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The oxidation behavior of Si- and/or Mn-Ni-25Cr alloys at 1050 °C in air was studied to provide new insights on the effect of minor additions on chromia-scale growth. Four nickel alloys with and without Si or Mn were prepared. Thermogravimetry and post-oxidation characterizations evidenced that manganese increased, whereas silicon addition decreased chromia growth rate. On the contrary, addition of both Si and Mn to the Ni-25Cr alloy led to an unchanged oxidation rate in comparison with the unalloyed Ni-25Cr material. Oxide scales were characterized by photoelectrochemistry (PEC) to evaluate the role of the additions on the point defects nature. The n and p contributions of chromia forming in air were not changed by Si. On the opposite, Mn modified the scale conducting properties since only n contributions were observed for both chromia and Mn1+xCr2−xO4 spinel oxide with Mn-containing samples. In the latter case, the PEC contribution at ~ 3.2–3.4 eV was allocated to the signature of the Mn1+xCr2−xO4 spinel oxide. These data suggested that manganese enhances and silicon limits grain boundary diffusion in chromia scale as reactive element does.

A test rig consisting of a quartz tube placed in a horizontal furnace made of five independently regulated heating zones allowed the evaluation of the metal dusting attack of a FeNiCr alloy. Fifteen samples were exposed in a temperature gradient from 504° to 704°C, 1.1 bar, to a highly carburizing atmosphere for 1500 h. Below 561 °C the high density of pits led to uniform corrosion whereas localized pits developed at higher temperatures. In depth pit growth rate raised to a maximum at 632 °C, then decreased showing that it depends both on temperature and on the carbon activity gradient.

International audience; Oxidation of chromia-forming alloys (pure chromium, nickel–chromium alloy and ferritic stainless steel alloy) at temperature ranging from 800 to 1050 °C was conducted on samples with or without gold markers applied prior to testing. Gold is often used as a marker in order to identify oxide growth direction. Its localization inside the oxide scale can demonstrate an inward or an outward growth. It is found that gold particles can be found either at the internal metal/oxide interface or inside the scale or again at the external oxide/gas interface on the same sample (on NiCr and stainless steel alloys). For pure chromium oxidation, gold is found inside the chromia scale, at the equiaxed-columnar interface, as expected from ASTAR-TEM characterization. However, photoelectrochemical characterization has shown that gold can modify the semiconducting chromia properties. Finally, gold is not inert in these several conditions and should not be considered in these present cases, but also more generally in most cases, as an appropriate marker. On the contrary, ASTAR maps may permit to access the oxide growth direction without interfering the studied system.

International audience; The aim of this work is to show the influence of the gaseous environment on the ferritic Fe-Cr-Si model alloy oxidation during 70 h, at 900 °C and 950 °C. Two different atmospheres have been used: air or nitrogen containing 5 Vol.% hydrogen (N 2-5%H 2). After air oxidation, a non-adherent chromia scale formed. In the N 2-5%H 2 gaseous environment, it clearly appears that silicon segregation near the internal scale-metal interface is favoured. In this low oxygen containing gas, adherent chromia and silica scales have been formed. Silica subscale associated to an adherent chromia scale obtained in low oxygen conditions are a good protection barrier against carburisation.

To clarify the influence of manganese on the oxidation behavior of Ni–Cr alloys, high resolution transmission electron microscopy was used to examine the microstructure and chemistry of oxide scales forming at 1050 °C on Ni–25Cr–xMn (x = 0.5, 1 and 1.5% wt) in 200 Pa O2. The scales consisted of two layers of chromia and one layer of spinel Mn1+xCr2-xO4. The internal layer exhibited an equiaxed grain structure, whereas columnar grains developed in the outer layer. The results demonstrated that the spinel composition at the chromia-spinel interface was similar regardless of the manganese concentration.

Pure chromium oxidized at 900 °C at low oxygen partial pressure (10−12 atm) gives duplex Cr2O3 scale with an internal part made of equiaxed grains and exhibiting an n-type conduction, and an external part made of columnar grains and exhibiting a p-type conduction. Spalled regions occurring during cooling have been studied with photoelectrochemical techniques at a microscale. New information in the form of a specific image (structural quality image) could be obtained and revealed a level of structural defect density in the internal chromia subscale higher than that measured in the non-spalled region. The results complement the spallation scenario proposed in part I of this work.

Chromia forming Alloy 718 and AD730™ have been submitted to isothermal long-term air exposure up to 10 000 h at 700 °C. The resulting very low specific mass gain led to consider two alternatives to determine the oxidation kinetics. Measurements of the external layer thickness and of the intergranular oxide amount were carried out on SEM micrographs evidencing the intergranular oxidation contribute around 30% to the total specific mass change. AD730™ with a thicker external scale and a higher amount of intergranular oxide showed a twofold faster oxidation rate than Alloy718. The results demonstrated the need of even longer tests.

The possibility to tune the semiconducting properties of chromia scales thermally grown on pure chromium as a function of temperature and oxygen partial pressure p (O 2 ) has been demonstrated in the present paper. While at 800 °C and a p (O 2 ) of 10 −14 atm, a single n -chromia is observed, at 900 °C and a p (O 2 ) of 10 −12 atm, a duplex n - and p -chromia is obtained. Between these two situations, a stoichiometric chromia exhibiting insulating properties could be identified at 850 °C and a p (O 2 ) of 10 −13 atm. In every case, the morphology of the oxide scale is duplex and seems to depend only on the growth direction: equiaxis grains for an inward (anionic) growth and columnar grains for an outward (cationic) growth. The nature of point defects, linked to the n or p semiconducting character, governs the oxide growth but surprisingly, does not seem to have any influence on the oxide scale morphology. Finally, the control of the growth of an insulating stoichiometric chromia layer should permit to optimize its protective character.

