Microtextural Analyses of the Ni/NiO System after High Temperature Oxidation

Role of Minor Element Addition in the Formation of Thermally Grown Alumina Scales

In-situ time resolved X-ray diffraction study of the formation of the nanocrystalline NbAl3 phase by mechanically activated self-propagating high-temperature synthesis reaction

The mechanically activated self-propagating high-temperature synthesis (MASHS) technique was used to produce a NbAl3 intermetallic compound. This process results from the combination of two steps: a mechanical activation of the Nb 3Al powder mixture which is followed by a self-propagating high-temperature synthesis (SHS) reaction, induced by the exothermal character of the reaction Nb3Al. An original experiment was designed to study in-situ the formation of the NbAl3 phase in the combustion front: time-resolved X-ray diffraction coupled with an infrared imaging technique and a thermocouple measurement were performed to monitor the structural and thermal evolution during the SHS reaction. Ow…

The Role of Zr in the High-Temperature Oxidation of Fe<sub>3</sub>Al

The paper describes an examination of the effect of the addition of zirconium as a third element on the heat-resisting properties and explains the high temperature oxidation mechanism of Fe3Al intermetallic compounds. The Fe3Al and Fe3Al-0,05Zr specimens have been isothermally oxidized in the temperature range of 1173-1473 K in synthetic air for 100 hrs. The formed oxide layer, about 1,5-2 μm thick, was Al2O3. An examination of the cross-sectioned scales by SEM-EDS showed that the alumina layer consisted of a small inner columnar layer and an outer equiaxed grain layer. Additionally, very fine (50-150 nm) oxide grains rich in Zr, further identified as ZrO2, were found across the alumina sca…

Synthesis of nanocrystalline NbAl3 by mechanical and field activation

Abstract The mechanically-activated, field-activated, and pressure-assisted synthesis (MAFAPAS) process, which combines the simultaneous synthesis and densification of nanophase materials, was utilized to produce nanocrystalline NbAl3 material from Nb+3Al mechanically activated powders. Nb+3Al elemental powders were co-milled for a short time in a specially designed planetary ball mill to obtain nanoscale distributed reactants but to avoid the formation of any product phases. These were then subjected to high AC currents (1500–1650 A) and uniaxial pressures (56–84 MPa). Under these conditions, a reaction is initiated by field activation and completed within a short period of time (3–6 min).…

Matter Transport and Corrosion Kinetics of Metals and Alloys at High Temperature

Presentation, Calibration and First Experiments Using a New Thermobalance for Experiments in Sulfur Vapor Atmospheres

Synthesis of niobium aluminides using mechanically activated self-propagating high-temperature synthesis and mechanically activated annealing process

The mechanically activated self-propagating high-temperature synthesis (MASHS) technique and the mechanically activated annealing process (M2AP) were used to produce NbAl3 intermetallic compound. The MASHS process results from the combination of two steps: first, a mechanical activation of the Nb 3Al powders mixture; second, a self-propagating high-temperature synthesis (SHS). The M2AP process also results from the combination of two steps: the first is the same; the second consists of the annealing of as-milled powders. Based on X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM) and X-ray energy dispersive spectroscopy (EDXS), the as-milled powders, MASHS, and M2AP end-pr…

Effect of Impurities (Carbon and Manganese) on Iron Oxidation at High Temperature: Impurities-Rare Earth Element (Cerium) Interactions

Apres oxydation du fer pur revetu d'un depot de CeO 2 , (T = 7000C, pO 2 = 0,04 Pa), le cerium se trouve localise dans la couche de wustite sous la forme de CeFeO 3 . La dissolution de cette phase assure l'introduction homogene du cerium a l'interieur de la couche de FeO. Sa formation est attribuee a un processus d'oxydo-reduction entre le depot de CeO 2 et les premiers germes de FeO. Dans le cas des echantillons d'acier Fe-Mn-C, recouverts de CeO 2 , le cerium n'est pas incorpore dans la couche d'oxyde de fer de facon homogene et se retrouve sous la forme de CeO 2 a l'interface oxyde-gaz. Le role de chaque impurete est maintenant bien etabli: le manganese inhibe l'incorporation du cerium d…

Mechanisms Involved by Reactive Elements upon High Temperature Chromia Scale Growth

