High-Temperature Oxidation Behaviour of a TiAl-Based Alloy Subjected to Aluminium Hot-Dipping

Intermetallic light weight TiAl-alloys are expected to replace the heavy Ni-based super alloys in several high temperature applications. However until now they cannot be used at temperatures above 700°C for longer times due to their insufficient oxidation resistance. The high temperature oxidation behavior can be improved drastically for the use at temperatures up to at least 1050°C by small amounts of fluorine in the surface region of TiAl-components. A thin protective alumina layer is formed after an optimized fluorine treatment during exposure in oxidizing high temperature environments. Results of isothermal and thermocyclic high temperature oxidation tests of untreated and halogen treated TiAl-samples of new types of TiAl-alloys containing Mo, Cu and Si will be presented in this paper. These results will be compared and discussed considering the beneficial effect of fluorine for a later use as e.g. turbine blades in jet engines. Key words: Titanium aluminides, high temperature oxidation, halogen effect,

Enhancing the wear and oxidation behaviors of the Inconel 718 by low temperature aluminizing

Oxidation behaviour of a Mo(Si,Al) 2 based composite at 1500 °C

Anodic bonding is a widely used process in research and industry for MEMS Applications. The main advantage is that it is a very reliable process, that results in a strong and hermetically sealed bond. The bonding process is also very stable to perform. These properties are due to the process sequence of heating the wafers, to be bonded, to a temperature in the range of 300 to 450°C. One of these two wafers has to be made of borosilicate glass (e.g., SCHOTT Borofloat33®). In such glasses, sodium ions (network modifier) are dissolved out of the glass network in the temperature range mentioned and thus become free and mobile. If now a negative voltage of typically 1000V is applied to the glass wafer, the free positively charged sodium ions will move away from the contact area of both wafers, the later bonding interface, under the influence of the electric field. This results in a displacement current, which decreases again after a maximum is reached, due to the limited available number of charge carriers (sodium ions). This ultimately leads to a depletion of the charge carriers in the glass. It is generally assumed that a linked effect occurs in the bond field, namely that oxygen ions migrate from the glass to the bond interface, where they cause the actual bond formation via anodic oxidation.Even though this basic concept of the anodic bonding corresponds to state of the knowledge of how the anodic bond is actually formed [1], it is difficult to prove in practice. In order to gain a deeper insight into the formation of anodic bonding and thus open up new applications and even wider utilization of anodic bonding, a first basic experiment was done as follows: A glass wafer was provided with a thin aluminum layer by a sputtering process and another glass wafer was anodically bonded to this aluminum layer. The typical bonding current (depletion and current drop) was observed and the resulting composite of two glass wafers with a very thin intermediate aluminum layer was examined and tested for bonding quality. First of all, it can be stated that bonding formation has taken place locally between the the 6" wafers. Unfortunately, particle contamination could not be completely avoided during processing leading to larger unbonded areas, especially in the center of the wafer. Nevertheless, the bonded areas allow for evaluation of bonding quality. The sputtered aluminum layer has a higher thickness in the center, due to the fact that the glass wafer was eccentrically rotated below a small sputter target. This inhomogeneity effect enables the characterization of different Aluminum layer thicknesses in one sample. The sputter time was set, that at the wafer center an untransparent Aluminum layer was deposited, while at the wafer edge just a few nanometers of aluminum forming a semitransparent layer. After the bonding experiment it was noticed that in all the bonded area the aluminum appears rather homogeneously transparent. According to that it can be concluded that the aluminum layer is oxidized during anodic bonding transforming into transparent Aluminum oxide. This finding is a clear confirmation of the general thesis, that during anodic bonding an anodic oxidation takes place at the bond interface. In further examinations the influence of the bonding parameters on this oxidation process will be investigated. It is expected that with higher temperature and bonding voltage the oxidation will be enhanced, ending up with an optically highly transparent layer of Aluminum oxide. This might be relevant for emerging optical applications requiring anodic bonding as reliable bonding process together with optical transparence for visual light. As blade tests have shown the bonding conditions in the first process are not yet optimal – the bonding strength was rather low, but darker spots in the crack opened area indicate a transfer of the aluminum oxide layer. By variating and improving bonding conditions, a deeper insight into the details of the oxidation process during anodic bonding will be available. So far it is known that easy to oxidize materials (Silicon and Aluminum) can be well anodically bonded while rather inert materials (Silicon Nitride) are hardly or not bondable with this procedure. For that reason, some materials like Platinum, Gold, and Titanium these predictions will be evaluated in practice. With this more profound knowledge it might be possible to predict the anodic bonding behavior of different materials in the future.Literature:[1] Lapadatu, A. C., & Jakobsen, H. (2015). Anodic bonding. In Handbook of Silicon Based MEMS Materials and Technologies (pp. 599-610). William Andrew Publishing. Figure 1

