Spark plasma sintering (SPS) enables the manufacturing of TiAl alloys with an exceptional combination of low density and mechanical properties such as acceptable ductility at room temperature and high strength at high temperature. However, TiAl alloys are known to exhibit low oxidation resistance above 700°C. The oxidation of a Ti-48Al-2W-0.1B (at. %) processed by SPS was investigated at 800 °C. Coupons were oxidized in air and in Ar-21O2 (vol. %), in isothermal and cyclic tests. In air, the alloy formed a mixture of Ti and Al oxides, but oxidation was slower than typically observed for W-free alloys. The oxide scale and underlying alloy were characterized by X-Ray diffraction and electron microscopy in order to examine the beneficial role of W in the oxidation resistance. The main constituents of the reaction product after reaction in air may be described as follows (from gas to alloy): TiO2 / porous Al2O3 / TiO2 + Al2O3 / TiN + Al2O3 + W / TiAl2 + W / TiAl. Close examination suggests that the relatively good oxidation resistance of the alloy is related to W doping of TiO2 in the mixed Al2O3 + TiO2 layer. The alloy formed an Al-rich oxide scale with much slower kinetics in Ar-21O2, confirming the detrimental role of N in the oxidation process.
Hypersonic airliner would be exposed to temperatures that are beyond the limits of classical aircraft materials. In order to handle this problem the latest developments of new materials and composite structures suitable for high temperature application need to be taken into account. The focus of the European Research program ATLLAS is on advanced light-weight, high-temperature material development strongly linked to a high-speed passenger aircraft design. ATLLAS stands for Aerodynamic and Thermal Load Interactions with Lightweight Advanced Materials for High Speed Flight. The 4.5 years program ATLLAS-II is a logical continuation project built upon the experience and technology development gained within ATLLAS-I. The corresponding work related to combustor structures and material integration deals with the opportunity to investigate at academic level, both in basic and relevant environment, different solutions possibly usable to ensure the long range cruise of a high speed airliner. Different materials (UHTC, CMC, metallic) and different cooling techniques (radiation, convective, transpiration) are studied. Available 2 numerical or semi-empirical tools are used to prepare the test, to design the different architectures. A pin fin experiment allows to better know the pressure drop and the heat transfer for different channel patterns with thermal crystal techniques. The ERBURIG K long duration test facility allows to characterize different ceramic matrix composite uncooled samples to realize, at small scale, a long duration (several hours) investigation of cooled ceramic structure in PTAH-SOCAR technology. A multifunctional metallic transpiration cooled HSS panel using Hollow Spheres Stacking as core material was designed and preliminary tested in cold conditions with GN2 and in hot conditions with infra-red lamps under 1 MW/m² heat flux before successful METHYLE testing in supersonic reacting flow. CMC and UHTC materials are used to design, manufacture and test generic fin injectors usable in high speed combustors. Industrial hypersonic METHYLE test facility is used to test in relevant Mach 6 combustor environment CMC and HSS panel as well as advanced fin injectors. Hot and cold permeability of composites is also documented with GN2 and GH2, taking into account the mechanical stress possible effect. Numerical models are used in accordance with the experiments, some examples are also given in the present paper. NomenclatureC/C-SiC = carbon fibre reinforced silicon carbide CFD = computational fluid dynamics CMC = ceramic matrix composite ERBURIG K = Environmental Relevant Burner Rig -Kerosene HSS = Hollow Spheres Stacking (sandwich) FEM = finite elements (mechanical computational) method GN 2 = gaseous nitrogen METHYLE= French acronym for long duration hypersonic technology test facility O 2 = oxygen SiC/SiCN = silicon carbide fibre reinforced silicon carbonitride TLC = Thermochromic Liquid Crystals UHTC = ultra high temperature ceramics
This paper focuses on manufacturing process of regular Metallic Hollow Sphere Structures (MHSS) through brazing technique. As a large stress level is generally confined into the necks formed by brazed spheres, the influence of the filler material on mechanical behavior of cellular metal has been studied. The microstructures of joints resulting from nickel hollow spheres brazing with different commercial fillers "MBF 30" and "MBF 1006" were compared by Scanning Electron Microscopy (SEM) and microhardness testing. These studies revealed a wide boron diffusion into nickel shells through grain boundaries for "MBF 30" brazing, with the formation of borides in a fine brittle eutectic structure. Conversely it was observed that the eutectic structure concentrates at the necks for "MBF 1006" and can be completely eliminated by diffusion-brazing, despite of the shells thinness. The uniaxial compressive tests of HSP specimens have shown two different strain mechanisms depending on brazing process.
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