Three steels were exposed in carburizing sodium at 600 and 650 degrees C. The kinetics and extent of carburization were characterized. Numerical simulations using the coupled thermodynamic-kinetic modeling software DICTRA were performed. It was proposed that the observed carbon diffusion profiles were induced by the combined diffusion of carbon in the grains and at grain boundaries coupled with the slow formation of carbides. The blocking effect of carbides on the carbon diffusion was observed to evolve as a function of time and microstructure. Acceptable agreement between experimental and simulated intragranular carbon profiles was achieved by optimizing the labyrinth factor and phases.

One of the hypothetical accidents studied in the field of the safety studies of Pressurized light Water Reactors (PWR) is the Loss-Of-Coolant Accident (LOCA). In this scenario, zirconium alloy fuel claddings could undergo a high level of oxidation at high temperature in a steam environment. Cladding tubes constitute the first confinement barrier of radioelements and then it is essential that they keep a certain level of ductility after quenching to ensure their integrity. These properties are directly related to the growth kinetics of both the oxide and the sub-oxide αZr(O) layers and especially to the oxygen diffusion profiles in the residual prior-β layer after HT oxidation and final water quenching. \r\nThis study was focused on the influence of a pre-annealing thermal treatment performed on first generation of 6-8 µm thick Cr-coated zirconium based specimens and on their further HT oxidation behavior. \r\nThe effect of pre-annealing temperature on the structure and morphology of the Chromium coating microstructure was investigated using scanning electron microscopy. During the annealing treatment, the chromium coating experienced recrystallisation and the initial columnar grains morphology became more equiaxed. After oxidation at HT, the weight gains of pre-annealed Cr-coated specimen were generally lower than the non-annealed ones. Glow discharge mass spectrometry, Electron Probe Micro Analysis and Raman spectroscopy measurements have confirmed that the thickness of ZrO2 and αZr(O) phases were significantly reduced for some of the pre-annealed samples, indicating that the grain size and morphological evolutions of the chromium coating upon the pre-annealing thermal treatment may influence its further HT oxidation behavior.

In-situ time-resolved Synchrotron X-ray diffraction analyses were performed on zirconium alloy (Zircaloy-4) sheet samples, during their heating, isothermal oxidation at 700, 800 and 900 degrees C under a flowing mixture of He and O-2 and cooling. The oxide growth and the evolution of the oxide structure as a function of time and temperature were studied with suitable time resolution. Oxide layer thicknesses of approximately 10 mu m were formed during the experiments. The incident X-rays penetrated the whole oxide thickness. The samples were examined after the experiments by field emission gun scanning electron microscopy, electron backscatter diffraction and electron-probe microanalysis. The results showed that the oxide contains a mixture of monoclinic and tetragonal zirconia evolving during heating, oxidation and cooling. The average volume fraction of tetragonal zirconia decreases during oxidation. This fraction is larger at 900 degrees C than at 700 and 800 degrees C. For oxide layers thinner than approximately 5 mu m, this fraction is larger at 800 degrees C than at 700 degrees C, but it is rather equivalent for both temperatures when the oxide thickness ranges between 5 and 8 mu m. Some of the tetragonal zirconia crystals transforms into the monoclinic phase during cooling after oxidation. This fraction of transformed tetragonal zirconia is larger after oxidation at 900 degrees C than after oxidation at 700 and 800 degrees C. It is suggested that these evolutions of the oxide crystallographic structure are related to micro-stresses and to temperature dependences of the critical size of zirconia crystals below which tetragonal zirconia is stabilized.

Oxide dispersion-strengthened ferritic stainless steels are foreseen as fuel cladding tube materials for the new generation of sodium fast nuclear reactors. Those materials, which exhibit remarkable creep properties at high temperature, are reinforced by a dense precipitation of nanometric oxides. This precipitation is obtained by mechanical alloying of a powder and subsequent consolidation. Before consolidation, to obtain a fully dense material, the powder is vacuumed to outgas trapped gases and species adsorbed at the surface of the powder particles. This operation is commonly done at moderate to high temperature to evacuate as much as possible volatile species. This paper focuses on the influence of outgassing conditions on some properties of the further consolidated materials. Chemical composition and microstructural characterization of different materials obtained from various outgassing cycles are compared. Finally, impact toughness of those materials is evaluated by using Charpy testing. This study shows a significant influence of the outgassing conditions on the mechanical properties of the consolidated material. However, microstructure and oxygen contents seem poorly impacted by the various outgassing conditions.

