the studied particles. EDX analysis indicated that 70% of the particles appeared to be Al-Y-O oxide. Previous research has also found compositions of a similar size [5,9]. The large particles occupied 21% and were composed mostly of Al–Fe–Y–O chemical elements. Shen et al. [9] described these dispersoids in their research. The large clustered particles were Al₂Fe₂O₆, Al₃FeO₆, Al₂O₃, and Al₃Ti₅O₂.

Figure 3. Diameter distribution of nanosized oxide particles in as-rolled PM2000 steel.

The TEM micrographs of carbon replica samples that were prepared from PM2000 steel (Figure 4(a)) show large particle P1 with a 65 nm diameter along with many nanosized particles, including particle P2. The chemical composition of particle P1 was 20.92Al, 0.02Ti, 0.10Fe, 18.16Y, 60.76O (atomic percent). This composition indicates that particle P1 belongs to a Y–Al oxide phase with chemical formula YAlO₃. Figure 4(b) shows the selected-area electron diffraction (SAED) pattern of particle P1. This pattern matched the diffraction pattern from yttrium aluminum oxide (YAlO₃, orthorhombic P Bravais lattice, space group: Pbn* with the lattice parameters a/b/c = 5.179/5.329/7.37 Å, JCPDS file 08–0147/33-0041) in the zone axis of [201] [24]. Based on the pattern indexing and EDX analysis, particle P1 was identified to be the YAlO₃ phase. Previous research has shown the presence of the YAlO₃ phase in the as-rolled PM2000 steel with the observed YAlO₃ particle size ranging from 35 to 84 nm in diameter [9].

Figure 4. (a, b) TEM micrographs of carbon replica prepared from as-rolled PM2000 steel, showing large YAlO₃ oxide particle P1 and nanosized Al₂Y₄O₉ oxide particle P2 marked with arrows; (c) SAED pattern of particle P1 in the zone axis of [201]; (d) FFT pattern generated from particle P2 in the zone axis of $\left[23\overline{1}\right]$.

EDX analysis does not provide reliable stoichiometric atomic ratios of the nanosized particles in Figure 4(a, b) and hence the nature of these particles is speculative. During exposure in air, carbon replicas may absorb oxygen and build up a layer of oxygen contamination on the replica surface, which results in an increase in oxygen concentration in the EDX data of nanosized oxide particles. In this investigation, the qualitative analysis of the nanosized oxide phase was based mainly on an analysis of the diffraction patterns.

Figure 4(c) shows the fast Fourier transformation (FFT) pattern that was generated from the arrow-marked particle P2 in Figure 4(a, b). This pattern could not be matched to the JCPDS database patterns for YAlO₃ (JCPDS file 08–0147/33-0041), Y₃Al₅O₁₂ (JCPDS file 09–0310), Al₂Y₄O₉ (monoclinic P Bravais lattice, space group: P2₁/c with parameters a/b/c = 0.747/1.053/1.119 nm and β = 108.88°, JCPDS file 83–0935) [25] or Y₄Al₂O₉ (JCPDS file 22–0987) [26] oxide phases, but instead it matched well with the diffraction pattern from Al₂Y₄O₉ (orthorhombic P Bravais lattice with parameters a/b/c = 1.046/1.054/0.3684 nm, JCPDS file 14–0475) [27] in the zone axis of $\left[23\overline{1}\right]$. This pattern indicated estimated interplanar spacings of ∼d₁/d₂/d₃ = 0.2630/0.3230/0.3130 nm, which show an agreement with the interplanar spacings, d₁/d₂/d₃ = 0.2896/0.2908/0.3300 nm, for $\left(2\overline{1}1\right)$/$\left(\overline{3}20\right)$/$\left(\overline{1}11\right)$ lattice planes in the zone axes of $\left[23\overline{1}\right]$ for the Al₂Y₄O₉ phase, respectively. Therefore, the observed nanosized particle P2 was identified to be Al₂Y₄O₉ with an orthorhombic P crystal structure.

