Oxidation Of TiAl Research Articles

Study on the effect of promotors in CO2 utilization for syngas production via dry reforming of methane over Co-MOX/TiO2-Al2O3 (MOX = La, Ce, Mg, and K) catalysts

In this study, Co-based catalysts supported over Ti-Al oxide and promoted with La, Ce, Mg, and K metals were assessed for CO2 reforming of methane reaction to produce syngas. Titania-alumina mixed oxide supports were prepared using the template-assisted-solvothermal method, and then Co and promotors were co-impregnated over the as-prepared support. Different characterizations of catalysts showed that variation in promotor metal impacts these catalysts' physical and chemical properties. The Ti-Al oxide support possessed the perfect hexagonal morphology. Potassium-promoted catalysts possessed the highest number of basic sites, whereas the La-promoted catalyst possessed the highest number of acidic sites. La promotion improved the Co dispersion, while Mg promotion enhanced the metal support integration. La-promoted catalysts are deactivated because of active metal oxidation and the generation of hard carbon. The carbon was deposited in all catalysts; however, the activity of the Mg-promoted catalyst was unaffected. The intermediate surface basicity and strong metal support interaction improved the Mg-promoted catalyst's stability. The La and Mg-promoted catalysts possessed lower apparent activation energies.

Effect of TiAlNbCrSi Alloy Microstructure Produced by Mould Casting (MC) or Electron Beam Powder Bed Fusion (EB-PBF) on Scale Grown Under Dry Air and Steam

TiAl parts fabricated with additive manufacturing have begun to find applications in car and aviation industry. Optimization of their properties concentrates on introduction of new alloying additions improving the TiAl ductility and oxidation resistance. Their effect on the mechanical properties was worked out quite well, but the knowledge how they affect the oxidation processes is still limited. Therefore the present experiment was aimed at investigation of scale grown over mould cast (MC) and electron beam powder bed fusion (EB-PBF) Ti–48Al–2Nb–0.7Cr–0.3Si (at. pct) alloys. It was conducted at 650 °C for 1000 hours both in dry air and steam covering early stages of this process. The scale microstructure, chemical and phase composition was examined with the transmission electron microscopy (TEM). Applied treatment caused development of three-layer scale, i.e. with major portions of R-TiO2 + α-Al2O3/R-TiO2/α-Al2O3. The one formed during dry-air oxidation of the EB-PBF alloy was most compact. Steam oxidizing changed morphology of rutile present at its surface from rod/plate-like into whiskers. The Si turned out to be especially active during scale growth diffusing up to its surface. The presence of steam further increased mobility of both Si and Cr rising their presence in the upper part of the scale. The Nb was found to accumulate within the substrate area adjoining to the scale. Refinement of EB-PBF microstructure as compared with the MC alloy resulted in promoting reaction at the scale/substrate front contributing significantly to development of thicker oxide coating and nitrides bearing oxidation affected zone.

A comparative study on the oxidation behaviours of a TNM alloy in argon and oxygen atmospheres at 650 °C

This study investigated the oxidation behaviours of a TiAl (TNM) alloy oxidized at 650 °C in argon and oxygen. The oxidation mass gains were similar in both atmospheres up to 200 h of oxidation. Between 200 and 500 h, the mass gains increased by 20 % in the O2 atmosphere. This study examined the triplex oxide scales of outer (Ti–Al)O layer-by-layer and Mo/Nb-rich TiAl oxides on the TNM alloy after oxidation in Ar and O2. The scale structures formed in Ar and O2 were similar but differed from those formed in air, and the respective TNM samples’ oxidation behaviours were different.

