# Precipitation pathway in oxide dispersion-strengthened steels: From metastable precursors to pyrochlore

J. Ribis

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J. Ribis. Precipitation pathway in oxide dispersion-strengthened steels: From metastable precursors to pyrochlore. Materialia, 2026, 45, pp.102673. 10.1016/j.mtla.2026.102673. hal-05504690

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Full Length Article

# Precipitation pathway in oxide dispersion-strengthened steels: From metastable precursors to pyrochlore

J. Ribis

Université Paris-Saclay, CEA, Service de Recherches en Matériaux et procédés Avancés, 91191 Gif-sur-Yvette, France

ARTICLE INFO

Keywords:

Oxide dispersion strengthened (ODS) alloy
High-resolution electron microscopy (HREM)
Precipitation
Metastable phases
Crystal structure

ABSTRACT

Oxide Dispersion Strengthened (ODS) materials are promising candidates for applications as core components for the next generation of nuclear power plants, including both GEN IV reactors and fusion reactors. These materials are characterized by a fine distribution of nano-oxides, which imparts excellent creep resistance. In this paper, a comprehensive characterization of the nano-oxides is proposed using Cs-corrected transmission electron microscopy. We show that three types of oxides exist within the ferritic matrix: nanoclusters with a structure identical to that of the matrix, pyrochlore-structured nano-oxides, and, for the first time, intermediate-sized nano-oxides exhibiting a structure in the process of transforming into the pyrochlore phase. We conclude that the formation of nano-oxides follows a precipitation sequence in which metastable nanoclusters act as precursors to the pyrochlore-like nano-oxides. Understanding this pathway is crucial, as it not only enables control over oxide distribution (in terms of size and density), but also provides insights into the formation of interface structure, which governs point defect and helium trapping. Moreover, it contributes to understanding the irradiation resistance of nano-oxides, since nanoclusters may be more resistant than pyrochlores, or vice versa.

1. Introduction

Oxide Dispersion Strengthened (ODS) materials are promising candidates for applications in harsh environments, particularly as core components for the next generation of nuclear power plants, including both GEN IV reactors and fusion reactors. These materials are characterized by a high density of nano-oxides finely dispersed within a ferritic matrix [1]. The primary purpose of adding these nano-oxides is to enhance the mechanical properties of the alloy, as the nano-precipitates are known to act as obstacles to dislocation motion. To achieve optimal mechanical properties after fabrication and thermo-mechanical treatment, one of the main challenges is ensuring homogeneous nucleation of the oxides during thermal annealing, which leads to their fine dispersion within the ferritic grains. Therefore, understanding the nucleation pathway of oxide precipitates is essential to address this issue effectively. However, this understanding requires precise characterization of the smallest oxides, particularly using high-resolution transmission electron microscopy (HRTEM), which is both difficult and challenging due to their extremely small size. For precipitates larger than for an arbitrary size of 3 nm, literature data are unambiguous and consistently show that they exhibit a $Y_2Ti_2O_7$-type face-centered cubic (FCC) structure [2,3]. One notable feature of the pyrochlore precipitates is that all

chemical analyses have shown they do not conform to the $Y_2Ti_2O_7$ stoichiometry [4–8], even though they exhibit the same structure with a lattice parameter of 1.01 nm. One possible explanation is that $Y_2Ti_2O_7$ compounds may tolerate deviations from stoichiometry by accommodating a high concentration of point defects [9]. However, this non-stoichiometric or near-stoichiometric composition must eventually relax toward equilibrium after annealing as observed by Sakasegawa et al. [10]. It is also worth noting that a size-dependent Cr-rich shell often surrounds these pyrochlore precipitates [11–14].

