Anthony Durand
email: anthony.durand2@cea.fr

Denis Sornin

Yann De Carlan

Gabriel Spartacus

François Brisset

Ludovic Delbes

Benoît Baptiste

Thierry Baudin

Roland Logé

Yann De Carlan

Characterization of untransformed ferrite in 10Cr and 12Cr ODS steels

Keywords: ODS steel, untransformed ferrite, ferrito-martensitic steels, microstructure, SAXS, creep

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Introduction

Oxide dispersion strengthened (ODS) steels have been developed, for nuclear applications, to improve the very high-temperature properties of body-centered stainless steel. The addition of very fine stable particles at high temperatures is a promising solution [START_REF] Bremaecker | Past research and fabrication conducted at SCK•CEN on ferritic ODS alloys used as cladding for FBR's fuel pins[END_REF][START_REF] Miller | Stability of ferritic MA/ODS alloys at high temperatures[END_REF]. ODS steels were considered as a potential structural and cladding material for GEN IV sodium-cooled fast reactors (SFR) [START_REF] Dubuisson | ODS Ferritic/martensitic alloys for Sodium Fast Reactor fuel pin cladding[END_REF][START_REF] Ukai | Perspective of ODS alloys application in nuclear environments[END_REF], and fusion reactors [START_REF] Oksiuta | Development and characterisation of a new ODS ferritic steel for fusion reactor application[END_REF]. Not only ODS steels exhibit a convenient void swelling resistance under irradiation but also they display interesting high-temperature mechanical properties [START_REF] Huet | Fabrication and Mechanical Properties of Oxide Dispersion Strengthening Ferritic Alloy Canning Tubes for Fast Reactor Fuel Pins[END_REF]. Indeed, body-centered cubic lattice, like ferrito-martensitic matrix, exhibit very good intrinsic resistance to swelling compared to face-centered cubic lattice. If ferritic ODS steels are a good candidate for SFR development [START_REF] De Carlan | CEA developments of new ferritic ODS alloys for nuclear applications[END_REF], manufacturing remains challenging. Manufacturing thin cladding tubes requires cold forming operations. Material recovery between rolling passes is necessary, at least to soften the materials and control the grain morphology [START_REF] Ukai | Development of 9Cr-ODS Martensitic Steel Claddings for Fuel Pins by means of Ferrite to Austenite Phase Transformation[END_REF]. Regarding this problem, a ferrito-martensitic grade exhibits the great advantage of a phase transformation at high temperatures which is efficient to recover microstructures [START_REF] Toualbi | Assessment of a new fabrication route for Fe-9Cr-1W ODS cladding tubes[END_REF][START_REF] Vakhitova | Texture evolution in Oxide Dispersion Strengthened (ODS) steel tubes during pilgering process[END_REF]. Nevertheless, regarding fuel reprocessing envisaged in the SFR framework, low chromium grades are not the most relevant because of their excessive corrosion during fuel acid dissolution. Thus, with a phase transformation at a sufficiently high temperature, 10Cr to 12Cr ODS steels are promising candidates offering easier manufacturing steps and interesting creep properties. Recent studies have evidenced improved creep properties of ODS steel attributed to untransformed ferrite above the ferrite to austenite phase transformation (α→γ) [START_REF] Yamamoto | Formation of residual ferrite in 9Cr-ODS ferritic steels[END_REF][START_REF] Yamamoto | Reverse phase transformation from α to γ in 9Cr-ODS ferritic steels[END_REF][START_REF] Yamashiro | Microstructural stability of 11Cr ODS steel[END_REF]. This enhancement is attributed to the nano-sized oxides which are smaller in ferrite than in martensite [START_REF] Ukai | Hightemperature strength characterization of advanced 9Cr-ODS ferritic steels[END_REF]. Particularly, such ODS steels containing untransformed ferrite are known to contain a double dispersion with smaller nano-sized oxides into the untransformed ferrite and bigger into the martensite matrix [START_REF] Yamamoto | Formation of residual ferrite in 9Cr-ODS ferritic steels[END_REF][START_REF] Yamamoto | Reverse phase transformation from α to γ in 9Cr-ODS ferritic steels[END_REF]. In that view, the chemical composition of the 12Cr steel of this study was adapted trying to retain some untransformed ferrite at high temperatures.

This study highlights the untransformed ferrite existence. For two different ODS steels, varying chromium content, a methodology based on dilatometric data and microstructural observations is proposed to quantify the untransformed ferrite fraction. Creep properties and nano-sized oxides dispersion are compared for the two materials.