Hsiung et al. [28,29] suggested that nanoparticles in the MA956 steel belong to the Y₄Al₂O₉ (monoclinic P Bravais lattice, space group: P2₁/c with lattice parameters a/b/c = 0.7375/1.0507/1.1113 nm and β = 108.58°, JCPDS file 22–0987) phase, which is similar to Al₂Y₄O₉ (JCPDS file 83–0935). We suggest that the identified nanosized phase in the as-rolled PM2000 steel belongs to the Al₂Y₄O₉ with an orthorhombic P Bravais lattice, rather than to the Al₂Y₄O₉ (JCPDS file 83–0935) or the Y₄Al₂O₉ with a monoclinic-simple crystal structure.

Figure 5 shows additional TEM micrographs of the carbon replica from the as-rolled PM2000 steel with FFT patterns. The FFT patterns in Figure 5(c,d) were generated from particles P3 and P4, which are marked with arrows, respectively. These two patterns did not match any of the patterns for YAlO₃ (JCPDS file 08–0147). The pattern in Figure 5(c) matched the pattern from the Al₂Y₄O₉ (JCPDS file 14–0475) in the zone axis of [135], which indicates that particle P3 is also the Y₄Al₂O₉ phase. For the pattern in Figure 5(d), the pattern is inconsistent with the Al₂Y₄O₉ (JCPDS file 83–0935) and shows no better match with the patterns for the Al₂Y₄O₉ (JCPDS file 14–0475). However, the pattern matches that from Y₃Al₅O₁₂ (tetragonal P Bravais lattice with the lattice parameters a/c = 0.751/0.424 nm, JCPDS file 09-0310) in the zone axis of [132], if the Y₃Al₅O₁₂ has a tetragonal-simple lattice. Therefore, particle P4 is a Y₃Al₅O₁₂ phase with a tetragonal-simple crystal structure.

Figure 5. (a) TEM micrograph of carbon replica prepared from as-rolled PM2000 steel, showing nanosized Al₂Y₄O₉ oxide particle P3 and Y₃Al₅O₁₂ oxide particle P4 marked with arrows; (b) FFT image generated from particle P3; (c,d) FFT patterns generated from particles P3 and P4 in zone axes of [135] and [132], respectively.

Shen et al. [9] confirmed the presence of the Y₃Al₅O₁₂ phase in the as-rolled PM2000 steel. They observed several Y₃Al₅O₁₂ particles with 33–173 nm diameters and an average diameter of 80 nm in the steel matrix. They speculated that the crystal structure of Y₃Al₅O₁₂ may have a tetragonal-simple lattice. Our investigation confirmed the presence of a smaller nanoscale (< 10 nm diameter) Y₃Al₅O₁₂ phase in the as-rolled PM2000 steel, that is, a tetragonal-simple crystal structure.

Figure 6(b) shows a FFT pattern from arrow-marked particle P5 in Figure 6(a), which was also taken from the carbon replica of the as-rolled PM2000 steel. The pattern did not match those from the Al₂Y₄O₉ (JCPDS file 14–0475/83–0935). The match did not improve when the pattern was indexed in terms of patterns from the Y₃Al₅O₁₂ (JCPDS file 09-0310). The pattern could be matched better with that from the YAlO₃ (JCPDS file 08–0147/33-0041) in the zone axis of $\left[23\overline{1}\right]$, which shows that particle P5 belonged to the YAlO₃ phase. Thus, except for the large YAlO₃ particles (> 35 nm diameter) as observed in the investigation [9], YAlO₃ phase particles are smaller (∼10 nm diameter) and were present in the as-rolled PM2000 steel.

Figure 6. (a) TEM micrograph of carbon replica prepared from as-rolled PM2000 steel, showing nanosized YAlO₃ oxide particle P5 marked with arrow; (b) FFT pattern generated from particle P5 in the zone axis of $\left[23\overline{1}\right]$.

A large amount of complex Y–Al–O/Al₂O₃ and Y–Al–O/TiC and pure Al₂O₃ inclusions that are significantly larger than ODS particles have been identified in PM2000 steel [10]. In the as-rolled PM2000 steel, some YAlO₃ and Y₃Al₅O₁₂ (33–173 nm in diameter) and a small amount of Al₂Fe₂O₆ and Al₃FeO₆ (233–374 nm in mean diameter) in addition to Al₃Ti₅O₂ and Al₂O₃ (160–500 nm in diameter) were observed [9]. These results indicate that during the milling of PM2000, exposure of the powder to air occurred, which led to the internal oxidation of Al, Fe and Ti. Al has a higher oxygen affinity compared with Ti, Cr and Fe, i.e. Al > Ti > Cr > Fe [30]. No previous reports exist on the Cr–O and Y–Cr–O oxide phases, such as Cr₂O₃ and YCrO₃, as identified in PM2000 steel. Besides Al₂O₃ oxide, Al₂Fe₂O₆, Al₃FeO₆ and Al₃Ti₅O₂ complex-oxide phases also formed in PM2000 steel. Because oxide phases larger than PM2000 steel were not the focus of this investigation, we did not study large oxide particles during this TEM examination.