To provide protection against corrosion at high temperatures, silicide diffusion coatings were developed for the V-4Cr-4Ti alloy, which can be used as the fuel cladding in next-generation sodium-cooled fast breeder reactors. The multilayered coatings were prepared by halide-activated pack cementation using MgF 2 as the transport agent and pure silicon (high activity) as the master alloy. Coated pure vanadium and coated V-4Cr-4Ti alloy were studied and compared as substrates. In both cases, the growth of the silicide layers (V 3 Si, V 5 Si 3 , V 6 Si 5 and VSi 2 ) was controlled exclusively by solid-state diffusion, and the growth kinetics followed a parabolic law. Wagner's analysis was adopted to calculate the integrated diffusion coefficients for all silicides. The estimated values of the integrated diffusion coefficients range from approximately 10 −9 to 10 −13 cm 2 s −1 . Then, a diffusion-based numerical approach was used to evaluate the growth and consumption of the layers when the coated substrates were exposed at critical temperatures. The estimated lifetimes of the upper VSi 2 layer were 400 h and 280 h for pure vanadium and the V-4Cr-4Ti alloy, respectively. The result from the numeric simulation was in good agreement with the layer thicknesses measured after aging the coated samples at 1150 °C under vacuum.

The energetic stability and oxidation behaviour of quaternary chromium silicides mainly depends on their chromium content. In this work, the pseudolamellar crystal structures of six representatives of the X3T2(T1-xCrx)2Si6 compounds (X = Nb or Ti and T = Fe, Co or Ni) are determined by resonant X-ray diffraction using the synchrotron facility, combined with calculations based on the density functional theory. Experimentally, the transition metal positions are determined using the anomalous scattering effect close to the energy absorption threshold. First principles calculations carried out on the as-determined structures show that the preferential site occupations are explained by thermodynamic arguments. Theoretical calculations highlight the influence of Cr atoms on the lattice parameters, and reveal that the experimental Cr content corresponds to the amount required to reach the equilibrium between X3T2(T1-xCrx)2Si6 and X4T4Si7.

Environmental/thermal barrier coatings (E/TBCs) of yttria partially stabilised zirconia (YSZ), gadolinium zirconate (GZO) and a combination of Y2SiO5 and GZO were applied on a Nb/Nb5Si3-based alloy with the nominal composition of Nb-25Ti-8Hf-2Cr-2Al-16Si (at%). A B-containing M7Si6-based (M=Ti, Nb, Fe, Cr) layer produced by pack cementation was used as bond coat. The ceramic topcoats were manufactured by electron-beam physical vapour deposition (YSZ and GZO) and magnetron sputtering (Y2SiO5).The different E/TBC systems were tested under cyclic oxidation conditions in air at 1100 and 1200°C. Lifetimes exceeding 1000 cycles of 1h dwell time at 1100°C were determined. The YSZ, GZO and combined Y2SiO5+GZO topcoats did not spall off during the maximum exposure time, exhibiting good adherence to the thermally grown oxide scale which consisted of mixed oxides of Fe,Ti, Cr and Nb embedded in silica. For the E/TBC systems with YSZ and GZO which were thermally cycled at 1200°C, lifetimes up to 850 cycles were measured; that of the system with a combined Y2SiO5+GZO topcoat exceeded again 1000 cycles.

Corrosion by molten phases leads to severe corrosion of heat exchangers in waste-to-energy plants. In addition, the presence of heavy metal chlorides in ash deposit increases degradation at low temperature due to the formation of highly corrosive molten phases. In this study, two heat exchanger materials, a low alloy steel (16Mo3) and a nickel based alloy (Inconel 625) were exposed in air to three different synthetic ashes, with various chloride contents, including ZnCl2 at isothermal temperatures of 450 and 650 °C in a muffle furnace. After the test, thickness and mass losses were evaluated on two separate samples, and metallographic cross sections of the specimens were characterized via SEM/EDX analyses. Both measurement results were in good agreement and showed that the corrosion observed on both materials was higher in the presence of zinc chloride in ash at 450 °C than in ashes without heavy metal chloride at 650 °C.

International audience; The microstructure and oxidation resistance of Laser Beam Melted (LBM) and Conventionally Manufactured (CM) Inconel 625 alloys were studied at 900 °C and 1050 °C. The microstructure of the LBM samples was cellular, with Nb and Mo segregations located at the cell walls. At 900 °C, the oxidation rate was similar for both materials but was clearly higher for the LBM material at 1050 °C. This high oxidation rate induced poor oxide scalecompactness, void formation in the subsurface region and the formation of a high amount of Nb1.5Cr0.5O4 at the alloy-oxide interface.

Oxidation of chromium at 800 °C for 30 min at oxygen activity from 5.10−14 atm to 5.10−13 atm have been conducted to grow an insulating chromia. Microstructural and semiconducting properties have been investigated using electron microscopy and photoelectrochemistry. The scales always exhibit a duplex microstructure with equiaxed and columnar grains, as well as a both n-type and insulating semiconduction type. Successive oxidation tests have demonstrated the higher protective nature of this chromia scale compared to the air grown one. Chromia semiconducting properties are finally described vs. oxygen activity in a large range of variation.