Materials and methods

Both ODS steels, of this study, have a very close chemical composition. The first one is 10 wt% chromium and the second one is 12 wt%. The pre-alloyed powder was mechanically alloyed in a high-energy attritor with 0.1 wt% of TiH2 powder and 0.3 wt% of Y2O3 powder under a helium atmosphere. The milled powder was sealed into a 64 mm diameter can and outgassed. Then, the powder was consolidated into a rod by hot-extrusion at 1050°C. The final product is a 23 mm diameter rod, the extrusion ratio is 9.8. The X-ray diffraction (XRD) at low and high temperatures was conducted on Panalytical Xper't Pro MPD equipment coupled to a controlled-atmosphere furnace from Anton Paar HTK. The X-ray source is a cobalt source with a wavelength of 1.79 Å. To avoid excessive sample oxidation the heating chamber is maintained under vacuum (10 -3 mBar). Dilatometric experiments were conducted on a DT1000 ADAMEL-LHOMARGY dilatometer on 12x2x2 mm 3 samples picked-up in the longitudinal direction of the rod. Differential scanning calorimetry (DSC), using a SETARAM MHTC96 equipment, measures phase transformation specific enthalpy.

Scanning electron microscopy using the electron backscatter diffraction (EBSD) detector were performed on gas-quenched samples, heat-treated into the dilatometer. EBSD maps analysis were completed with quantitative electron probe micro analyzer (EPMA) mapping using Cameca SX100 equipment.

Creep properties were evaluated at 650°C, under air atmosphere, on cylindrical samples of 4 mm diameter and 20 mm gauge length. Samples were machined with the tensile direction collinear to the hot-extrusion flow. Materials tested were heat-treated at 1050°C for one hour, completed by a furnace cooling (2°C/min). ODS steels are well known to exhibit a nano-sized oxides dispersion [START_REF] Tanno | Evaluation of mechanical properties and nano-meso structures of 9-11%Cr ODS steels[END_REF], whose size and density were evaluated by small angle X-ray scattering (SAXS) measurements in transmission mode. In this study, the samples thickness is grinded down to 25 µ𝑚 in order to achieve a suitable transmission regarding the energy used. Experiments were performed at the cSAXS beamline, Swiss light source (SLS), at the Paul Scherrer Institute (PSI). The energy of the X-ray beam is 4.766 𝑘𝑒𝑉 with a detector to sample distance around 2 m. Scattering patterns were acquired on two distinct 2D detectors in order to cover a wider scattering vector q-range. Acquisition time was set to 10 seconds and 15 frames were measured and averaged for each specimen. The two scattering patterns coming from the two detectors were both azimuthally integrated, background-corrected, normalized by incident intensity, sample thickness, transmission and their respective solid angle. Both intensities were then reduced to absolute units thanks to a secondary glassy carbon standard.

The interaction between the X-ray beam and the sample produces the scattered signal, which is collected by the detector. The signal obtained is intensity as a function of the scattering vector q,

|𝑞 ⃗| = 4𝜋 𝜆 sin(𝜃) ( 1
)
where 𝜆 is the wavelength of the applied radiation and 2𝜃 the scattering angle.

The intensity profile is linked to several characteristics of the scattering object [START_REF] Schnablegger | The SAXS Guide[END_REF]. The scattering of 𝑁 dispersed particles and correctly diluted into a matrix is:

Δ𝐼(𝑞) = 𝐼 0 * ∑ 𝑉 𝑖 2 * (Δ𝜌) 𝑖 2 * 𝑃 𝑖 (𝑞) 𝑁 𝑖=1 ( 2
)
where 𝐼 0 is the incident beam intensity, 𝑉 𝑖 the volume of the particle 𝑖, 𝑃 the form factor of precipitates assumed as spherical. Δ𝜌 the electronic contrast (difference in scattering length) between the particles and the matrix and depends on the X-ray beam wavelength (𝜆). The scattering length density is also dependent on the chemical composition and the mean atomic volume according to the equation (3). Then precipitates are assumed mainly Y2Ti2O7 pyrochlores. In order to determine the value of Δ𝜌, knowing that the matrix is a ferritic Fe-10/12Cr-1W system, scattering length density was determined thanks to tabulated data from Henke et al. study [START_REF] Henke | X-Ray Interactions: Photoabsorption, Scattering, Transmission and Reflection[END_REF].

𝜌 = ∑ 𝑥 𝑖 𝑉 𝑚𝑜𝑙𝑒𝑐𝑢𝑙𝑎𝑟 * 𝑏 𝑖 (𝜆) 𝑐ℎ𝑒𝑚𝑖𝑐𝑎𝑙 𝑒𝑙𝑒𝑚𝑒𝑛𝑡𝑠 (3)
Where 𝑏 𝑖 (𝜆) is the scattering cross-section of the element i, proportional to the atomic scattering factor.