To increase cycle length and/or fuel burnup, several theoretical and experimental studies have been performed at CEA. Among them, prospective neutronic calculations have shown that the addition of a few weight percents of erbium into the cladding materials could be a promising alternative to the introduction of the neutronic poison directly into the nuclear fuel pellets. Thus, fabrication of homogeneous Zr-Er alloys has been assessed, at least up to 10 wt. % of erbium and, based on the as-received mechanical properties, an optimum erbium concentration ranging from 3 to 6 wt. % has been derived. However, because of the high-oxygen thermodynamic affinity of erbium, thermal treatments have to be controlled during the fabrication route to limit Er2O3 precipitation and coarsening, which may have detrimental effects on the ductility/toughness of Zr-Er alloys. In parallel, to get more fundamental insights into the underlying phase diagrams, thermodynamic studies have been devoted to experimental assessment and modeling of the Zr-Er-(H-O) system. Because of the detrimental influence of erbium on the corrosion resistance, a three-layer sandwich clad prototype has been developed using corrosion-resistant inner/outer Zr-1Nb layers to protect the intermediate Zr-Er layer from direct water exposure. Compared to a reference Zr-1Nb(O) alloy that has been subjected to the same fabrication route, the three-layer clad prototype shows limited decrease in ductility because of pre-hydriding or after high-temperature steam oxidation e.g., in the case of a loss-of-coolant accident). Moreover, the studies performed so far have shown a spectacular hydride trapping capacity of the intermediate Zr-Er layer both for hydrogen coming from nominal outer corrosion or because of massive secondary hydriding in case of the direct access of water to the Zr-Er intermediate layer. Using μ-ERDA (elastic recoil detection analysis) measurements, detailed studies of the hydrogen spatial redistribution upon thermal cycling has been done. A simple model has been successfully used to characterize the cooling rate influence on the through-wall clad thickness partitioning of hydrogen/hydrides between the three layers, after cooling from a temperature corresponding to full dissolution of hydrides

The aluminum alloy AlFeNi used as fuel cladding for the Jules Horowitz Reactor (JHR) may undergo corrosion in the reactor environment. In order to qualify the corrosion behavior of the fuel elements of the JHR in accidental conditions, several specimens of AlFeNi have been corroded at 250°C for different durations (9–34days) in distilled water at various pH (4.9, 5.2 and 5.6) chosen to simulate that currently considered for the JHR. On all specimens, the only crystalline corrosion product formed is boehmite (AlOOH). The corrosion film is composed of three oxide layers which show through thickness chemical composition variations. The iron–nickel precipitates pre-existing in the metal matrix are present in the inner and intermediate oxide layers though oxidized. For long corrosion times, some of the iron and nickel particles are released in the water and some precipitation is observed at the surface of the oxide layer. The effect of surface finish (as received or polished) and thermal treatment (annealed and not annealed) on the oxide growth rate has also been investigated. For durations over 25days, pH=5.6 appears to be more favorable than pH=5.2 and 4.9 in terms of oxide thickness and weight gain limitation. This effect of pH is however reduced on unpolished specimens. The effect of surface finish on the corrosion behavior as measured by optical microscopy appears to be strong, especially for pH=4.9 where polished samples exhibited an accelerated evolution of the oxide thickness and of the mass gain. This could be due to the combined effect of a strong acid solution (pH=4.9) and of the local microstructural changes formed at the interface through polishing. The effect of thermal treatment on the behavior of unpolished AlFeNi specimens during corrosion tests in the conditions investigated was found to be small. In this study, microstructural and chemical analyses were performed on the corroded specimens in order to get a better understanding of the corrosion kinetics. The crystallographic nature of the boehmite layers investigated by X-ray diffraction is unaffected by the pH of the solution. Iron precipitates were identified on the oxide surface beyond 34days of corrosion by Environmental Scanning Electron Microscope (ESEM). Finally, Electron Probe Micro-Analysis (EPMA) was used to determine the chemical composition of the metal matrix and of the different oxide layers and precipitates versus the pH of the solution.

The effects of the thermal ageing at 400 °C, 500 °C and 600 °C during 5000 h on the mechanical properties of a 18%Cr ODS ferritic steel are investigated. A hardening effect is observed after ageing at 400 °C and 500 °C, probably due to the presence of chromium rich α′ particles as suggested by the literature. The impact resistance and the ductility of the material are strongly lowered by the ageing at 600 °C. This embrittlement is characterized on the fracture surfaces by the presence of cleavage facets on the whole range of testing temperatures. The intermetallic σ phase is found to be responsible for the occurrence of cleavage fracture on the material aged at 600 °C, and thus for the significant embrittlement of this material. M23C6 carbides are also observed before and after thermal ageing. The lattice parameters of the σ phase and the M23C6 carbides observed in this 18%Cr ODS steel aged at 600 °C during 5000 h are measured.