The oxidation behaviour of four model alloys with the composition Ni–25Cr–xMn (with x = 0, 0.5, 1 and 1.5 wt%) was investigated at 1050 °C in air by thermogravimetry and by in situ observations in an environmental scanning electron microscope (ESEM). The addition of manganese modifies the oxidation rate of Ni–25Cr alloys by (1) increasing the parabolic constant k p compared to that for Ni–25Cr and (2) lowering the short-time oxidation rate. Regardless of the Mn concentration, in situ ESEM observations indicated the formation of spinel crystallites from the very beginning of the oxidation process. The size of the spinel crystallites was directly linked to the initial Mn concentration. The obtained results suggested that the formation of the spinel layer at the top of the chromia oxide scale rather than at the metal–oxide interface as thermodynamically expected must be attributed to the higher diffusion rate of Mn than Cr both in the lattice and at the grain boundary.

International audience; Environmental/thermal barrier coatings (E/TBCs) were applied on an Nb/Nb5Si3-based alloy. The substrate material with the nominal composition Nb-23Ti-4Hf-3Cr-6Al-20Si (at.%) was manufactured by plasma melting. Rectangular platelets of this alloy were coated with chromia-silica forming M7Si6-based (M = Ti, Nb, Cr, X with X = Ni, Co, Fe) bond coats produced by pack cementation. The Co- and Fe-modified M7Si6-based layers also contained boron. On these grit blasted bond coats, ceramic topcoats of 7 wt.% yttria partially stabilised zirconia (YSZ) and gadolinium zirconate (GZO) were deposited using electron-beam physical vapour deposition. The E/TBC systems were thermally cycled at 1100 °C and 1200 °C in laboratory air (1 h at high temperature, 10 min at ambient temperature). Cross-sectional examinations of the samples after cyclic testing were carried out using scanning electron microscopy to study the protection capability of the E/TBC systems and failure mechanisms.The lifetimes of the E/TBC systems exceeded the maximum exposure period of 1000 cycles at 1100 °C. When cyclically tested at 1200 °C, the samples with Ni-modified and boron containing Co-modified M7Si6-based bond coats failed within 700 cycles. Failure was caused by spallation of thick oxides scales thermally grown on the substrate after degradation of the bond coats. The E/TBC systems with boron containing Fe-modified M7Si6-based bond coats exhibited lifetimes approaching 1000 cycles at 1200 °C. For both YSZ and GZO topcoats, chemical reactions with the TGOs were observed.

International audience; A thermal oxide scale has been grown on a high-chromium iron-nickel alloy under very low oxygen partial pressure (1050 degrees C, 10(-10) Pa). In this paper, a special attention has been paid to morphological and chemical characterizations of the scale by scanning transmission electron microscopy and energy dispersive X-ray analysis at high resolution on a cross-section thin lamella beforehand prepared by using a combined focused ion beam/scanning electron microscope instrument. The complexity of the oxide layer is highlighted, and the correlation between the present results and the ones of a photoelectrochemical study is discussed. (C) 2015 Elsevier Ltd. All rights reserved.

International audience; The high-temperature oxidation behavior of a binary Ni-30Cr model alloy has been studied at 1050 °C at three oxygen partial pressures p(O2): ~10−14, ~10−10, and ~10−5 atm. The oxide scales were characterized using grazing X-Ray diffraction, Raman spectroscopy, EDX spectroscopy, and TEM microscopy. The semiconducting properties of the oxide scale have been investigated using photoelectrochemistry (PEC). Classical characterizations show that chromia is the main oxidation product whatever maybe the oxygen activity. PEC reveals additionally the presence of (i) n-type Cr2O3 at p(O2) ~ 10−14 atm and (ii) both n-type and p-type Cr2O3 at higher oxygen pressures. The mechanisms of oxidation are discussed as a function of the major point defects that govern the oxide scale growth.

International audience; In the present work, liquid-solid phase equilibria of the V-Cr-Si phase diagram were experimentally studied. The samples were prepared by arc melting and as-cast microstructures were characterized by SEM, WDS and XRD. No new ternary phase was observed. Seven primary solidification regions were identified in these alloys: (V,Cr) solid solution, M3Si, M5Si3, MSi, M6Si5, MSi2 and (Si). Microstructural observations suggest two class II invariant transformations associated to the phases Liquid, M5Si3, MSi2 and MSi, and Liquid, M5Si3, MSi2 and M6Si5, respectively. The existence of a maximum on the monovariant line between Liquid, M5Si3 and MSi2 was evidenced.

International audience; The present work is focused on the oxidation at 1050 degrees C of a model chromia-forming alloy, paying special attention to the control of the oxygen partial pressure. Taking benefit of a recently developed approach to describe and fit photocurrent energy spectra, photoelectrochemical characterizations allowed to reveal the presence, in the scale formed at low P(O-2) (10(-15) atm), of an unexpected high number of oxide phases. This result underlines the complexity of the scale in term of chemical composition as well as of the semiconducting types of the individual phases.