The influence of Y 2 O 3 , Pr 2 O 3 , Nd 2 O 3 , Sm 2 O 3 and Yb 2 O 3 coatings on Fe-30Cr alloy oxidation behaviour was investigated at 1000°C in air under atmospheric pressure. Isothermal exposures indicated that the Y 2 O 3 coating was the most protective after 100 hours. Pr 2 O 3 , Nd 2 O 3 and Sm 2 O 3 coatings were less effective, but the less beneficial effect was observed when Yb 2 O 3 coating was applied onto the Fe-Cr alloy surface. Two-stage oxidation experiments in 16 O 2 and then 18 O 2 were performed to get information about the chromia growth phenomena with and without reactive elements. The 18 O-tracer distribution was determined by secondary ion mass spectrometry (SIMS) and…

Protection of iron-based alloys likely to be exposed to corrosive atmospheres

Summary This study examines the influence of temperature on oxidation kinetics of the alloys Z2CT 17 and Z4CN 18-09. A thin layer of aluminium oxide was laid down on these, using chemical, vapor deposition, in order to give improved protection.

Behaviour of the Alloys F17Ti and AISI 304 in an Atmosphere Containing Hydrogen Sulphide at High Temperature

Role of the Microstructure on the High Temperature Oxidation Properties of the Intermetallic Compound NbAl<sub>3</sub>

The influence of the NbAl 3 microstructure on its oxidation mechanism was investigated in air under atmospheric pressure over the temperature range 500-1080°C. Different processing techniques as induction melting and mechanically-activated annealing processes (M2AP) were used to produce the intermetallic compound NbAl 3 . A protective external alumina scale grew only on Al-enriched NbAl 3 between 700 and 1080°C. Stoichiometric NbAl 3 exhibited the pesting phenomenon between 550-900°C, whereas a non protective lamellar oxide scale formed above 900°C. The proposed oxidation mechanism explains these observations which are in agreement with the oxidation study of powders with different crystall…

The effect of rare earths deposited on steel surfaces, by different processes (sol/gel, electrophoresis, OMCVD), on high temperature corrosion behaviour

Abstract In this paper, different methods of rare earth oxides deposition on the surface of AISI 304 steel are described: sol/gel, electrophoresis in aqueous or organic medium, OMCVD of rare earth β-diketonates. After the deposition, all the samples were reheated. This treatment was absolutely essential for a strengthening effect of the protective oxide scale. Oxidation of coated and non-coated samples was performed under isothermal and cyclic conditions. The effect of rare earth deposition is significant in isothermal conditions. However, in thermal cyclic conditions, the effect of a rare earth deposited on surface is spectacular. Scale spallation is then completely avoided under the chose…

The Effect of Deposited Rare Earth Oxide Films on High Temperature Corrosion Behaviour of Different Metals and Alloys

Reactive Element (Y,Ce) Effect on Stresses Determined In-Situ during Iron Oxidation at 800°C

In-situ stress determinations by X-ray diffraction have been performed during pure iron oxidation (p(O 2 ) = 2 x 10 -3 Pa, T = 800°C). On yttrium implanted specimens the stress level is close to zero before oxidation. On CeO 2 coated specimens, the initial compressive stress due to surface polishing is not modified by the coating. During oxidation, the in-situ compressive stress value determined in the Fe (1-x) O scale, is not strongly dependent upon the reactive element nature. Nevertheless, the stress evolution during cooling and residual stresses determined after cooling to room temperature are very different. Blank and CeO 2 coated specimens show relatively high compressive stresses whe…

Self-Induced Stress and Matter Transport Interactions during Metal Oxidation

Effect of Microstructure on the High Temperature Oxidation and Pesting Behaviour of MoSi<sub>2</sub>

Investigations of the formation mechanism of nanostructured NbAl3 via MASHS reaction

Abstract The nanostructured NbAl3 intermetallic compound was synthesized using the mechanically-activated self-propagating high-temperature synthesis (MASHS) technique. This process results from the combination of two steps: a short duration ball-milling of a pure elemental Nb+3Al powder mixture followed by a self-propagating high-temperature synthesis (SHS) reaction induced by the Nb+3Al reaction exothermicity. Synchrotron time-resolved XRD coupled with a 2D infrared camera were used to investigate the structural and thermal evolutions during the SHS reaction, and to study in situ the mechanism of NbAl3 formation. The influence of the incoming heat flux and the mechanical activation effect…

High Temperature Oxidation Behaviour of Aluminide Coatings Obtained by Pack Cementation