Finally, the scattering length density values for 10Cr ODS and 12Cr ODS are 5.95 .10 -5 Å -𝟐 and 5. 93 .10 -5 Å -𝟐 respectively. For the Y2Ti2O7 particles, the value is 3.18 .10 -5 Å -𝟐 . Those equations are employed to fit the experimental values using a Monte Carlo algorithm. McSAS software is used to perform this fit [START_REF] Pauw | Improvements and considerations for size distribution retrieval from small-angle scattering data by Monte Carlo methods[END_REF][START_REF] Bressler | McSAS: software for the retrieval of model parameter distributions from scattering patterns[END_REF]. The output data are the radius number distribution and the radius volume fraction distribution of the scattered particles. From those outputs, the mean radius and the number density of precipitates are estimated.

Results and discussion

Untransformed ferrite highlight

The Fe-Cr binary diagram presented in Figure 1 is computed based on ThermoCalc software using TCFE5 database. This diagram is based on the chemical composition, identified in Table 1. The minor elements (Al, Si and N) are not taken into account. Yttrium is also not used in the chemical composition, supposed to be contained only in nano-sized oxides. Thus, the presented diagram is equivalent to the materials of the study without the oxide dispersion contribution. This diagram shows, at 1050°C, a fully austenitic material for 10Cr ODS. The 12Cr ODS is also fully austenitic but very close to the dual-phase domain. Dilatometric tests were conducted to identify experimentally the α→γ transformation temperatures. The thermal expansion was measured in hot-extrusion direction (ED). The heating rate was set to 1°C/s from room temperature to 1100°C to approach the equilibrium transformation avoiding an excessive dwell time that could influence the sample microstructure (decarburization). Before cooling, the samples were kept at 1100°C for two minutes. The α→γ temperatures transformation during heating are presented in Table 2. These temperatures are close to ODS steels data from the literature [START_REF] Ukai | Production and properties of nano-scale oxide dispersion strengthened (ODS) 9Cr martensitic steel claddings[END_REF]. The shift to higher temperatures compared to steels without oxide dispersion is also in agreement with the observations on 9 wt% Cr ODS steel [START_REF] Ukai | Development of 9Cr-ODS Martensitic Steel Claddings for Fuel Pins by means of Ferrite to Austenite Phase Transformation[END_REF][START_REF] Vakhitova | Texture evolution in Oxide Dispersion Strengthened (ODS) steel tubes during pilgering process[END_REF] and ODS EUROFER [START_REF] Cayron | Microstructural evolution of Y2O3 and MgAl2O4 ODS EUROFER steels during their elaboration by mechanical milling and hot isostatic pressing[END_REF]. The temperatures of the beginning and the end of the transformation are shifted up by about 20°C for the 12Cr ODS compared to the 10Cr ODS. This is consistent with the shape of the Fe-Cr binary diagram, which illustrates such a shifting with Cr content. To detect the presence of ferrite into the austenitic matrix upon the transformation temperature Ac3, XRD measurements are done.

The two materials are prepared with the same protocol to highlight the contribution of chromium content. Samples were prepared from a bulk part heat-treated at 1050°C for one hour followed by slow cooling ensuring a ferritic state at the beginning of the XRD measurements. Four different measurements were conducted at various temperatures: the initial state at 25°C, the heated material at 1050°C, the heated materials at 1050°C twenty minutes after reaching shelf temperature and finally the ferritic cooled state back to 25°C. The prospected angle extent is [48°, 56°] including the (111) austenite and (110) ferrite diffraction peaks. The theoretical position of those two peaks is 50.2° and 52.2°, respectively, according to the Bragg law with a cobalt source. The XRD results of 10Cr ODS and 12Cr ODS are presented in Figure 2. At 25°C both materials are constituted of ferrite and only one peak at 2θ = 52.2° is noticed corresponding to the (110) ferritic peak. This peak is strongly enhanced by the crystallographic fiber inherited from hot-extrusion. At 1050°C, two peaks are noticed: one at 2θ = 50.0° for both materials and a second one at 2θ = 51.4°. According to Bragg's law, the first one is associated with the austenitic phase diffraction and the second one is a ferritic peak. This second peak offsets to lower angles compared to room temperature. This is a consequence of the thermal expansion of the sample. The two peaks evidence the coexistence of both ferrite and austenite above the α→γ transformation temperature. Moreover, twenty minutes after reaching 1050°C the ferrite is still present on XRD measurements. The ferritic phase seen at 1050°C is arguably the initial ferrite that does not undergo the phase transformation during the heating procedure.