International audience; This paper reports the effects of adding from 2 to 8 at.% tin on the oxidation behaviour of Nb/Nb5Si3 composites at 815 degrees C and at higher temperatures (1100 and 1200 degrees C). The role of tin in the elimination of pesting and in the oxidation process at high temperatures was established. The consumption of elements with a higher affinity for oxygen than Sn induces the accumulation of tin at the oxide/internal oxidation zone boundary. Low melting point phases (NbSn2 and/or pure Sn) form at 815 degrees C, whereas a layer of M5Si3 and Nb5SiSn2 forms at 1100 and 1200 degrees C. Once these products are formed, they generate an oxygen diffusion barrier and allow the elimination of pesting. However, for long oxidation processes at 1100 degrees C, the oxidation rate of Nb/Nb5Si3 composites containing tin should be higher than that for tin-free composites. Moreover, some oxidation results have suggested that the presence of A15-(Nb,Ti)(3)(Sn,Ti) in the microstructure of composites with at.%Sn > 2 can severely impact the low temperature fracture toughness of these composites.

International audience; Chromia grown on pure chromium at 900 A degrees C for 30 min at an oxygen partial pressure p(O-2) of 10(-12) atm has been characterized using photoelectrochemical and electron microscopy techniques. This study reveals a duplex scale: n-chromia with equiaxis morphology in the internal part (similar to 650 nm thick) and p-chromia with columnar morphology in the external part (similar to 900 nm thick). Grain orientation maps also revealed the presence of a c-oriented chromia layer at the interface between the n- and the p-subscales. This aOE (c) 0001 > textured layer was identified as the first-grown chromia layer. It means that internal n-equiaxis chromia grew by anionic transport governed by oxygen vacancy diffusion, whereas external p-columnar chromia layer grew outwards and was controlled by chromium vacancy diffusion.

International audience; Nickel and iron aluminide coatings were identified as possible candidates for hardfacing materials in Sodium Fast Reactors. Both coatings were developed on two steel grades of interest for the next French Sodium Fast Reactor prototype, 316L(N) and T91. Pack cementation and slurry were employed as aluminization processes. The compatibility of all coatings with purified Na was evaluated at 550 A degrees C for exposure times up to 4250 h. All coatings evidenced high chemical stability in Na even though Na penetration and slow coating dissolution could be evidenced. The penetration depth of Na was observed to depend on the coating nature induced by the deposition process.

International audience; A comparison of the corrosion behaviour of simplified chromia- and alumina-forming alloys (Ni-30Cr and Ni-8Al-28Cr) in molten glass is performed via electrochemical methods. A pre-oxidation treatment in air prior to immersion ensured the formation of a 2-pm-thick oxide scale of Cr2O3 or Al2O3. The lifetime of Ni-30Cr depended on the competition between the oxide growth and its dissolution in the melt. For Ni-8Al-28Cr, the high solubility of alumina in the melt studied here led to severe aluminium loss in a few minutes and clearly demonstrated the higher efficiency of chromia-forming alloys for molten glass applications.

The halide-activated pack-cementation technique was employed to fabricate a diffusion coating that is resistant both to isothermal and to cyclic oxidation in air at 650 °C on the surface of the V–4Cr–4Ti vanadium alloy that is a potential core component of future nuclear systems. A thermodynamic assessment determined the deposit conditions in terms of master alloy, activator, filler and temperature. The partial pressures of the main gaseous species (SiCl4, SiCl2 and VCl2) in the pack were calculated with the master alloy Si and the mixture VSi2 + Si. The VSi2 + Si master alloy was used to limit vanadium loss from the surface. The obtained coating consisted of multi-layered VxSiy silicides with an outer layer of VSi2. This silicide developed a protective layer of silica at 650 °C in air and was not susceptible to the pest phenomenon, unlike other refractory silicides (MoSi2, NbSi2). We suggest that VSi2 exhibits no risk of rapid degradation in the gas fast reactor (GFR) conditions.

International audience; Two TaC-strengthened cobalt-based alloys, with compositions Co-10Cr-0.25C-4.4Ta and Co-10Cr-0.5C-8.7Ta (wt.%) underwent Cr-deposition by pack-cementation, followed by heat-treatment to allow Cr diffusing deeper in their sub-surface. Thereafter they were tested for 50 hours at 1200°C in a thermobalance. These alloys were successfully enriched in chromium, with a maximal content on surface of about 30wt.% and a depth of enrichment of several hundreds of micrometers. This allowed a chromia-forming behavior of the cemented alloys despite the 10wt.%Cr of the bulk. This was especially true for the {C,Ta}-richest alloy the oxidation kinetic of which was wholly parabolic and analogous to a Ni-30Cr alloy known for its good behavior in oxidation at 1200°C. The parabolic constants and chromia-volatilization constants were at the same levels of the Ni-30Cr ones. However, the chromium enrichment step, especially the diffusion heat-treatment following the cementation, causes fragmentation and surface fraction reduction of the tantalum carbides which strengthen the alloy.

This work focuses on the effect of tin additions (2, 5 and 8%) to the MASC alloy (Nb-25Ti-8Hf-2Cr-2Al-18Si) on the microstructure and the oxidation behaviour at 815°C in air. The alloys are mainly constituted of a niobium solid solution plus the () Nb5Si3 silicides. For the higher Sn additions (5 and 8%), a fourth phase is evidenced: it is enriched in Sn and has a crystal structure close to Nb3Sn. The oxidation resistance of these alloys is clearly improved by tin additions: the oxygen inward diffusion is hindered and consequently the fragmentation of the silicides is avoided. Cracks in silicides are no longer observed for the MASC containing 8%Sn. This effect is not attributed to a better efficiency of the oxide scales but rather to the reduction of the niobium solid solution fraction with tin additions.

