Jean Pierre Larpin

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-products were characterized. Various process controlling parameters such as mechanical activation milling conditions have been studied.

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 on the phase transformation kinetics induced by the SHS process were studied. Owing to the temporal resolution of 100 and 200 ms between two consecutive diffraction patterns and IR images, respectively, it was shown that solid niobium reacts with liquid aluminium via a heterogeneous nucleation reaction, to form the NbAl3 compound by successive combustion fronts.

Synthesis of nanocrystalline NbAl3 by mechanical and field activation

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). Using XRD analyses, the MAFAPAS end-product was identified as NbAl3. Back-scattered electron SEM observations coupled with EDXS analyses showed the presence of small amounts of alumina precipitates together with unreacted niobium in the NbAl3 matrix. The end-product relative density ranged from 85 to 96%. The NbAl3 crystallite size, determined by XRD line-broadening analysis using the Langford method, was in the range of 57–150 nm.

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. Owing to the temporal resolution of 100 ms between two consecutive diffraction patterns, it was possible to observe several steps before obtaining the niobium aluminide compound. Indeed, the phase transformations corresponding to the aluminum melting plateau, the subsequent temperature increase to the ignition temperature, and the fast reaction between niobium and molten aluminum at such a temperature were well-identified. The NbAl3 intermetallic compound resulting from the MASHS process is nanostructured.

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 chosen experimental conditions. The adherence of the protective scale could be due to an inversion of the defect structure oxide as it has been suggested in literature, but no experimental evidence to support this can be put forward from this work.

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 sputtered neutral mass spectrometry (SNMS). The experiments performed on uncoated samples clearly demonstrated that chromia growth mechanism was controlled by chromium cationic diffusion, whereas on reactive element coated samples the external diffusion of chromium ions was not predominant.

Influence of reactive element oxide coatings on the high temperature cyclic oxidation of chromia-forming steels

Abstract The beneficial effects of reactive element oxide coatings, Y2O3, Nd2O3 and Yb2O3, on the cyclic oxidation behaviour of a Fe–30Cr model alloy, were studied at 950xa0°C in air under atmospheric pressure. The oxide scale grown on uncoated samples started to spall after 2 cycles, because of its poor adherence. The coated specimens remarkably resisted to the thermal shocks, since no oxide scale spallation or cracks could be observed, even after 1000xa0h of cycling experiments. The surface oxide scale consisted of facetted oxide grains on the uncoated samples. On the contrary, it exhibited a fine-grained structure on the coated specimens. The reactive element beneficial effect was ascribed to reactive element ions or second phase particles segregation along the chromia grain boundaries, minimising the chromium cation outward diffusion.

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

The effect of Y+Zr+Hf and La+Ce on the oxidation behavior of FeCrAl alloys was tested at 1373 K in laboratory air under atmospheric pressure. Oxidation tests performed under different oxygen isotope atmospheres indicated that after 8 h of oxidation, the mixed diffusion of oxygen and aluminum controlled the alumina scale growth. After 96 h of oxidation, the oxide scale mainly grew via oxygen diffusion. The chemical analyses of the alumina scales allowed location of the reactive elements at the alumina grain boundaries and at the metal-oxide interface after the longer oxidation tests, but never after the shorter tests. These results are discussed in relation with the transient oxidation stage necessary for incorporation of the reactive elements into the growing oxide scale.

The influence of yttrium implantation on the oxidation of impure iron containing C and Mn at 700°C

A study of impurity-yttrium interactions hasbeen performed during iron oxidation [p(O2)= 0.04 Pa, T = 700°C]. Yttrium-implanted specimensalways exhibit better oxidation behavior compared withblank specimens. On pure iron or the Fe 0.054 wt.%C alloy thebeneficial effect is attributed toFe2YO4 formation. With themanganese-containing alloys (Fe 0.2 wt.%Mn), theprotective effect of yttrium is attributed to YMnO3 formation. The best oxidationbehavior is obtained with implanted Fe0.18 wt.%Mn-0.041wt.%C alloys due to the formation of an YMnO3oxide subscale at the scale-alloy interface. Yttriumimplantation also hinders carbon segregation at theoxide-alloy interface. This effect ensures better scaleadherence. With the most-impure alloy, yttriumimplantation also changes the growth process fromexternal cation diffusion to predominant inward-oxygendiffusion.

High Temperature Oxidation Behaviour of Aluminide Coatings Obtained by Pack Cementation

Currently, aluminide compounds are the most effective materials to resist under high temperature conditions because they combine many beneficial properties (low density, high temperature mechanical properties...). Thus, an Al2O3 scale can be formed. Moreover, the addition of small particles of oxygen-active-elements into materials can allow to improve their corrosion resistance thereby increasing the adhesion of the oxide scale. The aim of our study was to form aluminide coatings on the surface of a model alloy (Fe-30Cr) by aluminisation at 900°C or 1000°C for 3 to 5 hours using a pack-cementation process. Some of these materials were doped with yttria which was introduced by metal organic chemical vapour deposition (MOCVD) in addition to the pack cementation. The high temperature oxidation resistance of doped and undoped aluminised alloys was studied by oxidising the substrates at 1000°C for 100 hours in a thermogravimetric analyser (TGA) in air under the atmospheric pressure. The samples were also submitted to cyclic oxidation tests at 1000°C. The aluminide materials exhibit an excellent high temperature oxidation resistance at 1000°C. Their general behavior is close to that of commercial alumina-forming steels. The resistance of the samples doped with yttria is more or less improved.