Oxide dispersion strengthened steels are new generation alloys that are usually processed by hot isostatic pressing (HIP). In this study, spark plasma sintering (SPS) was studied as an alternative consolidation technique. The influence of the processing parameters on the microstructure was quantified. The homogeneity of the SPSed materials was characterised by electron microprobe and microhardness. A combination of limited grain growth and minimised porosity can be achieved on semi-industrial compact. Excellent tensile properties were obtained compared to the literature.

Oxide Dispersion Strengthened (ODS) steels are promising candidate materials for fission and fusion applications thanks to their improved properties related to both their fine grained microstructure and high density of Y-Ti-O nanoscale clusters (NCs). The Fe-14Cr-1 W-0.3Ti-0.3Y(2)O(3) ODS ferritic steel was produced by powder metallurgy: Iron-base gas atomized powders were mechanically alloyed with 0.3% Y2O3 particles in an attritor. Then, the ODS powders were encapsulated in a soft steel can, consolidated by hot extrusion and cold rolled under the shape of tube cladding.The present work investigates the evolution of the chemical composition and the microstructure after each stage of the fabrication route (i.e. mechanical alloying, extrusion and cold rolling). Chemical analysis indicates a significant increase of the carbon content and a moderate increase of oxygen and nitrogen after mechanical alloying compared to initial atomized powders. After extrusion, the measured oxygen content corresponds mainly to the oxygen coming from yttria addition during MA process. In addition, electron microprobe analyses are performed after hot extrusion to determine the concentration and the distribution of the constitutive elements (Cr, Ti, W, Y, O). The microstructure was investigated by transmission electron microscopy (TEM) and small angle neutron scattering (SANS) in order to characterize the size distribution of Y-Ti-O particles. TEM results reveal a fine microstructure (average grain size of 600 nm in the transverse direction) including Y-Ti-O NCs with a mean diameter close to 3 nm after extrusion. A slight coarsening of Y-Ti-O NCs is evidenced by SANS after cold rolling and heat treatments. (C) 2011 Elsevier B.V. All rights reserved.

During the last fifteen years, CEA has acquired much experience in the control of the microstructure and the mechanical properties of ODS alloys for nuclear applications. Each major step of the production process has been studied to get the best compromise for the fabrication route of ODS materials. From this scientific background, two new Fe-13/18CrWTi ferritic ODS alloys have been designed to meet the needs of the fusion or GEN-IV programs. These new materials have been investigated at a semi- industrial scale with different industrial partners and consolidated as small plates.The aim of this paper is to present the recent CEA developments on ODS materials, and to show the first results obtained on the Fe-18Cr1WTi new ferritic ODS alloy. The fabrication route for these new materials is presented, along with the measured mechanical properties and the preliminary microstructure characterizations. These new materials look promising for nuclear applications and are considered by CEA as reference materials for the development of new ODS alloys. (C) 2009 Elsevier B.V. All rights reserved.

A new ODS composition (Fe-14Cr-2W-0.3Ti-0.3Y2O3) developed in the ExtreMat integrated Project has been produced by mechanical alloying techniques and consolidated by hot extrusion. This study summarizes some results of characterization and cold workability tests carried out at CEA and EPFL. It appears that the microstructure is fine and uniform after hot extrusion. According to microprobe analysis, solute elements are homogenously distributed in the matrix. However, the relatively high hardness level measured after hot extrusion and heat treatment may be detrimental in case of additional cold processing which is required to produce final shape like thin plates or cladding tubes. An assessment of the cold workability and the effect of the degree of cold work by rolling on recrystallisation temperature are addressed here. It is found that this material can be successfully cold rolled with a high degree of cold work (up to 60% of thickness reduction) without any damage. According to optical micrographs and Differential Scanning Calorimetry (DSC) measurements, it seems that the recrystallisation temperature remains always very high (above 1400°C) even though cold work level increases (up to 66% of thickness reduction). However, the hardness values begin to decrease for heat treatment temperatures above 1200°C for hot worked conditions and below 1000°C for cold worked conditions, respectively.