Austenitic nickel-iron-chromium based superalloys are materials of choice for high temperature applications as they provide high temperature creep resistance associated with a suitable oxidation behavior in the temperature range of 600-1100°C. However, these properties are not sufficient for applications as Steam Methane Reformer (SMR). As a consequence, aluminum and chromium coatings are developed by the pack-cementation technique to improve their corrosion resistance. The oxidation behavior of the coated samples has been carried out in air at 1050°C. Chromium deposition leads to a layer of bcc chromium-iron solid solution. Oxidation tests indicated that a too high chromium concentration induces a too rapid growth of the chromia layer. In the case of aluminum coatings, a layer of -NiAl is formed at the surface of the alloy. It permits a significant decrease of the oxidation rate.

The feasibility of surface chromium enrichment by pack-cementation was assessed for different low chromium-containing cobalt alloys, in order to improve their resistance against high temperature. A binary Co-10Cr alloy, two ternary Co-10Cr-0.5C and Co-10Cr-1.0C alloys and two TaC-containing Co-10Cr-based alloys were elaborated by foundry for the study. 7.5h-long and 15h-long cementations at 1050°C, followed or not by a 75h-long heat treatment at 1200°C were performed on these alloys. Microstructure examinations performed using a Scanning Electron Microscope and concentration profiles using Electron Probe Micro Analysis-Wavelength Dispersion Spectrometry were realized in order to analyze the level of Cr-enrichment of the sub-surface region, with as studied criteria: the nature of the external Cr-enriched zone, the maximal chromium content on surface and the depth of chromium enrichment. The Cr-enrichment of the sub-surface succeeded for the Co-10Cr alloy and for the two tantalum-containing alloys, with the formation of an external metallic zone containing around 30wt.%Cr. In contrast the chromium carbides-containing alloys were effectively enriched in chromium in surface but in the form of a continuous chromium carbide layer which can induce other problems such as spallation and then possible fast oxidation of the denuded alloy. Finally it appeared that only the carbon-free alloys, and the alloys reinforced by carbides more stable than chromium carbides, are potentially able to be successful enriched in chromium in their sub-surface by pack-cementation.

Chromium can be a candidate of interest as a base component for refractory alloys for high temperature applications. Few data are available about oxidation kinetic of chromium in air for temperature as high as 1300 °C. Moreover, rare papers take the nitridation of chromium into account in the description of the oxidation process or in the calculation of the kinetic constants. In the present study, global methods like thermogravimetry were linked to thickness measurements and microstructural characterisation to evaluate oxidation and nitridation contribution to weight gain. High stress levels were induced in the oxide layer during its formation. The kinetic constant associated to the nitridation, at this temperature, was one order of magnitude higher than the parabolic oxidation constant. Experiments demonstrated that the presence of nitrogen in the substrate is always a precursor to breakdown of the oxide layer and does not result from diffusion through the Cr2O3 layer.

In this study, chromium based alloys containing a hardening NiAl phase are investigated from 1100°C to 1300°C in air. The influence of the NiAl content and of the Al/Ni ratio on microstructure modification and on oxidation behaviour are characterized by metallography and thermogravimetry. Increasing the Al/Ni ratio leads to a higher Al content in the chromium solid solution but does not modify the amount of primary NiAl. At high temperature, and for NiAl content exceeding 16 at%, a duplex oxide layer forms at the surface of the alloys exposed in air, alumina in the inner part, and chromia in the outer part. The oxidation behavior is discussed according to oxidation profile and to the thermodynamic predominance diagram of the involved species.

The oxidation and nitridation of pure chromium and of chromium alloys containing 0.5 to 5at% silicon was investigated at 1300°C in several atmospheres. Global methods like thermogravimetry were associated to thickness measurements and microstructural characterisation to evaluate oxidation mechanisms. The contribution of nitridation to weight gain during high temperature exposure is discussed, examining nitride volume fraction. Experiments demonstrate that the presence of nitrogen in the substrate is always consecutive to a breakdown of the oxide layer and does not result from diffusion through the Cr2O3 layer. Silicon, when present in chromium solid solution, slows down the oxidation kinetics and limits the progression of nitrogen in the chromium lattice and at grain boundaries. The absence of the Cr2N layer beneath the metal/oxide interface does not benefit to the adherence of the oxide scale. Oxidation of silicon in chromium leads to the formation of discontinuous particles of SiO2 at the metal/oxide interface, at the grain boundaries and dispersed in the bulk in the alloy sub-surface.

In the present work, an assessment of the V–Cr–Si system was done. Mainly based on previously ternary experimental data on the isothermal section at 1200 °C and on the liquid–solid equilibria, a thermodynamic modeling of the ternary system was performed within the CALPHAD approach using the Thermo-Calc and the Pandat software packages and taking into account data from existing assessment of binary sub-systems. Four intermediate phases were considered as stoichiometric compounds (Cr,V) x (Si) y with substitution of Cr and V at Si ratio constant: (Cr,V)Si, (Cr,V) 5 Si 3 , (Cr,V) 11 Si 8 and (Cr,V) 6 Si 5 . Two others phases (Cr,V)Si 2 and (Cr,V) 3 Si presenting a slight domain of homogeneity, were described as (Cr,Si,V) 1 (Cr,Si) 2 and (Cr,Si,V) 3 (Cr,Si,V) 1 respectively. The metallic liquid phase and the terminal solid solutions of chromium and vanadium were described by the disordered solutions model as (Cr,Si,V) 1 (Va) 3 . The silicon admitting any solubility, its description was limited to (Si). The resulting thermodynamic optimization led to a good agreement with experimental data.