High temperature oxidation of Ti–46Al–6Nb–0.5W–0.5Cr–0.3Si–0.1C alloy

High-temperature oxidation behavior of Ti2AlNb alloy with PEO/hBN composite coating at 1000 °C

High-temperature oxidation behavior of any materials depends on the stability of the protective oxide formed on the surface of the materials. The chromium oxide, Cr2O3, formed on Fe-Cr alloys is unstable above 850 °C, whereas the alumina, Al2O3, scale formed on Fe-Al alloys is stable up to 1350 °C. Consequently, Fe-Cr alloys are not suitable for high-temperature applications, especially above 850 °C. In contrast, Fe-Al alloys containing a sufficient amount of Al exhibit an excellent oxidation resistance at high temperatures due to the formation of a continuous and robust layer of Al2O3. However, the high amount of Al required for the formation of a robust layer of Al2O3 on Fe-Al alloys makes the alloys brittle and restricts their structural application. The required critical content of Al for the formation of a protective layer of Al2O3 on Fe-Al alloys can be reduced by the addition of the “third element” like Cr, which improves the ductility of the alloy. In addition, the required critical Al content can further be reduced by decreasing the alloy grain size to the nano-regime. Based on the available literature, it can be hypothesized that the nanocrystalline (NC) structure can enhance the protective oxide formation, and further reduce the required critical content of Al for the formation of a continuous layer of Al2O3 on Fe-Cr-Al alloys. Although the oxidation behavior of Fe-Cr-Al alloys has been studied by several researchers, the studies are limited to the microcrystalline structure of the alloys. The effect of NC structure on the oxidation behavior of Fe-Cr-Al alloys has yet not been reported. Therefore, the present work primarily focuses on the study the oxidation resistance of NC Fe-Cr-Al alloys vis-a-vis the oxidation behavior of their microcrystalline (MC) counterparts.The NC Fe-Cr-Al alloys powders (of compositions Fe-20Cr-5Al, Fe-20Cr-3Al Fe-10Cr-5Al and Fe-10Cr-3Al) were synthesized using high-energy ball milling followed by a rapid consolidation using spark plasma sintering at 900 °C with the application of a pressure of 90 MPa. Out of these four alloys, Fe-20Cr-5Al and Fe-20Cr-3Al alloys were selected to study the effect of the NC structure on their high-temperature oxidation behavior. The oxidation behavior of the NC Fe-20Cr-(3,5)Al alloys at temperatures range (500-900 °C) in 60 h of oxidation was compared with that of their MC counterparts. The oxide scales formed on the NC and MC alloys were analyzed for morphology, chemical composition, and thickness of oxide using different characterization techniques. The post-oxidation characterization shows a remarkable effect of NC structure on the oxidation behavior of the Fe-20Cr-(3,5)Al alloys. The NC Fe-20Cr-(3,5)Al alloys exhibit superior oxidation resistance at high temperatures than that of their MC counterparts due to the formation of a continuous protective layer on the NC alloy. Contrary to the oxidation behavior of common steels at high temperatures, the Fe-20Cr-(3,5)Al alloys show better oxidation resistance at high temperatures (800 and 900 °C) than that at relatively low oxidation temperatures (500 and 700 °C) due to the formation of a considerably more protective oxide layer at high temperatures (800 and 900 °C). Further, the NC structure also influences the “third element effect” of Cr. Consequently, the protective oxide formed on NC Fe-20Cr-5Al alloy with the assistance of the “third element effect” of Cr at 500 and 700 °C, whereas the oxide formed on the NC Fe-20Cr-5Al alloy without the assistance of the “third element effect” of Cr at 800 and 900 °C. On the other hand, the oxide formed on MC Fe-20Cr-(3,5)Al alloys without the assistance of the “third element effect” of Cr at all oxidation temperatures (500-900 °C). Based on the post-oxidation characterization and available literature, the mechanisms for the formation of oxide scale on the Fe-Cr-Al alloys are proposed. The thesis provides the evidence to validate the hypothesis that NC structure can extensively enhance the formation of protective oxide on the Fe-Cr-Al alloys at high temperatures. In addition, the thesis also presents that the NC structure influences the role of the “third element effect” of Cr for the formation of protective oxide on the Fe-Cr-Al alloys at high temperatures. Thus, the present research work has provided a comprehensive overview of the oxidation behavior of NC and MC Fe-Cr-Al alloys at high temperatures.

