Critical Temperature Range in Standard and Ni-bearing Spheroidal Graphite Cast Irons

Describing the conditions for reaustenitization of spheroidal graphite cast irons is of interest for their heat-treatment after casting, e.g. for manufacturing austempered ductile irons. Differential thermal analysis has been used to characterize the direct eutectoid transformation and the reverse transformation, i.e. the reaustenitization. This has been applied to a standard and a Ni-bearing alloy, with a ferritic matrix for the former, both a ferritic and a pearlitic matrix for the latter. The results are discussed in relation with the stable and metastable three phase fields. While earlier description of the direct eutectoid transformation is confirmed, the one for reverse eutectoid has been found more complex and is amended

Pt-modified Ni aluminides, MCrAlY-base multilayer coatings and TBC systems fabricated by Spark Plasma Sintering for the protection of Ni-base superalloys

Pt-modified Ni aluminides and MCrAlY coatings (where M=Ni and/or Co) are widely used on turbine blades and vanes for protection against oxidation and corrosion and as bond coatings in thermal barrier coating (TBC) systems. The present work shows the ability of a new fabrication technique, the Spark Plasma Sintering, to develop rapidly new coating compositions and microstructures. This technique allows combining powders and metallic foils on a superalloy substrate in order to obtain multilayered coatings in a single short experiment. Fabrication of MCrAlY overlays with local Pt and/or Al enrichment is shown, as well as fabrication of coatings made of ζ-PtAl2, ε-PtAl, α-AlNiPt2, martensitic β- (Ni,Pt)Al or Pt-rich γ/γ′ phases. The realization of a complete TBC system with a porous and adherent Yttria Stabilized Zirconia (YSZ) layer on a γ/γ′ low mass bond coating is also demonstrated. Difficulties of fabrication are reviewed and discussed, such as Y segregation, risks of carburization, local overheating, or difficulty to coat complex shape parts. Finally, some first results of cyclic oxidation are given

Characterisation of rosette formation in an aluminium–silicon alloy

Differential thermal analysis has been used to investigate the effect of cooling rate on rosette formation during solidification of a synthetic Al–Fe–Si alloy. Rosettes can be characterised as a very fine multiphase structure within more or less convex areas dispersed in the matrix. Their formation during solidification is related with liquid entrapment and high solidification undercooling associated with the need of independent nucleation events of second phases. It is here shown that their density and internal coarseness depend on cooling rate. Further, metallographic observation of rosettes in contact with large precipitate of b-Al9Fe2Si2 phase allowed to conclude that this latter phase does not help silicon nucleation

Preliminary Results of the Isothermal Oxidation Study of Pt-Al-NiCoCrAlYTa multi-layered coatings prepared by Sparks Plasma Sintering (SPS)

MCrAlY coatings (where M = Co, Ni or Co/Ni) are widely used on turbine blades and vanes as oxidation and corrosion resistant overlays or as a bond-coating in thermal barrier coatings systems. MCrAlY are usually fabricated by Plasma Spraying, Physical Vapour Deposition, High Velocity Oxy-Fuel spraying or electrolytic techniques. The use of emergent Spark Plasma Sintering technique as a preparation method for NiCoCrAlYTa coatings has been presented previously [1]. SPS technique allows fast development of new coatings with a one-step fabrication of multilayered coatings. This work presents first results of the long term isothermal oxidation behaviour of Pt-Ni aluminide/NiCoCrAlYTa multi layered coatings. The obtained coating is dense and homogeneous. Isothermal oxidation up to 500 h at 1100°C leads to the formation of an adherent alumina scale with Y-rich precipitates and deep intergranular oxidatio

A Process-Structure Investigation of Aluminum Oxide and Oxycarbide Thin Films prepared by Direct Liquid Injection CVD of Dimethylaluminum Isopropoxide (DMAI)