Only a few studies have attempted to investigate the structure of precipitates smaller than about 3 nm, which may already form during the earliest stages of processing, immediately after mechanical alloying [4,15]. Hirata et al. [16] were the first to report an atomic-scale characterization of the oxide nanoclusters and found that these precipitates possess a defective NaCl-type structure with high lattice coherency with the bcc steel matrix. Ribis et al. [17] and Ribis and de Carlan [18] confirmed that the oxide nanoclusters exhibit a cubic structure with a lattice parameter identical to that of the host matrix and are, therefore, fully coherent with it. From a chemical point of view, APT studies [13] reported that the oxide nanoclusters are composed of Y-Ti-O, but also appear to be rich in Cr. However, these results raise a question regarding the coexistence of two types of precipitates: one with an undefined cubic

<footnote>E-mail address: joel.ribis@cea.fr.</footnote>
<table><caption>**Table 1**
Chemical composition of the ODS steel (%wt).</caption><thead><tr><th>Fe</th><th>Cr</th><th>W</th><th>Ti</th><th>Mn</th><th>Si</th><th>Ni</th><th>C</th><th>Y<sub>2</sub>O<sub>3</sub></th></tr></thead><tbody><tr><td>Bal.</td><td>14</td><td>1</td><td>0.3</td><td>0.3</td><td>0.3</td><td>0.15</td><td>0.5</td><td>0.3</td></tr></tbody></table>

structure, Y-Ti-Cr-O rich, sharing the same lattice parameter as the matrix, and the other with a well-defined pyrochlore structure surrounded by a Cr shell. Ribis et al. [11] proposed that these two types of

precipitates are actually connected through a precipitation sequence, in which the fully coherent oxide nanoclusters act as precursors to the formation of Y₂Ti₂O₇-type precipitates. Ribis et al. [11] also proposed that, during the transformation from the metastable structure to the pyrochlore phase, Cr atoms—being insoluble in Y₂Ti₂O₇ [19]—are rejected, which may contribute to the formation of the Cr-rich shell. However, to confirm this precipitation pathway, the transforming precipitates with sizes lying between the fully coherent nanoclusters and the pyrochlore-type precipitates must be studied, although they have

<figure><img src="image_3.png" /><figcaption>**Fig. 1.** Distribution of nano-oxides dispersed in the ferritic matrix: (a) Bright-field TEM image showing the nano-oxides; (b) Nano-oxides smaller than 1 nm, hereafter referred to as nanoclusters; (c) Ti M-edge jump ratio image of nano-oxides (outlined in white) coexisting with nanoclusters (outlined in yellow); (d) Cr M-edge jump ratio image of nano-oxides (outlined in white) coexisting with nanoclusters (outlined in yellow); (e) Ti M-edge jump ratio image of nanoclusters (outlined in yellow); (f) Cr M-edge jump ratio image of nanoclusters (outlined in yellow).</figcaption></figure>
<figure><img src="image_4.png" /><figcaption>**Fig. 2.** Pyrochlore-like Nano-Oxide (Size > 3 nm): (a) Observed along the (001) zone axis; (b) Corresponding FFT pattern; (c) Observed along the (110) zone axis; (d) Corresponding FFT pattern.</figcaption></figure>

never been reported to date. Therefore, the present study aims to characterize the smaller precipitates using Cs-corrected transmission electron microscopy, with particular attention given to those lying in the intermediate size range.

## 2. Experimental details

The study was conducted on a Fe-14Cr-1W-0.3Ti-0.3Y₂O₃ (wt%) ODS ferritic steel developed at CEA/SRMA for nuclear applications [20]. Two pre-alloyed metal and oxide powders, supplied by Aubert & Duval (France), were mechanically alloyed in an attritor under a hydrogen atmosphere at room temperature at Plansee AG (Austria). The material was then consolidated at CEA/LTMEX by hot extrusion at 1100 °C, hot rolled up to 20 % at 650 °C and annealed for 1 h at 1050 °C. The chemical composition of the alloy is presented on Table 1.

The materials were characterized using a JEOL neoARM double Cs-corrected transmission electron microscope operating at 200 kV and equipped with a Gatan imaging filter. TEM samples were prepared by punching 3 mm disks from 60 µm-thick mechanically polished foils, followed by electropolishing in a 10 % perchloric acid-90 % ethanol solution at 10 °C using a Struers Tenupol thinning device.