To form Al₂Y₄O₉ nanoparticles in consolidated MA/ODS steels, the following three reactions should occur: (1) Y₂O₃ powder particle dissolution: Y₂O₃→2[Y]+3[O], (2) internal oxidation reaction of Al: 2[Al]+3[O]→Al₂O₃ because of the higher oxygen affinity of Al, and (3) a complex-oxide formation reaction: 2Y₂O₃+Al₂O₃→Al₂Y₄O₉, which can occur between 900°C and 1100°C [28,29]. In this investigation, we did not detect the Y₂O₃ oxide phase in the PM2000 steel, which indicates that initial Y₂O₃ dispersoids that were added to the starting powder dissolved or were transformed to other oxide phases during the PM2000 steel fabrication. We found no experimental evidence that initial Y₂O₃ particles dissolved during the process but it is possible that the Y₂O₃ phase transformed into other oxide phases. If nanosized Al₂Y₄O₉ particles formed through the reaction 2Y₂O₃+Al₂O₃→Al₂Y₄O₉, the Y₂O₃ and Al₂O₃ particles should be small. Although the size of the initial Y₂O₃ particles that were added to the starting powder for PM2000 steel fabrication was not available, it is believed that during mechanical alloying, the Y₂O₃ phase was small because of the particle fragmentation by ball milling. Through many TEM observations, we did not detect small Al₂O₃ particles. Instead, Al₂O₃ particles in PM2000 steel were confirmed to be large [9,10]. Therefore, nanosized Al₂Y₄O₉ particles in PM2000 steel may not form by the reaction of fine Y₂O₃ with large Al₂O₃.

Because the identified Al₂Y₄O₉/Y₃Al₅O₁₂/YAlO₃ phases in PM2000 steel have a nanoscale spherical morphology and a higher number density, and they disperse in the matrix, the formation of the Y₄Al₂O₉ phase appears to be related to initial Y₂O₃ particles that are added to the starting powder. Hsiung et al. [28,29] investigated the formation mechanism of Y₄Al₂O₉ or YAlO₃ oxide nanoparticles in MA956 (Fe-20Cr-4.5Al-0.34Ti-0.5Y₂O₃) and K3 (Fe-16Cr-4.5Al-0.3Ti-2W-0.37Y₂O₃) ODS ferritic steels. Oxide nanoparticles with a crystalline core and an amorphous shell were observed frequently in as-fabricated K3 and MA956 ODS steels. This result indicates a three-stage oxide nanoparticle formation mechanism: (1) fragmentation of starting Y₂O₃ particles during the early stages of ball milling, (2) agglomeration and solid-state amorphization of Y₂O₃ fragment mixing with matrix constituents during the later stages of ball milling, and (3) crystallization of amorphous agglomerates larger than ∼2 nm to form oxide nanoparticles with a (complex-oxide) core/(solute-enriched) shell structure during consolidation at 1150°C. In this investigation, nanosized oxide particles with a core/shell structure were not observed in the as-rolled PM2000 steel. As an example, Figure 7 shows a group of FFT images that were generated from randomly selected nanosized particles on the carbon replica that was prepared from the as-rolled PM2000 steel. Figure 7 shows that the boundary contour of these particles is clear, and no disordered region exists on the particle edge. These nanosized particles have complete crystal characteristics, and their interior and edge do not have an amorphous structure. Therefore, the formation mechanism of nanoparticles that was proposed by Hsiung et al. [28,29] appears unsuitable to explain the formation of nanosized oxide particles in the as-rolled PM2000 steel.

Figure 7. FFT images generated from nanosized particles on carbon replica prepared from as-rolled PM2000 steel, showing that these particles are crystalline.