International audience; The system Bi2O3-Al2O3 has been experimentally investigated above 600 degrees C by DTA, XRD and EPMA under air and low oxygen pressure. Only two compounds were found to exist in equilibrium, which are Bi2Al4O9(1Bi(2)O(3): 2Al(2)O(3)) and Bi25AlO39(25:1). The latter exhibits a sillenite structure and does not contain pentavalent bismuth. A peritectoid decomposition of (25:1) and a peritectic melting of (1:2) occur at 775 degrees C and 1075 degrees C respectively, while an eutectic transformation was observed at 815 degrees C for 97 mol% Bi2O3. On the basis of the results obtained within the present work as well as experimental data provided from literature, a thermodynamic modeling where the liquid phase is described by the two-sublattice ionic liquid model was performed according to the Calphad approach. The resulting thermodynamic optimization yielded good agreement with experimental results in the investigated region.

International audience; The oxidation resistance of MSi2 compounds with M = V, Ti, Cr was investigated from 450 to 950 degrees C in air under isothermal and cyclic conditions. Vanadium, chromium and titanium disilicide were not subjected to the pest phenomenon at 650 degrees C over 800 1-h cycles. The results demonstrated very low weight gains regardless of the testing conditions. Oxidation tests were also performed over long duration (1000 h) to identify the oxidation products. The MSi2 compounds were all subjected to the simultaneous oxidation of M and Si despite the formation of a protective silica scale. Increasing the duration of oxidation enhanced the protective properties of the silica scale. Therefore, short-term measurements (by thermogravimetry) did not allow an extrapolation of the MSi2 lifetime. The formation of molten V2O5 induced a higher oxidation rate of VSi2 and delayed the establishment of the protective silica scale compared with CrSi2 and TiSi2.

International audience; The oxidation behaviour of Nb silicide based alloys, considered as potential ultra-high temperature material for gas turbine engine applications, can be improved by chromia-silica forming coatings with M7Si6 structure. To stabilise the protective oxide scales against water vapour corrosion in combustion atmospheres, environmental barrier coatings (EBCs) of yttrium silicate were deposited on an FeB modified M7Si6 based bond coat using magnetron sputtering. The 15-20 mm thick ceramic top coats with an approximate chemical composition of 25Y-13Si-62O (at.-%) were dense and amorphous after deposition. They were annealed in vacuum to get a crystalline structure with the predominant phase Y2SiO5. Samples with this EBC system were tested in rapidly flowing water vapour at temperatures between 1100 and 1300 degrees C for up to 16 h. At 1100 and 1200 degrees C, significantly reduced mass gains were determined for samples with yttrium silicate top coat in comparison to those coated only with FeB containing layers. The EBC partially transformed into the Y2Si2O7 phase and exhibited microporosity. At 1300 degrees C, the yttrium silicate layer decomposed forming yttrium oxides.

International audience; Coupons of a Nb-silicide-based alloy with the nominal composition of 47Nb-25Ti-8Hf-2Cr-2Al-16Si (at-%) were coated with Co-, Ni- and FeB-modified M7Si6-based layers using packcementation. Subsequently, on one side of the coated samples, environmental/thermal barrier coatings (E/TBCs) of yttria partially stabilised zirconia (YSZ) and gadolinium zirconate were deposited by electron beam physical vapour deposition. The lifetimes of the different E/TBC systems were determined performing thermal cycling tests at 1100 and 1200 degrees C in air. At 1100 degrees C, lifetimes approaching 1000 cycles (corresponding to 1000 h of high temperature exposure) were measured. However, severe oxidation occurred at the edges and rear side of the samples (areas without ceramic topcoat) due to local degradation of the oxidation protective coatings, resulting in substantial material recession with prolonged testing. When thermally cycled at 1200 degrees C, the E/TBC systems exhibited lifetimes exceeding 200 cycles. The sample with FeB containing bond coat and Gd2Zr2O7 topcoat survived even 400 cycles at this exposure temperature.

The different constituents of an AZ91 alloy (α,β, and MnAl phases) were synthesized and their corrosion resistance was studied by electrochemistry in ASTM D1384 water, pH 8.3. The pure phases were characterised through the corrosion potential, the polarisation resistance, and polarisation curves, then systematically coupled to assess the galvanic corrosion occurring in the AZ91 alloy. The aluminium content of the oxide film was obtained by X-ray photoelectron spectroscopy measurements. The corrosion rate of the α solid solution alloys depends closely on their Al content. Aluminium enhances the corrosion resistance of the α-phase through the formation of an Al enriched superficial layer. The β-phase is 150 mV nobler than the α-phase, but their corrosion rates are similar. The galvanic currents are low (below 20 μA cm−2) whatever the implemented couples and close to the corrosion current previously measured for the AZ91 alloys.

International audience; Experiments and thermodynamic calculations were performed on five cobalt-base superalloys containing tantalum carbide and chromium carbides in order to evaluate the accuracy of thermodynamic calculation for this alloy family. The studied quantities were the solidus temperature and the phase fractions of the different carbides observed after a long time exposure at different high temperatures. The predicted and observed phases are in good agreement. However the measured phase-fractions and melting temperatures were sometimes higher than the calculated values. The disagreement in the phase fractions can be attributed to the fact that surface fractions and volume-fractions are not necessarily equal.