Thermally-induced nonlinear optical properties of Ti-Al oxide nano-films with double epsilon-near-zero behavior

This research examines the thermally-induced nonlinear optical properties of Ti-Al-oxide nano-films. The Ti-Al nano-layer system was sequentially evaporated on glass and c-Si(100) substrates at room temperature. The Ti layers were spontaneously oxidized by the residual oxygen of the background vacuum in the deposition chamber. The Al layer, on the other hand, was partially oxidized and contained metallic inclusions. However, when oxygen was deliberately introduced into the chamber, the Al layer was oxidized with smaller inclusions. We experimentally demonstrate the existence of double epsilon-near-zero (2ENZ) behavior in the oxidized Al films by directly measuring the dielectric permittivity through ellipsometric analysis. The Maxwell-Garnett theory for a composite film formed by a mixture of Al and TiO2 inclusions in Ti-Al-oxide matrix was used to predict the two cross-over points of the ENZ behavior. Enhancement in the nonlinear optical response near the ENZ points was obtained.

STEM investigations of γ-TiAl produced by additive manufacturing after isothermal oxidation

Short-term isothermal oxidation of TiAl 48-2-2 alloy produced by Electron Beam (EBM) was investigated at 750, 800, 850 and 900 °C with special focus on the oxidation kinetics and metal-scale interface evolution. It was shown that at the lowest oxidizing temperature a passivating and thin mixture of alumina and titania is formed with subsurface Z-phase formation. At higher temperatures a porous and non-protective oxide scales formed with subsequent nitridation at the metal-scale interface. Moreover, long-term isothermal oxidation at 750 °C up to 3000 h in 500 h intervals was performed and related to the initial band microstructure of the alloy resulting from EBM process. Internal oxidation was observed on lamellar α2 / γ colonies while α-alumina mixed with TiN and Ti2AlN was found to form on equiaxed γ-TiAl regions. Analytical Scanning Electron Microscopy (STEM) aided with Energy Dispersive Spectroscopy (EDS) and Electron Energy Loss Spectroscopy (EELS) were used for investigations of the oxide scales as well as nitride layers formation at different temperatures.

Synthesis, Characterization and Methyl Orange Degradation Activity of Ti-Al Oxides Nanomaterial

The present work reports the synthesis of Ti-Al Oxides (TAOs) nanomaterial by an organic solvent-free impregnation method. The as-prepared products were studied by XRD, SEM-EDAX, UV-Vis, and FT-IR analytical techniques. XRD patterns revealed the fact that both TiO2 (TO)and TAOs exhibited almost similar peaks with the exception to minor peak difference in width and height which could be attributed to the alteration in the size of particles. At the calcination temperature of 500 oC, the absence of peaks for Al2O3 (AO) in the TAOs XRD pattern indicates the amorphous nature of AO. However, the appearance of Ti, Al, and O on EDAX and redshift on UV-Vis spectra confirm the successful impregnation of AO on TO. The pollutant degradation ability of TAOs photocatalyst was tested on methyl orange (MO) dye. Compared to pure TO, TAO composite has greater degradation efficiency. However, as the percentage of AO increases the degradation efficiency decreases. The conducted Langmuir model test was found to fit well for the photocatalytic reaction process following first-order reaction kinetics. Comparing the values for kinetics constant with earlier work, this work showed good MO degradation efficiency with reaction rate constant (k) values of 0.023.

Agglomeration Mechanism of Complex Ti-Al Oxides in Liquid Ferrous Alloys Considering High-Temperature Interfacial Phenomenon

This work presents the agglomeration mechanism of complex Ti-Al oxides in the liquid ferrous alloy. Cluster characteristics were investigated using Al and Ti/Al-complex deoxidation method in lab scale. The time-dependent size distribution, total number per volume, average size, and circularity of the clusters were quantitatively analyzed. Furthermore, high-temperature confocal laser scanning microscopy was utilized to directly observe the cluster formation of Ti-Al oxides. A capillary force model including wettability parameters was applied to compare the agglomeration capabilities of different types of non-metallic inclusions. When a low Ti is added into melt, the agglomeration of TiOx·FeO liquid inclusions is one of the key factors to decrease the frequency of cluster formation. When the Al is added into melt, the heterogeneous precipitation on TiOx·FeO surfaces is the main reaction process. Ti-Al oxides have lower agglomeration ability than that of Al2O3, which in turn, contribute to a low agglomeration frequency as well.