EFTEM analytical data were obtained using jump ratio images (two windows method) from the low-loss regions. For the Fe-M edge jump ratio image the pre-edge energy was set at 48 eV while the post-edge energy was set at 64 eV. For the Cr-M edge jump ratio image the pre-edge energy was set at 36 eV while the post edge energy was set at 52 eV. For the Ti-M edge jump ratio image the pre-edge energy was set at 25 eV while the post edge energy was set at 45 eV. The slit width was 8 eV for all the jump ratio image acquisitions.

## 3. Results

Fig. 1(a) displays the microstructure of the studied Fe-14Cr ODS steel, revealing a homogeneously distributed population of nano-oxides. The size of these nano-oxides ranges from below 1 nm to approximately 6 nm, with a number density of $2.9 \times 10^{23} \text{ m}^{-3}$ [18]. Fig. 1(b) shows a detailed view of one of the smallest nano-oxides, hereafter referred to as a nanocluster oxide, with an estimated size of 0.8 nm. According to literature data [2,3], oxides larger than 3 nm are well established to exhibit a pyrochlore-like structure with a lattice parameter of 1 nm. However, it is difficult to believe that nanoclusters smaller than the theoretical lattice parameter of $Y_2Ti_2O_7$—such as the one shown in Fig. 1(b)—could adopt such a structure. Therefore, a distinction is made between nano-oxides above and below an arbitrary threshold of 3 nm. Both categories are investigated using high-resolution (HRTEM) and Energy-Filtered transmission electron microscopy (EFTEM) below.

### 3.1. The nano-oxides precipitates (> 3 nm)

First, nano-oxides larger than 3 nm—believed to exhibit a pyrochlore-like structure—are examined. The Fd3⁻m lattice structure of a $Y_2Ti_2O_7$ pyrochlore is closely related to the fluorite structure, with the main difference being that it contains two distinct cation sites and one-eighth of the anion sites (8a) are vacant [21]. In this structure, Y and Ti cations occupy the 16d (¹/₂, ¹/₂, ¹/₂) and 16c (0, 0, 0) positions, respectively, while the oxygen atoms are located at the 48f (x, 1/8, 1/8) and 8b (³/8, ³/8, ³/8) positions [21]. Fig. 2(a–d) show two precipitates along with their corresponding fast Fourier transforms (FFTs) (Fig. 2(b) and (d)), oriented along the (001) direction (Fig. 2(a)) and the (110) direction
<figure><img src="image_5.png" /><figcaption>Fig. 3. Metastable NanoCluster Oxides (Size < 3 nm): (a–c) Observed along the (110) zone axis; (d) Corresponding FFT pattern.</figcaption></figure>

(Fig. 2(c)), respectively.

In the FFT (Fig. 2(b)), the additional spot corresponds to the atomic planes of the oxide. The zone axis of the precipitates exhibits (001) face-centered cubic (fcc) symmetry, while the two measured interplanar spacings are 0.25 nm and 0.18 nm. These values are in good agreement with the (400) (0.252 nm) and (440) (0.178 nm) interplanar distances of the pyrochlore structure. In the FFT (Fig. 2(d)), the additional spot corresponds to the atomic planes of the oxide. The zone axis of the precipitates exhibits (110) face-centered cubic (fcc) symmetry, while the two measured interplanar spacings are 0.25 nm and 0.30 nm. These values are in good agreement with the (400) (0.252 nm) and (222) (0.291 nm) interplanar distances of the pyrochlore structure. Therefore, it can be concluded that these precipitates correspond well to the pyrochlore-like structure, in agreement with literature data. For each direction, (001) and (110), a detailed high-resolution image is presented along with a schematic illustrating the atomic arrangement of the pyrochlore-type lattice structure.

The images in the Fig. 1(c, d) correspond to jump ratio maps obtained at the Ti, and Cr edges, respectively. These images show Ti enrichment within the nano-oxides, as well as the presence of an irregularly shaped Cr-rich shell surrounding them (outlined in white).