The formation of nanosized oxide phases in the as-rolled PM2000 steel may be related closely to the high-temperature diffusion of steel constituents during steel production. The nanosized Al₂Y₄O₉/Y₃Al₅O₁₂/YAlO₃ particles in the PM2000 steel could be speculated to form from initial Y₂O₃ particle fragments that were mixed with steel constituent Al through diffusion during production. The Y–Al–O oxide phases should form in the Y–O-enriched regions during processing. Crushed fine Y₂O₃ particles are suitable in this context. Y₂O₃ particle fragments could act as nucleation sites for the Y–Al–O oxide phases. The nucleation and growth of the Al₂Y₄O₉/Y₃Al₅O₁₂/YAlO₃ phases require Al solute atom diffusion to their nucleation sites and nuclei during processing.

Commercial PM2000 steel was produced by Plansee GmbH through mechanical alloying, consolidation by hot isostatic pressing (HIP), and thermomechanical processing (hot-extrusion/hot-rolling). More detailed PM2000 steel processing procedures are unavailable. The fabrication processes, such as HIP and the hot-extrusion/hot-rolling of the steel, were probably performed between 900°C and 1100°C [31,32]. The activation energy for self-diffusion of Al, Cr and Ti substitutional atoms in α-Fe from 900°C to 1100°C, 612°C to 901°C, and 675°C to 901°C was reported to be 246 [33], 267 [34], and 293 [35] kJ/mol, respectively. Because the activation energy for the self-diffusion of solute Al in α-Fe at the PM2000 steel processing temperature was lower than that of solute atoms Cr and Ti, solute Al diffuses easily into the Y₂O₃ particle fragments compared with solute atoms Cr and Ti. With an increase in Al content to a certain critical minimum concentration of Al required for the nucleation of Al₂Y₄O₉/Y₃Al₅O₁₂/YAlO₃ phases, the Y–Al–O oxide phases would nucleate inside the Y₂O₃ particle fragments. Subsequently, these phase nuclei would grow to Y–Al–O oxide particles through solute Al atom diffusion from the steel matrix to the particles. This behaviour may show the formation mechanism of the nanosized Al₂Y₄O₉/Y₃Al₅O₁₂/YAlO₃ phases in the as-rolled PM2000 steel. The role of the Al₂Y₄O₉/Y₃Al₅O₁₂/YAlO₃ phase formation in the as-rolled PM2000 steel requires further investigation.

5. Conclusions

Dispersoids in the as-rolled PM2000 steel consist of large oxides and nanosized oxide particles. Large oxides are distributed heterogeneously. Nanosized oxide particles with a 10–50-nm diameter and a mean diameter of ∼20 nm distribute uniformly in the alloy matrix. Three nanosized oxide phases that consist of Al₂Y₄O₉, Y₃Al₅O₁₂, and YAlO₃ are identified in the as-rolled PM2000 steel. The identified nanoscale Al₂Y₄O₉ oxide phase in the steel has an orthorhombic-simple crystal structure, rather than a monoclinic-simple crystal structure as reported previously in MA956 steel. The nanosized Y₃Al₅O₁₂ oxide phase in the steel has a tetragonal-simple crystal structure. The probable formation mechanism of nanosized Al₂Y₄O₉/Y₃Al₅O₁₂/YAlO₃ particles in the steel is as follows. During HIP and thermomechanical processing (hot-extrusion/hot-rolling) of steel, Al diffusion from the steel matrix into the Y₂O₃ particle fragments induces the nucleation and growth of the Al₂Y₄O₉/Y₃Al₅O₁₂/YAlO₃ phases, which leads to a formation of oxide nanoparticles. The formation mechanism of the Al₂Y₄O₉/Y₃Al₅O₁₂/YAlO₃ phases in the steel requires further investigation.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (11805293), Key Program of National Natural Science Foundation of China (51034011), ITER-National Magnetic Confinement Fusion Program of The Department of Science and Technology of China (2011GB113001), and Shanghai Pujiang Program. The authors are grateful to Ms. Xuezhen Zhou, Shanghai University of Engineering Science, for preparing extraction carbon replicas that were used in this work. The authors thank Dr. Jinsung Jang, Nuclear Materials Research Center, Korea Atomic Energy Research Institute, for providing the steel that was used in this work.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This work was supported by National Magnetic Confinement Fusion Program of China: [Grant Number 2011GB113001]; National Natural Science Foundation of China: [Grant Number 51034011]; Youth Program of National Natural Science Foundation of China: [Grant Number 11805293].