To compare both materials, the quantification of the untransformed ferrite volume fraction is necessary. One way is to use dilatometric data. When considering the material above Ac3, composed of a volume fraction (x) of untransformed ferrite and a volume fraction (1-x) of austenite, the thermal expansion coefficient of this material, is assumed to be a fraction combination of the coefficient of the two phases. Then, after austenitization at high temperatures, the thermal expansion coefficient is expressed as:

𝛼 𝑂𝐷𝑆 = 𝑥 * 𝛼 𝑂𝐷𝑆 𝐹 + (1 -𝑥) * 𝛼 𝑂𝐷𝑆 𝐴 (4)
Where 𝛼 𝑂𝐷𝑆 is the material thermal expansion coefficient of the characterized ODS steel, 𝛼 𝑂𝐷𝑆 𝐹 is the thermal expansion coefficient of the ODS in the fully ferritic state and 𝛼 𝑂𝐷𝑆 𝐴 is the thermal expansion coefficient of a fully austenitic state.

𝛼 𝑂𝐷𝑆 is calculated with a temperature interval of 5°C during the heating (1°C/s) between 1000°C and 1100°C, the growth with the temperature is linear. Numerous measures are used, giving the standard deviation of 𝛼 𝑂𝐷𝑆 .

The materials follow an α→γ transformation around 950°C. To estimate 𝛼 𝑂𝐷𝑆 𝐹 , the thermal expansion coefficient of the ODS material is calculated between 250 and 800°C (i.e., fully ferritic range) with a temperature interval of 5°C during heating at 1°C/s. The expansion evolution with the temperature is linearly between 250°C and 800°C, and then it is assumed that the value can be extrapolated between 1000°C and 1100°C. The calculated values for the ferritic state are in agreement with data issued from literature : 12 -14 .10 -6 °𝐶-1 [START_REF] Yamamoto | Formation of residual ferrite in 9Cr-ODS ferritic steels[END_REF][START_REF] García De Andrés | Application of dilatometric analysis to the study of solid-solid phase transformations in steels[END_REF].

Using dilatometric data from 9 wt% of chromium ODS steels [START_REF] Hary | Investigation of the relationships between mechanical properties and microstructure in a Fe-9%Cr ODS steel[END_REF] which is fully austenitic above Ac3, 𝛼 𝑂𝐷𝑆 𝐴 is evaluated. The obtained value is 19.70 .10 -6 °𝐶-1 which is in agreement with other literature data [START_REF] Yamamoto | Formation of residual ferrite in 9Cr-ODS ferritic steels[END_REF][START_REF] Abe | Creep-resistant steels[END_REF]. Dilatometric data illustrate a volume fraction of untransformed ferrite significantly more important for 12Cr ODS compared to 10Cr ODS. To confirm this tendency, DSC measurements are performed during the α→γ transformation to compare the specific enthalpy of the phase transformation for 10Cr ODS and 12Cr ODS materials.

𝜶
The DSC heating rate chosen is very low (10°C/min) to approximate thermodynamic equilibrium conditions. The sample is heated from 25°C to 1350°C. Samples stayed 5 min at 1350°C before cooling at a rate of 10°C/min. In those conditions, the signal gives access to the specific enthalpy of phase transformation during heating. Figure 3 shows the evolution of the heat flow as a function of the temperature for both materials. As presented in Figure 3 the behavior of both materials is similar during the heating sequence. The first peak, around 750°C, is the Curie peak, characteristic of ferrous alloys. The second peak, around the 950°C, represents the α→γ transformation. The characteristic temperatures of both materials are slightly different. This difference is due to the increase of the chromium content that shifts up those temperatures. The main advantage of DSC measurement is the quantification of the phase transformation specific enthalpy. To get this quantity the onset/offset method is used with a linear regression between the beginning and the end of the phase transformation. The area between the regression and the DSC peak is calculated. Thus, that integration gives the specific enthalpy of the phase transformation. The specific enthalpy values are presented in Table 5.

𝚫𝑯 (𝑱/𝒈)

10Cr ODS 4.949 12Cr ODS

3.286

Table 5: Specific enthalpy of the α→γ transformation with a heating rate of 10°C/min for 10Cr ODS and 12Cr ODS.

The specific enthalpy is 1.5 higher for the 10Cr ODS than for the 12Cr ODS. This is in agreement with the dilatometric quantification of the untransformed ferrite. The higher specific enthalpy means that a higher volume of material changes phase. Additionally, the DSC measurements do not reveal the existence of δ-ferrite at 1350°C. There is no visible phase transformation above the ferrite to austenite transformation. The ferrite observed at 1050°C is untransformed ferrite during heating.

Microstructure

XRD measurements have revealed untransformed ferrite after austenitization heat treatment. EBSD maps give complementary information. They precise the spatial distribution of the untransformed ferrite. Microstructural observations are focused on a martensitic quenched state, which is an image at room temperature of the high-temperature microstructure. The Figure 4 shows the dilatation signals of the samples during heat treatment. Figure 4 shows that the entire amount of austenite is transformed into martensite during cooling. In fact, dilatometric curves only highlight a martensitic transformation. Quench transforms the austenite into martensite whereas ferrite is assumed to be unchanged. Consequently, after quenching, the two materials present a dual-phase structure with untransformed ferrite and martensite.