Study of the microstructure and small punch behavior of a 9Cr ODS tube

Abstract The microstructure of a 9Cr ODS tube (supplied by CEA) obtained by a powder metallurgy route followed by hot extrusion and cold pilgering of the final cladding, has been investigated by means of scanning and transmission electron microscopy. The alloy presents a weak fiber texture and a bimodal grain size distribution. Three different families of precipitates have been identified: isolated and large Ti-Al oxides, Cr rich precipitates placed preferentially at prior austenite grain boundaries and martensitic lath boundaries and a fine distribution of Y-Ti nano-oxides within grains. In order to correlate this microstructure with the mechanical properties, 3 mm diameter discs drilled from the tube wall were tested by small punch technique at room temperature, 300 °C and 500 °C to reproduce the load system to which the claddings will be subjected under service conditions. Changes in the slip character and failure mode as temperature increases were observed. At low temperatures the alloy exhibited a wavy slip character and a quasi-ductile fracture mode. At high temperature, 500 °C, the fracture tended to be more brittle, what may be attributed to an intergranular deformation mechanism leading to the weakening of grain boundaries.

Effect of Mo on microstructure and oxidation of NiCoCrAlY coatings on high Nb containing TiAl alloys

In order to improve the oxidation resistance of high Nb containing TiAl alloys, the microstructure and oxidation behavior of NiCoCrAlY(Mo) coatings were investigated at 900°C for 100h by isothermal oxidation tests. The oxidation resistance of TiAlNbCr alloys was significantly improved after adding coatings and the oxidation resistance of NiCoCrAlY coating was slightly better than that of NiCoCrAlYMo coating. A continuous and dense Al2O3 film was formed on the surface of NiCoCrAlY coating. Mo has negative influence on the oxidation resistance of NiCoCrAlY coating. It is found that Mo was oxidized to form MoO3, meanwhile the evaporation of MoO3 resulted in the spallation of oxide film and severe internal oxidation. However, Mo was beneficial to enhance the adhesion between coating and substrate due to the formation of Al2O3 pegs. Elemental interdiffusion at the interface of coating-alloy was inhibited by the MoO3 and Al2O3 diffusion barriers effectively.

A Study of the Frictional Layer of TiAl-12Ag-5TiB2 Composite During Dry Sliding Wear

Many studies have shown that the excellent tribological properties of materials are primarily attributed to the formation of expected frictional layer on the worn surface. This article is dedicated to explore the possible formation and acting mechanism of frictional layer of TiAl-12Ag-5TiB2 composite. At normal load of 12 N, a frictional layer that consists of wear-induced layer and plastic deformation layer is observed. The soft wear-induced layer supported by the harder plastic deformation layer leads to the low friction coefficient and high wear resistance. The harder plastic deformation layer is induced by repetitive tribo-contact and considerable plastic deformation. Its high hardness improves the wear resistance of composite, and fine-grained structure promotes the diffusion of lubricating phase during dry friction process. The soft wear-induced layer can be divided into tribofilm and mechanically mixed layer. The mechanically mixed layer that consists of Ag and Ti-Al Oxides can continuously be provided to the worn surface to form a tribofilm with low shearing stress junctions, lowering the friction coefficient.