We present the direct liquid injectionCVDof aluminum oxide and oxycarbide thin films using dimethylaluminum isopropoxide at high process temperature (500–700 8C) with the addition ofO2 gas, and at low temperature (150–300 8C) with the addition of H2O vapor. Very smooth films with typical roughness values lower than 2 nm are obtained. The thin films are composed of an amorphous material. The composition evolves as a function of temperature from that of a partial hydroxide to a stoichiometric oxide at low deposition temperature(150–300 8C), and from that of a stoichiometric oxide to a mixture of an oxide with an(oxy)carbide at higher temperature (500–700 8C)

Levures contre pourritures des fruits ::évaluation de levures indigènes contre les maladies post-récolte

La réduction du risque phytosanitaire posé par les pesticides chimiques de synthèse nécessite d’évaluer toutes les solutions alternatives dont les microorganismes. Ici, nous avons évalué 25 souches de levures indigènes isolées en Suisse romande comme agents de lutte biologique contre trois importants pathogènes post-récolte des fruits à pépins: Botrytis cinerea, Penicillium expansum et Penicillium digitatum

Fissuration par fatigue d’un tube de générateur de vapeur

Suite à une fuite sur un générateur de vapeur de la tranche 2 de la centrale de Fessenheim, une fissure circulaire de fatigue est observée au niveau de la plaque entretoise supérieure. Pour compléter les examens et le retour d’expérience international, un nouveau procédé d’extraction de tube a été développé, avant le remplacement du générateur de vapeur. Les examens en laboratoire ont permis d’exclure le scénario d’instabilité fluide-élastique et ont fourni de nouveaux éléments

Mechanism of olivine and glass alteration under experimental H2O-CO2 based supercritical gas: Application to modern and ancient Venus

International audienceExtreme conditions encountered in some geological contexts (deep serpentinization,interaction of Venus atmosphere with its basaltic surface, volcanic degassing) activate mechanismsand rates of silicate alteration that are poorly understood. In the present study, we investigate themechanisms of mineral reactions in a natural geological system at high temperature, under conditionswhere the low solvation of cations by fluids likely promotes surface reactions such as surface diffusionand/or local recrystallization. We focus on vitreous glasses and olivine, reputed to be the mostalterable phases in volcanic rocks, by reacting samples for one week in a Ni-based alloy experimentalvessel. For the framework of our experimental study, we chose to apply the deep atmosphereconditions on Venus: 470°C and 90 bar of reconstituted Venus-like gas. We also tested the effect ofwater (Early Venus or wet volcanic degassing) by adding water vapor at up to 320 bar total pressure.The mineral reactions affecting the samples were identified by a set of spectroscopic surface analysesof the altered samples: Scanning Electron Microscopy, Energy Dispersive X-ray Spectroscopy, X-RayDiffraction in grazing incidence mode, X-ray Photo electron Spectroscopy and Raman spectroscopy.Samples of obsidian and tholeiitic glasses are found to be sensitive to a threshold waterpressure, depending on glass composition, below which the reaction is limited to some elementalmobility in the glass (alkali enrichment, calcium loss) leading to a possibly more stable surface layerof tens to hundreds of microns. Above this threshold water pressure (ca. 50 bar H2O for the obsidianbut >250 bar H2O for the tholeiitic glass), water promotes the depolymerization of the glass and thecrystallization of stable minerals. This crystalline rim is less protective that the chemically modifiedlayer.Olivine samples react differently depending on whether the olivine is isolated or included in abasaltic rock. In the latter case only, iron coatings are formed, which are identified as hematite,suggesting that this phase is not fed by olivine itself but rather by surface diffusion from neighboringFe-rich phases. This supports the conclusions from experimental studies and orbital observations onthe short-term visibility of unaltered olivine in Venus lava flows: such a coating is enhanced when Febearingminerals are in the proximity of olivine. Under high water vapor pressure, Fe-bearing talc (andnot serpentine) forms by a likely topotactic reaction that also incorporates silica from the gas. This talclayer may form a protective layer, implying that serpentinization of ultramafic rocks at hightemperature may not be as prevalent as one might think in a gas-dominated system like the EarlyVenus surface