Figure 5 presents a large view of this metallurgical state, the EBSD map size is 400 x 400 µm² with a step size of 200 nm. A focus is also presented, which is a 100 x 50 µm² map acquired with a step size of 50 nm. 10Cr ODS and 12Cr ODS both present two different elementary microstructures. The first one, is constituted of elongated green grains corresponding to the fiber <110>//ED orientation (black arrows on Figure 5). The second is a cluster of grains with random orientation and an equiaxed shape. For 10Cr ODS, in the large view, there is an equiaxed distribution of these two elementary microstructures. On the contrary, the 12Cr ODS presents the two elementary microstructures distributed as elongated areas, like a columnar distribution. These elongated areas are parallel to the hot-extrusion direction. Nonetheless, the focused map of the 10Cr ODS shows a few grains elongated and parallel with the hot-extrusion direction (pointed by black arrow).

Firstly, the α→γ transformation is supposed to confer a microstructure without privileged orientation to the materials. Thus, the domain exhibiting randomly orientated grains, is assumed to constitute the part of the material that changes phase. Secondly, the elementary microstructure with α-fiber grains is characteristic of a hot-extruded microstructure. The α-fiber is a main characteristic of hotextruded material, especially for ferritic ODS steels [START_REF] Sornin | Competition between intragranular and intergranular deformation mechanisms in ODS ferritic steels during hot deformation at high strain rate[END_REF]. Hence, the elongated green grains domains present a microstructure inherited from hot-extrusion, consistent with a lack of phase transformation [START_REF] Aghamiri | Microstructure and texture evolution and ring-tensile properties of recrystallized FeCrAl ODS cladding tubes[END_REF]. These observations are strongly consistent with the untransformed ferrite evidence given by XRD measurements.

EBSD microstructures observations are in favour of the two phases coexistence at 1050°C. The solubility of chromium is different in ferrite and martensite. Besides, chromium is known to be a ferrite marker [START_REF] Atamert | Elemental partitioning and microstructural development in duplex stainless steel weld metal[END_REF]. Then, a chromium partitioning can be consistent with the two phases coexistence at 1050°C. To access the local chemical composition, the martensitic state is studied using an electron probe micro analyzer (EPMA). The EPMA maps of martensitic state are presented in Figure 6. For the 12Cr ODS, the chromium content gradient is evidenced as long strips. A chromium content gradient is also seen in the 10Cr ODS. Unlike the 12Cr ODS, the distribution of the high and low chromium areas is equiaxed. The chromium content gradient into the 12Cr ODS is more important than the one of 10Cr ODS. The chromium content deviation between the nominal composition and the enriched areas is more important for 12Cr ODS (+ 2 wt%) than for 10Cr ODS (+ 1.5 wt%) as illustrated by the chromium profiles of Figure 7. For 12Cr ODS, enriched titanium areas are aligned with the hot-extrusion direction. The titanium map of 10Cr ODS shows poor titanium segregation. Chromium-enriched areas are composed of elongated grains, like elementary microstructures. This is in agreement with the EBSD observations of the untransformed ferrite. EPMA maps present a uniform distribution of tungsten, another ferrite stabilizer. Here the tungsten does not appears segregated like chromium. This is not consistent with the work of Ohtsuka et al. [START_REF] Ohtsuka | Microstructure and hightemperature strength of high Cr ODS tempered martensitic steels[END_REF]. Probably because the added tungsten amount is lower in the present study (1 wt% instead of 2 wt% for Ohtsuka et al. [START_REF] Ohtsuka | Microstructure and hightemperature strength of high Cr ODS tempered martensitic steels[END_REF]) and the diffusion is too slow to induce a significant partitioning.