Hydrogen gas sensing properties of nanoporous Al-doped titania

Nanoporous aluminum doped titanium (TiAl) oxide thin film sensor materials with an Al concentration of 9–10at.% were produced on alumina substrates and their hydrogen (H2) sensing behaviors were investigated. The well-adherent and dense metallic TiAl thin films with equiaxed structure were deposited on alumina substrates using cathodic arc physical vapor deposition (CAPVD) method and the nanopores were grown on the TiAl thin films by anodic oxidation in ammonium fluoride (NH4F) containing ethylene glycol (EG) electrolyte. With the deposition method utilized, it became possible to anodize the films without spalling and defects. The metallic film on alumina was totally anodized in order to eliminate short-circuit problems between the metallic layer below the oxide and Pt pads used for resistance measurements. The amorphous nanoporous structures were converted into anatase by heat treatment at 480°C for 3h and the fabrication of sensor was completed by depositing Pt contact pads on the surface of nanopores using magnetron sputtering technique. The hydrogen sensing performance of thin film nanoporous TiAl oxide sensor was studied in the range of 50–2500ppm hydrogen at different temperatures (25–350°C). The sensor characteristics in terms of sensitivity, response, recovery and stability were examined in detailed analysis and a possible hydrogen detection mechanism was proposed. The nanoporous TiAl oxide sensor exhibited promising sensing performance towards H2 gas concentrations and quick response and recovery behavior with high stability.

Band gap structure modification of amorphous anodic Al oxide film by Ti-alloying

The band structure of pure and Ti-alloyed anodic aluminum oxide has been examined as a function of Ti concentration varying from 2 to 20 at. %. The band gap energy of Ti-alloyed anodic Al oxide decreases with increasing Ti concentration. X-ray absorption spectroscopy reveals that Ti atoms are not located in a TiO2 unit in the oxide layer, but rather in a mixed Ti-Al oxide layer. The optical band gap energy of the anodic oxide layers was determined by vacuum ultraviolet spectroscopy in the energy range from 4.1 to 9.2 eV (300–135 nm). The results indicate that amorphous anodic Al2O3 has a direct band gap of 7.3 eV, which is about ∼1.4 eV lower than its crystalline counterpart (single-crystal Al2O3). Upon Ti-alloying, extra bands appear within the band gap of amorphous Al2O3, mainly caused by Ti 3d orbitals localized at the Ti site.

High Temperature Oxidation Behaviour of Tial8Nb Alloy

The goal of this work is to determine the effect of niobium on the kinetics and mechanism of Ti-Al oxidation in air. In order to compare the oxidation kinetics of Ti-Al and Ti-Al with the addition of niobium, isothermal oxidation was performed on Ti-48Al and Ti-46Al-8Nb (in at.%) alloys at 1073 K in synthetic air. Cyclic oxidation of Ti-46Al and Ti-46Al-8Nb alloys was carried out in laboratory air for 42 cycles (1 cycle, 24 hrs). The morphology, as well as chemical and phase composition of the oxidation products were investigated using X-ray Diffraction (XRD) and Scanning Electron Microscopy combined with Energy Dispersive Spectroscopy (SEM-EDS). From these investigations it can be concluded that niobium addition increases the corrosion resistance of TiAl and, furthermore, improves the adherence between the metallic substrate and the oxide scale. The oxidation mechanism of Ti-46Al-8Nb was studied via secondary neutral mass spectroscopy (SNMS) after two-stage isothermal oxidation (24 hrs in 16O2 followed by 24 hrs in 18O2) at 1073 K. From this analysis it can be assumed that the oxidation mechanism of Ti-46Al-8Nb alloy consists of simultaneous outward titanium and aluminum diffusion and inward oxygen transport.

Isothermal oxidation behaviour of beta gamma powder metallurgy TiAl–4Nb–3Mn alloys