### 3.2. The nanoclusters precipitates (< 3 nm)

Fig. 3(a-c) present a HRTEM image of a 0.8–1.3 nm-sized nano-cluster, in which atomic planes of both the cluster and the surrounding matrix are clearly resolved. The matrix is oriented along the (1-10) crystallographic direction. In the corresponding Fast Fourier Transform

(FFT) shown in Fig. 3(d), no additional diffraction spots from the nanocluster are discernible, indicating that its lattice structure and interplanar spacings are nearly identical to those of the matrix. This observation suggests that, at such a small size, the nanocluster adopts the body-centered cubic (bcc) α-Fe lattice structure imposed by the surrounding matrix, as previously reported in [17]. Furthermore, Fig. 3 (a-c) shows that the nanoclusters are fully coherent with the matrix, which is also consistent with the findings reported in [16–18].

The images in the Fig. 1(e, f) correspond to jump ratio maps obtained at the Ti, and Cr edges, respectively. These images show enrichment in both Ti and Cr within the nanoclusters, indicating that they are Y-Ti-Cr-O compounds, as already observed [11]. It can be noted in the Fig. 1(d) that Cr-rich nanoclusters (outlined in yellow) and nano-oxides surrounded by a Cr-rich shell (outlined in white) coexist within the ferritic matrix.

### 3.3. The intermediate size range precipitates (3-6 nm)

This study provides, for the first time, high-resolution images of intermediate-sized precipitates, a size range that has not previously been documented in the literature. Fig. 4(a–f) show high-resolution images of various precipitates within the intermediate size range. These images reveal that the precipitates no longer share the same structure as the matrix. They appear to exhibit the onset of a new structural arrangement, which, however, does not yet correspond to a fully developed pyrochlore structure. It gives the impression that an atomic-scale structural motif is beginning to emerge. Fig. 5(a-d) presents a closer examination of the precipitates presented in Fig. 4(b and d), along with
<figure><img src="image_6.png" /><figcaption>**Fig. 4.** Intermediate size range oxide precipitates (3-6 nm): (a and b) cube-on-edge type transforming precipitates (c, d, e and f) cube-on-cube type transforming precipitates.</figcaption></figure>

their corresponding Fourier transform. For the precipitate displayed in Fig. 5(a), the additional spots observed in the Fourier transform (Fig. 5(b)) provide clear evidence that its structure differs from that of the surrounding matrix. Upon closer examination, the crystal structure of the precipitate shown in Fig. 5(a) can be divided into three distinct regions. These regions are labelled from 1 to 4, with region 1 corresponding to the atomic structure of the matrix. Region 1 exhibits cubic symmetry, consistent with the body-centered cubic (bcc) lattice of the Fe-Cr matrix. Region 2, although part of the precipitate, still exhibits cubic symmetry. Region 3, on the other hand, exhibits a structure that is

no longer cubic but rather diamond-shaped, with an angle of approximately 85°. In Region 4, the diamond shape is even more pronounced, with an angle of about 76°, close to the 71° angle characteristic of the pyrochlore structure. For the precipitate shown in Fig. 5(c), additional spots observed in the Fourier transform (Fig. 5(d)) indicate that it has a structure different from that of the matrix. However, unlike the precipitate shown in Fig. 5(a), it does not divide into distinct regions but consists of a single region labelled as region 6, while region 5 still corresponds to the matrix structure. Region 6 exhibits a cubic shape, but its interplanar spacings do not yet correspond to those of a developed
<figure><img src="image_7.png" /><figcaption>**Fig. 5.** Intermediate size range oxide precipitates (3-6 nm): (a) structure analysis of a cube-on-edge type transforming precipitates (b) corresponding FFT pattern, (c) structure analysis of a cube-on-edge type transforming precipitates, (d) corresponding FFT pattern.</figcaption></figure>