This paper deals with the study of oxidation kinetics and the identification of oxygen diffusion coefficients of low-tin Zy-4 alloy at intermediate (973K⩽T⩽1123K) and high temperatures (T⩾1373K). Two different cases were considered: dissolution of a pre-existing oxide layer in the temperature range 973K⩽T⩽1123K and oxidation at T⩾1373K. The results are the following ones: in the temperature range 973–1123K, the oxygen diffusion coefficient in αZr phase can be expressed as Dα=6.798 exp(−217.99kJ/RT)cm2/s. In the temperature range 1373–1523K, the oxygen diffusion coefficients in αZr, βZr and ZrO2, were determined using an ‘inverse identification method’ from experimental high temperature oxidation data (i.e., ZrO2, and αZr(O) layer thickness measurements); they can be expressed as follows: Dα=1.543 exp(−201.55kJ/ RT) cm2/s, Dβ=0.0068 exp(−102.62kJ/ RT) cm2/s and DZrO2=0.115exp(−143.64kJ/RT)cm2/s. Finally an oxygen diffusion coefficient in αZr in the temperature range 973K⩽T⩽1523K was determined, by combining the whole set of results: Dα=4.604exp(−214.44kJ/RT)cm2/s. In order to check these calculated diffusion coefficients, oxygen concentration profiles were determined by Electron Probe MicroAnalysis (EPMA) in pre-oxidized low-tin Zy4 alloys annealed under vacuum at three different temperatures 973, 1073 and 1123K for different times, and compared to the calculated profiles. At last, in the framework of this study, it appeared also necessary to reassess the Zr–O binary phase diagram in order to take into account the existence of a composition range in the two zirconia phases, αZrO2 and βZrO2.

Hydrogen content and its distribution in in-core materials of nuclear plants are known to have a strong influence on their behaviour, especially on their mechanical properties but also on their corrosion resistance. This point has to be largely investigated in the case of the nuclear fuel cladding (Zr based alloys) of pressurized water reactors (PWR).Two situations have been considered here, with regards to the hydrogen content and its spatial distribution within the thickness of the tubes:(1) Irradiated fuel cladding tubes after a nominal period under working conditions in a PWR core.(2) Non-irradiated fuel cladding previously exposed to conditions representative of an hypothetical "loss of coolant accident" scenario (LOCA).As far as micrometric distributions of H were required, mu-ERDA has been performed at the nuclear microprobe of the Pierre Sue Laboratory. This facility is fitted with two beam lines. In the first one, used for non-active sample analysis, the mu-ERDA configuration has been improved to reduce the limits of detection and the reliability of the results. The second one offers the unique feature of being dedicated to radioactive samples. We will present the nuclear microprobe and emphasize on the mu-ERDA configuration of the two beam lines. We will illustrate the performance of the setup by describing the results obtained for Zircaloy-4 cladding both on non-irradiated and irradiated samples. (C) 2008 Elsevier B.V. All rights reserved.

L'etude ici illustree s'interesse au comportement thermo - metallurgique - mecanique des alliages de Zr des tubes de gainage du combustible des Reacteurs nucleaires a Eau Pressurisee (REP), lors de transitoires a hautes temperatures (HT) en ambiance vapeur d'eau, simulant des conditions hypothetiques accidentelles (dites APRP). Il apparait interessant d'approfondir la connaissance des phenomenes metallurgiques et thermomecaniques mis en jeu lors de tels transitoires, en particulier vis-a-vis du comportement mecanique residuel « post oxydation trempe ». Differentes teneurs en oxygene ont ete incorporees dans l'alliage etudie, pour simuler la diffusion de cet element dans le substrat metallique qui intervient lors de l'oxydation HT. Des caracterisations microstructurales par EBSD, MET, MEB... et des caracterisations mecaniques par essai de traction, mesures de constantes physiques... ont permis de decrire la microstructure et le comportement mecanique de la phase residuelle la plus ductile – phase dite « ex-beta » - resultant de l'oxydation HT des gaines en zircaloy-4.

The design of an Accelerator Driven System (ADS) requires that the "window", which separates the proton accelerator from the spallation target, be able to withstand very severe irradiation conditions. Fe-9/12Cr martensitic steels are good candidates for the window material due to their intrinsic stability under neutron irradiation, but the influence of iron spallation elements on their behaviour is not known. To elucidate the effects of the spallation elements titanium, phosphorus and sulphur on the behaviour of martensitic steels, it was decided to obtain different castings of 9Cr 1Mo steels doped with these elements. The aim of this paper is to present the data obtained on the physical metallurgy of these steels and to show the possible methods of obtaining titanium, phosphorus and sulphur in solid solution for subsequent study of the evolution of the microstructure and mechanical properties.