A new TiAl–4Nb–3Mn beta gamma alloy was synthesised by a powder metallurgy process. HIPed and vacuum heat treated specimens were isothermally oxidised at 800 and 900°C in air up to 500 h. The TiAl–4Nb–3Mn alloys oxidised parabolically up to 500 h at both 800 and 900°C. The oxides consisted of outer TiO2 layer, intermediate Al2O3 layer and inner TiO2 rich mixed layer and the oxidation mechanisms of the alloy were identical at both temperatures. During oxidation, the degradation of α2 lamellae in the vicinity of the interface forms a diffusion zone (lamellar depleted zone) leading to the formation of Nb and Mn rich white layers just below the interface by outward diffusion of Nb and Mn which are released from breakdown of α2 lamellae. As exposure time increases, Nb begins to diffuse earlier than Mn and diffuses more actively at higher temperature. The activation energy for oxidation of TiAl–4Nb–3Mn alloy was lower than that of Ti–48Al alloy and was higher than those of Ti–48Al–2Nb–2Cr and Ti–48Al–2Nb–2Cr–W alloys.On a synthétisé un nouvel alliage TiAl–4Nb–3Mn de type bêta gamma par un procédé de métallurgie des poudres. On a oxydé en isotherme à 800 et à 900°C à l’air, jusqu’à une durée de 500 heures, les spécimens pressés par HIP et traités thermiquement sous vide. Les alliages de TiAl-4Nb-3Mn s’oxydaient paraboliquement jusqu’à 500 heures tant à 800°C qu’à 900°C. Les oxydes consistaient en une couche externe de TiO2, en une couche intermédiaire d’Al2O3 et en une couche interne mixe riche en TiO2 et les mécanismes d’oxydations de l’alliage étaient identiques aux deux températures. Lors de l’oxydation, la dégradation de lamelles d’α2 dans le voisinage de l’interface forme une zone de diffusion (zone lamellaire appauvrie) menant à la formation de couches blanches riches en Nb et en Mn juste au-dessous de l’interface par diffusion vers l’extérieur du Nb et du Mn, qui sont relâchés par la dégradation des lamelles d’α2. À mesure que la durée de l’exposition augmente, le Nb commence à se diffuser plus tôt que le Mn et se diffuse plus activement à une température plus élevée. L’énergie d’activation pour l’oxydation de l’alliage de TiAl–4Nb–3Mn était plus basse que celle de l’alliage de Ti–48Al et était plus élevée que celle des alliages de Ti–48Al–2Nb–2Cr et de Ti–48Al–2Nb–2Cr–W.

High Temperature Oxidation of TiAl and TiAl8Nb Alloys in Air

Isothermal oxidation of two g-TiAl-based intermetallic alloys: Ti48Al and Ti46Al8Nb alloys was studied in synthetic air at 1073-1223 K for up to 240 hrs. Mass change per unit area for the oxidized samples followed approximately the parabolic rate law. The kp values for the studied temperature interval were in the range from 7.2×10-13 to 1.8×10-11 g2cm-4s-1. The activation energy for oxidation of Ti48Al alloy in air at 1073-1223 K was Ea = 165±12 kJ/mol. Niobium addition to Ti48Al alloy in the amount of 8% increased its oxidation resistance. Structure and chemical composition of the oxidation products, and morphology of the oxidized samples were investigated using XRD, SEM-EDS, and TEM. The oxide scales formed on Ti48Al and Ti46Al8Nb alloys were well adherent to the metallic substrates and exhibited a multilayer structure. Depending on the oxidation temperature and the alloy composition, the scale consisted of variable amounts of TiO2 and Al2O3. Additionally TiN, and niobium rich particles were also identified in the appropriate oxide scale. To understand the growth mechanism of oxide scale formed on Ti46Al8Nb alloy, two stage oxidation experiment was performed using 16O2/18O2, followed by SNMS and TEM-EDS. Particular attention was paid to the use of TEM in order to precisely characterize the reaction products on the Ti46Al8Nb alloy.

Formation and evolution of phosphorus-containing species during high-temperature oxidation of TiAl dipped in a low-concentrated phosphoric acid solution

TiAl samples were dipped in a low-concentrated phosphoric acid solution, air dried, then heated to 800 °C under argon gas before oxidation under reconstituted air at this temperature. At the end of the heating ramp, sample surfaces were covered by a titanium pyrophosphate (TiP 2O 7) layer, well adhered to the substrate. This pyrophosphate was shown to be detectable from 400 °C. Chemical reactions have been proposed for the formation of this compound. TiP 2O 7 coating strongly increased the oxidation resistance of TiAl at 800 °C for 100 h. After 100 h, a transition period occurred leading to the change of TiP 2O 7 into TiO 2, which was achieved after 230 h, total mass gain always remaining below that of raw substrate. This evolution was demonstrated not resulting from a thermal decomposition of pyrophosphate compound.