The microstructure and the corrosion behaviour of high pressure die-cast and semi-solid cast AZ91D magnesium alloys have been investigated. Semi-solid processing leads to a structure with large rounded grains of a solid solution of magnesium (α phase) whereas die-cast alloys are more homogeneous. Electrochemical measurements, particularly with impedance spectroscopy, have shown that the semi-solid cast alloy possesses a corrosion rate at least 35% below that of the die-cast alloy. This can be explained with considering the differences between the composition of the primary α phase and the volume fraction of the β phase, in the two types of alloy.

Équipe 103 : Composés intermétalliques et matériaux hybrides; International audience; The oxidation resistance and the crystal structure of the new Ti4NiSi4 compound were investigated. This compound was manufactured both as a single-phase and as the outer layer of a protective coating for vanadium alloys. The recorded oxidation rates were very low at 650 degrees C and 750 degrees C in air, which represents a more severe environment than those envisaged for the targeted application: fuel cladding for sodium-cooled fast reactors. The powder XRD measurements performed at the synchrotron SOLEIL allowed for the crystallographic structure determination of Ti4NiSi4 (S.C. Pnma; a = 15.63931; b = 5.08321; c = 12.75151) with high confidence factors (R-wp = 0.07; R-Bragg = 0.067). Its structure consists of atomic planes stacking along the b-axis with planes at coordinates y = 0, 1/4, 1/2 and 3/4; it can therefore be considered as a pseudolamellar structure. It has been suggested that this peculiar feature induces a low activation energy for the phase transformation of Ti4NiSi4 into Ti4Ni4Si7, which thus renders the transformation possible at moderate temperature. This structural rearrangement is accompanied by the liberation of Ti and Si, which leads to the formation of both TiO2 and SiO2 oxidation products, which are responsible for the high environmental resistance observed at 650 degrees C in air.

The aims of this study were to develop Nb 4 Fe 4 Si 7 coating for Nb-silicide composite and to assess their oxidation resistance. The Nb 4 Fe 4 Si 7 was first manufactured as single phase and its oxidation behaviour was characterised from 1100 to 1300 °C. Its oxidation resistance at 1300 °C in air was high because of the formation of a duplex protective oxide layer of Fe 2 O 3 and SiO 2 . Halide activated pack-cementation was successful to deposit the Nb 4 Fe 4 Si 7 compound as outer layer of the coating both on pure niobium and Nb-silicide composite. In both cases, the master alloy 10%FeSi 2 + 10%NbSi 2 + 80%Nb 4 Fe 4 Si 7 can be employed. Oxidation tests performed both cyclically and isothermally at 1100 °C demonstrated that the oxidation resistance of coated Nb-silicide composite was not as good as that observed on the single phase Nb 4 Fe 4 Si 7 compound. Nevertheless lifetime as long as 500 1 h-cycle at 1100 °C was obtained both on pure niobium and Nb-silicide composite.

International audience; Intermetallic single phases (TiXCr)(7)Si-6 (with X = Fe, Co or Ni) were manufactured by uniaxial hot pressing. Their full characterization (SEM + XRD) highlighted that the nature of the metal constituent X influenced the chromium content in the silicide compositions. Both isothermal and cyclic oxidation studies at a temperature as high as 1300 A degrees C evidenced very low degradation rate for all compounds and lifetime at 1100 A degrees C exceeded 4700 cycles. All phases developed a duplex protective layer of chromia and silica. It was observed that the chromia:silica ratio depended on the nature of X, and, particularly for the Ni-containing specimen, also on the temperature. Co- and Ni-containing specimens developed pure chromia during oxidation whereas Fe ones develop (CrxFe1-x)(2)O-3, reducing drastically the CrO3 volatilization during exposure at temperature higher than 1000 A degrees C.

International audience; This study focuses on the development of new protective coatings for the vanadium-based alloy V-4Cr-4Ti. Halide-activated pack-cementation (HAPC) technique was used to develop VxSiy multilayered diffusive suicide coatings. The outer layers (coatings) were formed of VSi2 doped with 27 at.% Cr or TiSi2. These compounds exhibited a very low oxidation rate at 650 degrees C, both in air and at a low oxygen pressure (He, 5 PPm O-2). The coatings formed mainly of MSi2 were found to be insensitive to pesting and largely unreactive to liquid sodium (