High-temperature oxidation behavior and mechanism of Inconel 625 super-alloy fabricated by selective laser melting

To achieve high thermal-to-electric conversion efficiency and to make solar technologies cost-competitive with conventional electric power generation, the U.S. Department of Energy (DOE) launched the SunShot Initiative in 2011. Unlike photovoltaic (PV) systems, concentrating solar power (CSP) technology captures and stores the sun’s energy in the form of heat, using materials that are low cost and materially stable for decades. This allows CSP with thermal energy storage (TES) to deliver renewable energy while providing important capacity, reliability and stability attributes to the grid, thereby enabling increased penetration of variable renewable electricity technologies. Today’s most advanced CSP systems are towers integrated with 2-tank, molten-salt TES, delivering thermal energy at 565°C using molten nitrates for integration with conventional steam-Rankine power cycles. Higher efficiencies are obtained integrating CSP plants with a supercritical CO2 (sCO2) Brayton power cycle. To achieve this integration the next generation CSP (Gen3 CSP) needs to operate at temperatures above 550°C requiring high-temperature advanced fluids in the range of 550°C to 750°C. New salts are required to operate in this higher temperature range because nitrates are unstable at temperatures above 620°C. Selection of a high-temperature molten salt is needed, especially with regard to its compatibility with containment materials with acceptable mechanical strength, durability, and cost targets at these high temperatures. Chloride and carbonate salt blends have been proposed and tested, but each brings new challenges. The corrosion mechanism differs among candidate salts and information is needed for CSP component designers. Because of their low cost, and high decomposition temperatures, molten chlorides are the top candidates. However, these molten salts introduce a set of technological and engineering challenges because of their very corrosive characteristics for typical materials. Corrosion mitigation approaches are been investigated to obtain degradation of containment materials around 20 µm/year or lower. From the salt handling, and thermal energy density point of views molten ternary eutectic carbonate Na2CO3/K2CO3/Li2CO3 is the best heat transfer fluid and TES for Gen3CSP, but its cost is extremely high. Corrosion in molten chlorides is controlled in atmospheres without oxygen and water. If these impurities are present, molten chlorides become very corrosive in the liquid and vapor phases. Catastrophic mechanical failure is then possible because intergranular attack is the corrosion mode. Some researchers have proposed the redox potential control using active-metals such as Mg to reduce corrosion rates to below 10 µm/year at 800°C but the use of no-oxygen/water in the atmosphere is also required. Other corrosion mitigation approach is the use of surface treatments such as in-situ passivation, pre-oxidation, coatings, and diffusive coatings such as boronizing and aluminizing. Several nickel-based (NiCo)CrAl(Y,Ta,Hs,Si) coatings have been tested in molten carbonates and chlorides from 650°C to 750°C using electrochemical techniques to determine corrosion rates, and mechanisms. Several high-alloyed stainless steels, nickel superalloys, and alumina forming alloys have been tested. Untreated In800H and 310SS alloys corroded rapidly (~2,500 to 4,500 µm/year) in molten chlorides and carbonates. The lowest corrosion rate in molten chlorides of 190 µm/yea was obtained for atmospheric plasma spray NiCoCrAlY coatings pre-oxidized in zero air (ZA) at 900°C for 24 h with a heating/cooling rate of 0.5°C/min. The corrosion of the alloys exposed to molten carbonate at 700°C in bone-dry CO2 atmosphere was reduced from ~2,500 µm/year to 34 µm/year when coated with high-velocity oxyfuel NiCoCrAlHfSiY and pre-oxidized (ZA, 900°C, 24 h, 0.5°C/min). Metallographic characterization of the corroded surfaces showed that the formation of a uniform thin alumina scale before exposure to the molten salts considerably reduced the corrosion of the alloy. However, the rates of corrosion determined herein are still large, highlighting the relevance of testing materials durability in solar power applications. Further electrochemical impedance spectroscopy tests and metallographic characterization showed that the best performing alumina forming alloy was In702 pre-oxidized in ZA at 1050 °C for 4 h due to the formation of protective, dense and continuous alumina layers. But these layers were unstable when argon was used as the carrier gas during corrosion evaluations. Corrosion results in static ZA are promising for next-generation CSP applications using molten chlorides because alumina scales were stable after 185 h of immersion in the oxygen-containing atmosphere. Alumina layers in pre-oxidized AFA In702 grew from 5 µm (before immersion) to 13 µm (after 185 h of immersion). The use of these alloys could be commercial feasibility and cost-effective because of the possibility of using oxygen-containing atmospheres instead of keeping enclosed systems with inert atmospheres to protect alloys from corrosion in molten chlorides.