Modelling of the long time stability of the fluorine effect in TiAl oxidation

Recently the target temperature of components manufactured from γ-TiAl alloys like turbine blades, turbocharger rotors or automotive valves has been increased to 900 °C. However, there is an insufficient oxidation resistance above 750 °C. One method used to improve the γ-TiAl oxidation behaviour is the so-called fluorine microalloying effect. After application of fluorine to the TiAl surface by ion implantation or treatment with diluted HF and oxidation at 900 °C in air a dense alumina layer is formed. However, after the treatments a distinct loss of fluorine was observed during heating and within the first hours of oxidation. In this work, the long time behaviour during isothermal and cyclic oxidation up to 1500 h/900 °C/air was investigated showing a slow fluorine decrease. The alumina layer acts as a diffusion barrier for fluorine, whereas fluorine diffuses into the metal. The diffusion coefficient was calculated. The results fit the theoretical model of the fluorine effect.

Effect of niobium on the structure and high-temperature oxidation of TiAl–Ti 5Si 3 eutectic alloy

Structure and high-temperature oxidation of Ti–40Al–5Si and Ti–39Al–5Si–2Nb alloys at 900 °C in air are studied. The structure, chemical and phase composition of the as-cast and oxidised alloys are determined by using scanning electron microscopy, energy dispersive spectrometry and X-ray diffraction. Oxidation kinetics is measured by weighing of samples and by using thermogravimetric analysis. It is found that the addition of 2 at.% of niobium to the eutectic TiAl + Ti 5Si 3 alloy has no significant effect on the structure consisting of TiAl matrix and silicide fibers or lamellae. Niobium homogeneously dissolves in both eutectic phases. In contrast, niobium markedly influences oxidation kinetics, since it reduces oxidation rate about six times. This reduction results from the doping effect and from the activation of Al. Silicon also plays a role in the oxidation process but its effect on the Al activation is weaker than of Nb. In addition, silicon is shown to stabilize anatase modification of TiO 2 in the scales.

Effects of Nb and Si on high temperature oxidation of TiAl

The isothermal oxidation behavior of TiAl based alloys at 900 °C in air with a combination of Nb (5%-10%, mole fraction) and Si (1%-5%, mole fraction) was investigated. The microstructure and the composition of the oxidation scale were studied by using XRD, TEM and EPMA. The results show that the combination of Nb and Si can improve the oxidation resistance of the alloys significantly. The element Si can change the typical microstructure of oxidation scale on TiAl based alloys. In alloys with Si addition, the compact Al 2O 3 forms in the interior side of oxidation scale. When x(Si)>3%, the Ti 5Si 3 phase forms and the coarse crystal TiO 2 forms on Ti 5Si 3 phase after oxidation. The increase of Nb content in the TiAl based alloys impedes the growth of Ti 5Si 3 phase, and the formation of TiO 2 on surface and on Ti 5Si 3 phase is also impeded.

(S)TEM study of different stages of Ti–45Al–8Nb–0.2W–0.2B–0.02Y alloy oxidation at 900 °C

The oxidation process of Ti–45Al–8Nb–0.2W–0.2B–0.02Y (at%) can be divided into three stages during 100 h at 900 °C. The change of ion diffusion path leads to the discontinuity of the oxidation curve between the second and third stage. Different stages of oxidation morphology have been investigated combined with TEM and STEM. AlNb 2 phase forms at the interface of oxide scale/base alloy. Contrary to a continuous formation of ‘x’ phase in γ-TiAl with Nb free at the interface of oxide scale/base alloy. AlNb 2 phase hinder continuous ‘x’ phase formation, which can promote TiAl oxidation protection.