The EBSD observations are also used to estimate the untransformed ferrite fraction into the materials. The image quality (IQ) parameter from EBSD maps of the martensitic state is used to identify the two domains. IQ is the OIM software parameter to name the Kikuchi band contrast. The IQ value of the ferritic grains is greater than the one of the martensitic grains [START_REF] Kumar | In-situ observation of strain partitioning and damage development in continuously cooled carbide-free bainitic steels using micro digital image correlation[END_REF][START_REF] Nowell | Differentiating Ferrite and Martensite in Steel Microstructures Using Electron Backscatter Diffraction[END_REF] due to lower geometrically necessary dislocations. To improve the contrast between the martensite and the untransformed ferrite, the average IQ per grain is preferred [START_REF] Mujica | Microstructure and mechanical properties of laser-welded joints of TWIP and TRIP steels[END_REF]. The average IQ per grain is presented in Figure 8 for both materials. Two main domains of average IQ values are highlighted. The thresholds identified onto the diagram are applied to split the EBSD maps between martensite and ferrite as presented in Figure 9. Grains with an average IQ value under the threshold are plotted in red and assumed grains of martensite. The untransformed ferrite grains, plotted in blue, are assumed to be above the threshold. The blue areas, in Figure 9, match with the elementary microstructure of the α-fiber grains identified on the EBSD maps. Thus, the IQ threshold is in agreement with the crystallographic texture observations. This identification method offers the possibility to quantify the surface fraction of untransformed ferrite. Those surface fractions, reported in Table 6, are closed to the macroscopic dilatometric volume quantifications summarized in Table 4. The tendency is the same with less untransformed ferrite into the 10Cr ODS compared to the 12Cr ODS.

𝑼𝒏𝒕𝒓𝒂𝒏𝒔𝒇𝒐𝒓𝒎𝒆𝒅 𝒇𝒆𝒓𝒓𝒊𝒕𝒆 (%)

𝟏𝟎𝑪𝒓 ODS

36

𝟏𝟐𝑪𝒓 ODS

43

Table 6: Surface fraction of untransformed ferrite in 10Cr ODS and in 12Cr ODS using IQ threshold. For the two considered loads, 10Cr ODS total stain is more important and the time to rupture (𝑇 𝑟 ) is lower than for 12Cr ODS. At the higher load, the creep rate of the 12Cr ODS is 1.5 times more important than for 10Cr ODS. This order is reversed for the lower load. The same observation was already reported by Muroga et al. [START_REF] Muroga | Fabrication and characterization of reference 9Cr and 12Cr-ODS low activation ferritic/martensitic steels[END_REF].

Creep properties

Creep properties are usually split up into diffusive and dislocation creep contributions [START_REF] Frost | Deformation-mechanism maps: the plasticity and creep of metals and ceramics[END_REF][START_REF] Klueh | Oxide dispersionstrengthened steels: A comparison of some commercial and experimental alloys[END_REF].

Diffusive creep is mainly dependent on the number of grain boundaries and then correlated to the grain size. To compare the contribution of the diffusive creep, the grain size of both materials is calculated using the intercept method and presented in Table 6. Because of an anisotropic microstructure, grain size is estimated in both longitudinal direction (collinear to the tensile direction) and in transverse direction. Vertically average intercept length is similar for both materials. Consequently, grain size cannot explain the differences observed in creep properties. Concerning the dislocation creep, the nano-sized oxide particles constitute barriers to the moving of the dislocations. A finer nano-sized oxides dispersion increase the dislocations pinning on the nanosized oxides, decreasing the dislocation creep [START_REF] Ukai | Production and properties of nano-scale oxide dispersion strengthened (ODS) 9Cr martensitic steel claddings[END_REF]. This effect is discussed in the section dedicated to oxide particle dispersion.

Average intercept length (µm) (longitudinal) Average intercept length (µm) (transverse)

𝟏𝟎𝑪𝒓 ODS Materials of this study are compared to other ODS steel with 9 wt% of chromium [START_REF] Muroga | Fabrication and characterization of reference 9Cr and 12Cr-ODS low activation ferritic/martensitic steels[END_REF][START_REF] Kaito | In-pile creep rupture properties of ODS ferritic steel claddings[END_REF][START_REF] Sakasegawa | Creep constitutive equation of dual phase 9Cr-ODS steel[END_REF] and 12 wt% of chromium but without Ni addition [START_REF] Muroga | Fabrication and characterization of reference 9Cr and 12Cr-ODS low activation ferritic/martensitic steels[END_REF][START_REF] Kaito | In-pile creep rupture properties of ODS ferritic steel claddings[END_REF]. Both ODS steels of this study are close to the mean behavior of literature data prospected. The dualphase 9Cr of Sakasegawa et al. [START_REF] Sakasegawa | Creep constitutive equation of dual phase 9Cr-ODS steel[END_REF] seems to present better properties. The authors attribute the main behavior difference between the dual and single-phase to the presence of residual ferrite. The 10Cr ODS of this study contains some untransformed ferrite and its creep behavior is between the 9Cr dual phase and the 9Cr single phase. This 9Cr dual-phase contains a volume fraction of about 25% of ferrite. This quantity is only half the amount of untransformed ferrite in 10Cr ODS or 12Cr ODS of the present study. A high volume fraction of untransformed ferrite is not a warranty of very high creep properties.

Oxide particles dispersion

The evaluation of the oxide particles size and number density is conducted for both materials using SAXS method. As referred to in the creep section, dislocation creep properties depend on the oxide particle sizes.