pyrochlore structure. Based on these observations, it is tempting to conclude that the structures of these intermediate-sized precipitates displayed in Fig. 4(a-f) are likely undergoing a transformation from a matrix-identical metastable structure toward the pyrochlore structure. Furthermore, it can be noted that the precipitates in Fig. 4(a and b) are developing a cube-on-edge orientation relationship with the matrix, whereas those in Fig. 4(c, d, e and f) are developing a cube-on-cube orientation relationship. Both of these orientation relationships have been previously reported for pyrochlore-like precipitates [18]. Our results suggest that a pyrochlore daughter phase with a different crystallographic orientation — either cube-on-cube or cube-on-edge — can originate from the same cluster structure. Thus, the relationship between nanoclusters and nano-oxides is now better understood, with a high probability that the former acts as the metastable precursor of the latter, as proposed [11].

The histogram in Fig. 6 presents the size distribution of all oxide types—nanocluster-type, transforming-type, and pyrochlore-type—as a function of their diameter. It is shown that the nanocluster-type oxides range from 0 to 3 nm, with sizes varying between 0.8 nm and 2.6 nm. The transforming-type oxides range from 3 to 6 nm, with sizes between 2.6 nm and 5.7 nm. Finally, the pyrochlore-type oxides exhibit sizes larger than 4 nm.

The nanocluster-type and pyrochlore-type oxides can be regarded as representative of the bulk material, since they have been repeatedly observed in various samples. In contrast, the transforming-type oxides are relatively rare, and those reported in this work were found in two distinct grains of the same thin foil.

Fig. 7 schematically summarizes the proposed precipitation pathway for pyrochlore formation in ODS steels, involving nanocluster-type, transforming-type, and pyrochlore-type oxides.

Now that the relationship between nanoclusters and nano-oxides has been established, we can discuss the behaviour of chromium atoms. It is

observed that Cr is present within the nanoclusters, whereas it is located at the interface of the nano-oxides. Since nanoclusters transform into pyrochlore, and chromium is not soluble in the latter [19], it can be assumed — as previously suggested [11,13,14] — that Cr is expelled from the nano-oxide during its transition to the pyrochlore state. Thus, the expulsion of Cr could be at the origin of the Cr-rich shell, which would explain its size dependence with respect to the oxide precipitate. However, Cr has a tendency to segregate toward high-energy regions such as grain boundaries [11]. Therefore, it cannot be excluded that the Cr-rich shells (not necessary size-dependent) also form due to Cr segregation from the matrix. A combination of both mechanisms — Cr expulsion from the nano-oxide and Cr segregation from the matrix — is also possible. This would explain why, even if Cr is expelled from the nano-oxide, it remains located at its interface. However, the hypothesis of Cr expulsion from the oxide must be verified by monitoring a Cr-rich nanocluster in real time during an in-situ heating experiment.

## 4. Conclusion

In conclusion, this paper has demonstrated the existence of three distinct types of oxides: nanoclusters with a structure identical to that of the matrix, pyrochlore-structured nano-oxides, and, finally, intermediate-sized precipitates exhibiting a transitional structure in which features of the pyrochlore structure appear to emerge. While the first two types have already been reported in the literature, the latter are newly observed and are presented here for the first time. As a result, it can be assumed that the formation of nano-oxides follows a precipitation sequence in which nanoclusters act as precursors to pyrochlore precipitates. This information is valuable and represents a new advance-ment in the understanding of nano-oxide formation mechanisms in ODS steels.
<figure><img src="image_8.png" /><figcaption>**Fig. 6.** Histogram of the size distribution of all oxides type—nanocluster-type, transforming-type, and pyrochlore-type—as a function of their diameter. The histogram was constructed based on the analysis of 19 nanocluster-type oxides from two distinct thin foils, 82 transforming-type oxides from two separate grains, and 8 pyrochlore-type oxides from two different grains.</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>**Fig. 7.** Schematic of the proposed precipitation pathway for the formation of pyrochlore.</figcaption></figure>

**CRediT authorship contribution statement**

**J. Ribis:** Writing – review & editing, Writing – original draft, Supervision, Investigation, Conceptualization.

**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.
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