During a typical LOCA transient, the fuel cladding tubes are subjected to high temperature oxidation and finally quenched because of the reflooding of the core. The global oxygen content and its distribution affect strongly the residual ductility/toughness of the cladding, which microstructure can be described in terms of zirconia phase, an oxygen enriched {alpha} phase (*) and an 'ex-{beta}' (*) phase. The main objective of this communication is to summarize some recent results concerning the mechanical behaviour at Room Temperature (R.T.) of Framatome low-tin Zy-4 and M5{sup TM} (Zr-NbO) alloys, after single face oxidation at 1100 deg. C in steam and quenching. The residual ductility/toughness properties at R.T. have been determined using impact, bending and compression tests. A metallurgical study has been made to support these mechanical results. It is observed that, after oxidations giving weight gains ranging from {approx}4 mg/cm{sup 2} ({approx}100s) up to {approx}23 mg/cm{sup 2} ({approx}3600s) and according to the measured residual ductility/toughness properties at R.T., the M5{sup TM} and Zy-4 alloys show comparable properties. Also, it is worth noticing that, even for the Zr-1%Nb (M5{sup TM}) alloy, no hydrogen pick-up is observed after oxidation at 1100 deg. C. All these results are different from some previous data. Besides, it was found that there is a residual ductility/toughness for both alloys even at the maximum oxidation. To get a better insight of the microstructural origins of the mechanical behaviors observed, microprobe analysis and metallographic studies have been also performed. In particular, it is shown that systematic diffusion of '{beta}-stabilizing' alloying elements (Fe, Cr, Nb) has occurred ahead of the oxygen stabilized {alpha}-phase front, within the residual {beta}-phase. Finally, complementary fractographic observations have been conducted on failed Charpy impact samples. (author)

Les proprietes d'usage (corrosion, proprietes mecaniques) des alliages de zirconium-niobium dependent etroitement des microstructures obtenues a l'etat final de fabrication. De fait, l'obtention a l'etat final d'une dispersion fine et homogene de phases precipitees d'equilibre β Nb dans la matrice a ameliore le comportement en service de ces alliages. Ce travail concerne donc l'etude de la cinetique de precipitation du β Nb en phase a (a 570°C) a partir de structures similaires a celles rencontrees a l'issue de traitements thermiques de fabrication ou de soudage (ZAT). L'utilisation de la calorimetrie et de la mesure du Pouvoir ThermoElectrique, a permis de suivre et de quantifier cette cinetique. Enfin, l'etude des mecanismes de precipitation du β Nb , au cours des vieillissements thermiques, a ete menee a partir d'une structure contenant initialement des phases β Zr metastables et ceci, en particulier grâce a l'utilisation de la dissolution anodique, protocole experimental original quant a son application aux alliages de Zr [1 ].

Mo and NiMo catalysts supported on stabilized zirconia and on alumina have been prepared and characterized by X-ray photoelectron spectroscopy (XPS) and high resolution electron microscopy (HREM). The catalytic properties have been evaluated in hydrodesulfurization and hydrogenation reactions. By comparison to Al2O3 supported samples, the higher activities observed for the zirconia supported catalysts have been related to the morphology of the sulfide phase. It is suggested that on zirconia the promoted sites are more numerous, easily formed (type II 'NiMoS' phase) and probably more accessible to the reactant molecules.

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Zirconia-alumina powders have been prepared by synthesis in molten salt, and their structural and textural properties have been characterized by X-Ray diffraction, scanning electron microscopy and N2 adsorption. The effect of the support chemical composition and of the temperature of preparation on the textural properties has been investigated.For an equimolar ratio between ZrO2 and Al2O3, a 3 mol. % amount of Y2O3 with respect to each oxide, and a 570-degrees-C temperature of preparation, it was found that: (i) no mixed phase was formed between zirconia and alumina, (ii) aluminium was present as crystallized aluminium hydroxide which is transformed into amorphous alumina after calcination at 600-degrees-C, (iii) the introduction of this amount of yttrium oxide allowed the structural stabilization of zirconia under its tetragonal structure, (iv) the solid presented a high surface area (220 m2/g) and a large porosity (pore radii in the range 2-10 nm, pore volume of 0.35 cm3/g), (V) these textural properties were kept for heat treatment up to 600-degrees-C.