Figure 11 presents the scattering curves of both grades acquired in the same experimental conditions. The shape of the scattering curve looks very similar for the two materials. As illustrated by Figure 11, the linear asymptote observed at lower q is modified between 0.4 𝑛𝑚 -1 and 1.1 𝑛𝑚 -1 which is known to be the contribution of the nano-sized objects. Therefore, nano-sized oxides contributions are very similar for both materials. Using the scattering data, Monte Carlo analyses are conducted. McSAS software is used to obtain the fit between the Monte Carlo analysis and the experimental data. The mean radius and the number density are calculated and referred to in Table 9.

𝒓 𝒎𝒆𝒂𝒏 (𝒏𝒎) 𝑵 (𝟏𝟎 𝟐𝟑 𝒎 -𝟑 ) The mean radius and number density are very close to each other in both grades. Indeed, the two ODS grades follow the same manufacturing route. These values are in the range of common values referred to in the literature for such addition of Y203 [START_REF] Tanno | Evaluation of mechanical properties and nano-meso structures of 9-11%Cr ODS steels[END_REF][START_REF] Zhong | Study of the thermal stability of nanoparticle distributions in an oxide dispersion strengthened (ODS) ferritic alloys[END_REF]. The SAXS measurements do not emphasize any double distribution of nano-sized oxides.

As mentioned in creep properties section, nano-sized oxides play a crucial role on dislocation creep. SAXS measurements disclose a similar size and density for both materials. Consequently, the effects of the nano-sized oxides on the dislocation creep is probably equivalent for the two materials.

Only the distribution and fraction of untransformed ferrite are different in the microstructure of both materials.

Conclusions

The present study investigates the presence of untransformed ferrite at high temperatures into two ODS steels containing 10 wt% and 12 wt% of chromium respectively. These steels were elaborated with the same manufacturing route. XRD at 1050°C was used to evidence the existence of untransformed ferrite above the α→γ transformation temperature. This untransformed ferrite is still observed even after twenty minutes at 1050°C. The microstructure of both materials is studied using quenched samples to distinguish untransformed ferrite from martensite. The martensite corresponds to the fraction of materials that changes phase. Each grades is constituted of two different elementary microstructures. One of the two presents characteristics of a ferrite inherited from the hot-extrusion process. This elementary microstructure is superimposed with chromium-enriched areas, consistent with partitioning at high temperatures. The second elementary microstructure, consistent with a martensitic structure, is assumed to be inherited from phase transformation. In addition, the IQ analysis of EBSD maps also reveals these two elementary microstructures. Both materials are tested in creep conditions. Results illustrate a slightly better behavior of the 12Cr ODS compared to 10Cr ODS. Regarding grain size and nano-size oxides dispersion, assumed to monitor diffusive and dislocation creep behavior respectively, these differences in creep properties cannot be explained. The only microstructural difference between both materials seems to be the distribution and fraction of untransformed ferrite.

Further creep investigations are necessary to confirm the effect of the untransformed ferrite on the mechanical properties.

Figure 1 :

 1 Figure 1: Calculated Fe-Cr binary diagram corresponding to the chemical composition of the experimental materials.

Figure 2 :

 2 Figure 2: XRD at 25°C of the initial ferritic state, 1050°C and 25°C after a slow cooling for (a) 10Cr ODS and (b) 12CrODS.

Figure 3 :

 3 Figure 3: DSC curves for a heating and cooling rate of 10°C/min for 10Cr ODS and 12Cr ODS.

Figure 4 :

 4 Figure 4: Thermal expansion measurements during heating at 1°C/s from 25°C to 1100°C following by quenching at 10°C/s until 25°C for (a) 10Cr ODS and (b) 12Cr ODS.

Figure 5 :

 5 Figure 5: Inverse pole figure maps <uvw> direction parallel to the extrusion direction of the martensitic state of the (a) 10Cr ODS and (b) 12Cr ODS.

Figure 6 :

 6 Figure 6: Quantitative EPMA mapping of the martensitic state of the (a) 10Cr ODS and (b) 12Cr ODS for Cr, Ti and W chemical elements.

Figure 7 :

 7 Figure 7: Chromium content evolution along lines marked on Figure 6-Cr of the (a) 10Cr ODS and (b) 12Cr ODS, the dotted line represents the measured mean composition fromTable 1.

Figure 8 :

 8 Figure 8: Average IQ distribution of the martensitic state of (a) 10Cr ODS and (b) 12Cr ODS.

Figure 9 :

 9 Figure 9: Inverse pole figure maps <uvw> direction parallel to the extrusion direction of (a) 10Cr ODS and (c) 12Cr ODS in martensitic state and IQ threshold map of (b) 10Cr ODS and (d) 12Cr ODS, martensite area are in red and untransformed ferrite area are in blue.