In many industrial applications water is present in the hot oxidant gases to which the metallic components can be exposed in service. The presence of vapor may influence the progress of high temperature oxidation by modifying the structural characteristics of the growing external oxide scales and the kinetics of mass gain as well as of mass loss. The chromia-forming refractory alloys or superalloys are especially concerned since, in vaporcontaining hot gases, the volatilization of their protective oxide scale is enhanced by the formation of oxy-hydroxides according to the equation ½ Cr2O3 (s) + H2O(g) + ¾ O2(g) = CrO2(OH)2(g), with as result a minimization of the mass gain rate when the oxidation progress is followed by thermogravimetry. When the temperature is very high the volatilization of the platinum wire to which the sample is hung may also contribute to this linear mass loss by volatilization, while, one the contrary, a linear mass gain may occur in case of redeposition of a part of the previous volatilized species on the suspension in colder places of the thermobalance. It is thus important, in the mass variations recorded by the thermobalance, to separate on the one hand the parabolic contribution of the oxidation of the alloy, and on the other hand all of the linear mass losses (volatilization) or linear mass gains (recondensation), in order to better characterize the resistance of the alloy to high temperature oxidation. This uncoupling can be done for example by exploiting the mass gain curves according to a method previously adapted to the assessment of both the parabolic and volatilization constants from the mass gain curves recorded during oxidation in dry air. The aim of the present study is to test Ni25Cr-type samples cut in the same ingot obtained by equiaxed solidification, in dry air and in a humidified air with the same thermo-balance and with the same parameters (heating and cooling rates, temperature and duration of the isothermal stage), in order to clearly reveal the differences of mass variation kinetic and of characteristics of the oxides formed (composition, external morphology, …), induced by the presence of water vapor.The presence of water vapor in hot air, which is generally known to have a great importance on the high temperature oxidation behavior of the alloys, generally in the bad way, obviously had rather a good influence in the present case, for the studied cast Ni-25Cr alloy and for the water concentration. Indeed the oxidation rate was slowed (but the chromia-volatilization rate probably accelerated), as shown by the thermogravimetry records, oxides thicknesses and quantifications of alloy’s chromium consumption. Furthermore the oxides were of pure chromia, displayed a greater fineness, and a significantly better adherence on the substrate (at least better resistance against spallation which can be also due to a better plasticity of the oxide as often reported), an important point for alloys which are exposed to thermal cycling. After these observations one can imagine that it can be interesting to artificially enrich hot air with a precise quantity of water to improve both the isothermal oxidation resistance of such alloy and its resistance against cycling oxidation. However these observations need to be confirmed by additional investigations, including for example in situ XRD measurements during certain periods of oxidation or technical improvements of the thermogravimetric device for avoiding the lack of precision due to the re-deposition of volatile species.

International audience; In this work, we describe the fabrication of coatings containing (TiNbXCr)(7)Si-6 silicide (with X = Fe, Co and Ni) by halide-activated pack-cementation on Nb and silicide composites. The coating oxidation performance is evaluated isothermally at temperatures ranging from 1100 degrees C to 1300 degrees C and cyclically at 1100 degrees C In all the coating systems examined, the M7Si6 suicide formed in the protective coating and constituted the outmost layer for coatings containing Co, whereas M6Si5 and a Cr-rich suicide constituted the outmost compounds for Fe and Ni coatings, respectively. The results of isothermal oxidation tests show that all these compounds form protective oxide layers leading to very low mass gains, both at 1100 degrees C and 1200 degrees C. Under cyclic conditions at 1100 degrees C, coatings containing Co and Ni exhibited very long lifetimes, exceeding 2600 1-hour cycles. The beneficial effect of Ni and Co diminished at 1300 degrees C, due to rapid interdiffusion between substrate and coating in comparison to that found in the Fe containing coating.

For energy production and also for the glass industry, finding new refractory alloys which could permit to increase the process temperatures to 1200°C or more is a permanent challenge. Chromium base alloys can be good candidates, considering the melting point of Cr itself, and also its low corrosion rate in molten glass. Two families of alloys have been studied for this purpose, Cr-Mo-W and Cr-Ta-X alloys (X= Mo, Si..). A finer selection of compositions has been done, to optimize their chemical and mechanical properties. Kinetics of HT oxidation by air, of corrosion by molten glass and also creep properties of several alloys have been measured up to 1250°C. The results obtained with the best alloys (Cr-Ta base) give positive indications as regards the possibility of their industrial use.

Electrochemical measurements demonstrate that magnesium surfaces can be protected by alkyl carboxylate. In a nearly neutral pH solution of sodium decanoate, the reduced corrosion rate and a passivation behaviour are attributed to the formation of Mg(C10H19O2)2(H2O)3 (Mg(C10)2) at the magnesium surface whereas heptanoate Mg(C7H13O2)2(H2O)3 (Mg(C7)2) is not efficient in such media. The crystal structures of the two metal carboxylates Mg(C7)2 and Mg(C10)2 are determined by X-ray diffraction. Single crystal data: Mg(C7)2, P21/a, a = 9.130(5) A, b = 8.152(5) A, c = 24.195(5) A, β = 91.476(5)°, V = 1800.3(15) A3, Dx = 1.242 g cm−3, Z = 4. Synchrotron powder data: Mg(C10)2, P21/a, a = 9.070(3) A, b = 8.165(1) A, c = 32.124(1) A, β = 98.39(1)°, V = 2353.85(8) A3, Dx = 1.188 g cm−3, Z = 4. Their layered structures are quite similar and differ mainly by the length of the hydrophobic chains. They consist of two planes of O-octahedra centred by Mg atoms, parallel to (001). The distorted octahedra are constituted by three oxygen atoms from carboxylate groups and by three oxygen atoms coming from water molecules. The layers are connected by hydrogen bonds. The carboxylate chains are located perpendicularly and on both sides of these planes. One carboxylate chain is bridging the Mg atom along [010] while the other is monodendate. The presence of structural water is confirmed by thermal analyses.

International audience; Several cast strengthened superalloys, Ni base and Co base, were exposed to high temperature oxidation for long times and metallographically examined. Different phenomena occurred in the sub-surface microstructure, depending on both alloy and temperature. Thermodynamic modelling was used to know what it happened for carbon during oxidation, then to explain the observed microstructural changes. It appears that carbon atoms either quit the alloy probably after its oxidation into gases, or on the contrary go deeper into the bulk where they promote the precipitation of new carbides by solid state transformation. Thereafter, thermodynamic modelling allowed to know the new local refractoriness of the zones affected by oxidation, then to appreciate the new mechanical properties in the sub-surface.