Figure 10

 10 Figure 10 presents a Larson Miller graph taking into account several ODS steels from the literature.Materials of this study are compared to other ODS steel with 9 wt% of chromium[START_REF] Muroga | Fabrication and characterization of reference 9Cr and 12Cr-ODS low activation ferritic/martensitic steels[END_REF][START_REF] Kaito | In-pile creep rupture properties of ODS ferritic steel claddings[END_REF][START_REF] Sakasegawa | Creep constitutive equation of dual phase 9Cr-ODS steel[END_REF] and 12 wt% of chromium but without Ni addition[START_REF] Muroga | Fabrication and characterization of reference 9Cr and 12Cr-ODS low activation ferritic/martensitic steels[END_REF][START_REF] Kaito | In-pile creep rupture properties of ODS ferritic steel claddings[END_REF].

Figure 10 :

 10 Figure 10: Larson Miller graph of the two ODS steel 10Cr ODS and 12Cr ODS compared with literature references.Arrow denotes ongoing test.

Figure 11 :

 11 Figure 11: Scattering curve of the 10Cr ODS and 12 Cr ODS obtained at 4.766 keV.

  Table1shows the chemical composition of the bulk materials after hot-extrusion. The manufacturing route is the same for both materials 10Cr ODS and 12Cr ODS.

		10Cr ODS (wt%)	12Cr ODS (wt%)
		(Ref. CEA P42)	(Ref. CEA P43)
	Cr	9.7	11.5
	W	0.88	0.87
	Mn	0.26	0.26
	Mo	0.45	0.44
	Ni	0.31	0.3
	Ti	0.22	0.25
	C	0.11	0.12
	O	0.12	0.1
	Al	0.01	0.01
	Si	0.06	0.07
	N	0.004	0.009
	Y	0.21	0.21

Table 1 :

 1 Chemical composition of the bulk produced materials. Ti and Y are measured by plasma emission spectrometry. C, O and N are respectively measured by combustion infrared absorption, reducing melting infraredabsorption and reducing melting thermal conductivity. The other elements are measured by optical emission spectroscopy.

Table 2 :

 2

Temperature of the α→γ transformation for a 1°C/s heating rate with standard deviation in the brackets.

Table 3 :

 3 Calculated thermal expansion coefficient of 10Cr ODS and 12Cr ODS in a fully ferritic state at low temperature and in the austenitic domain at high-temperature with standard deviation in the brackets.Therefore, eq. 4 gives the volume fraction of untransformed ferrite, reported in Table4.

		𝒖𝒏𝒕𝒓𝒂𝒏𝒔𝒇𝒐𝒓𝒎𝒆𝒅 𝒇𝒆𝒓𝒓𝒊𝒕𝒆 (%)
	𝟏𝟎𝑪𝒓 ODS	41.2 ± 1.2
	𝟏𝟐𝑪𝒓 ODS	48.8 ± 3.7

Table 4 :

 4 Volume fraction of untransformed ferrite in the 10Cr ODS and in the 12Cr ODS calculated from eq. 4 using dilatometric data.

Table

Table 7

 7 presents the results of the thermal creep tests of the two materials. The tests are performed at 650°C under stresses of 210 MPa and 180 MPa.

		𝝈	𝑻 𝒓	𝑪𝒓𝒆𝒆𝒑 𝒓𝒂𝒕𝒆	𝑻𝒐𝒕𝒂𝒍 𝒔𝒕𝒓𝒂𝒊𝒏
		(𝑴𝑷𝒂)	(𝒉)	(𝟏𝟎 -𝟔 . 𝒉 -𝟏 )	(%)
		210	206	26.3	5.02
	𝟏𝟎𝑪𝒓 ODS
		180	511	6.31	3.32
	𝟏𝟐 𝑪𝒓 ODS	210	286	42.4	4.61
		180	1275	2.75	2.74

Table 7 :

 7 Thermal creep properties (Time to rupture, secondary creep rate and total strain) at 650°C of 10Cr ODS and 12Cr ODS for two different stress.

Table 8 :

 8

Grain size of the creep-tested microstructures using the intercept method, in the longitudinal direction and the transverse direction.

Table 9 :

 9 Mean radius obtained from the number distribution and number density of particles in 10Cr ODS and 12Cr ODS.

Acknowledgements

We are thankful to Thomas Guilbert for dilatometric and DSC measurements. The authors are also grateful to Véronique Lezaud-Chaillioux and Florent Lefebvre for supporting the creep tests.

We acknowledge the Paul Scherrer Institut, Villigen, Switzerland for provision of synchrotron radiation beamtime at beamline cSAXS of the SLS and would like to thank Andreas Menzel for his assistance during the experiments.