[EN] TiAl intermetallic have demonstrated excellent behavior at high temperature, however, the processing for producing coatings is not easy due to its high melting point, otherwise the coaxial laser cladding process promise to be an excellent tool for obtaining extensive overlapping coatings, achieving complete fusion and deposition of alloys with high melting point on surfaces with complex shape. In this work we study the parameters of coaxial laser process and preheating the substrate to achieve Ti48Al2Cr2Nb intermetallic coatings on Ti6Al4V sheet 3 mm thick, in order to improve the tribological, oxidation and corrosion behavior of the Ti6Al4V alloy. The geometrical and chemical dilution analysis of the single tracks obtained at different levels in the laser processing variables were able to identify combinations that minimize defects such as cracks, high dilution and inadequate aspect ratio. It found a direct relation between the cooling rate and the coaxial laser process parameters such as the powder feeding rate and scanning velocity. Thus the process was optimized by minimizing the cooling rate with decreasing the velocity. After this was selected as appropriate preheating temperature 350 ºC and were obtained coatings with 40% overlap, using process parameters which generate laser specific energy of 70, 80, 90 and 180 J/mm2, then they have been evaluated by optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffraction (XRD), Vickers micro-hardness (HV) and nanoindentation. The microstructure of the coatings consists gamma-TiAl phase and alfa2-Ti3Al.\nPreheating the substrate has allowed obtaining coatings with good metallurgical bond, although cracks and pores are observed for some conditions. It is noted that the expected variation in chemical composition from coating surface to the substrate was found, with low dilution of vanadium. The hardness of the TiAl laser coatings is higher than the substrate and the bending tests results shown that the coatings have good adhesion but with limited ductility. The tribological properties of the coatings shows that in the wear tests at room temperature a lower wear rate is obtained compared to the substrate. In the case of high temperature, the coatings have a lower coefficient of friction; however, a higher wear rate is obtained when compared with the substrate. The coatings have good resistance to oxidation evaluated by isothermal oxidation tests in air at 800 ºC, when compared with the substrate, the thermal growth oxide up to 12 microns thick for 150 hours were obtained. The structure of the oxide layers is complex and comprises the growth of successive layers from the outer surface of the coating. We also studied the electrochemical corrosion behavior of the coatings obtained. The results indicate that the coaxial laser cladding can be a good alternative to obtain extensive TiAl intermetallic coatings, dense coatings with good substrate bonding and minimal defects were obtained, that improve the oxidation and wear behavior of Ti6Al4V alloy.

Influence of process parameters and initial microstructure on the oxidation resistance of Ti48Al2Cr2Nb coating obtained by laser metal deposition

High temperature oxidation behavior of directionally solidified Fe(Al,Ta)/Fe2Ta(Al) eutectic composite

High-temperature oxidation of Q235 low-carbon steel treated by plasma electrolytic borocarburizing

A comparative study of the high-temperature oxidation behavior and mechanism of 0Cr25Ni20 austenitic heat-resistant stainless steel (AHSS) and 0Cr18AlSi ferritic heat-resistant stainless steel (FHSS) at 800°C, 900°C, and 1000°C in air up to 140 h was performed using isothermal oxidation tests. The oxidation kinetics of 0Cr25Ni20 AHSS and 0Cr18AlSi FHSS followed the parabolic law. The oxide films on 0Cr25Ni20 AHSS were composed of continuous and dense Cr2O3, MnCr2O4, and a small amount of NiMn2O4, whereas silicon exhibited internal oxidation and deteriorated the adhesion between the oxide film and substrate. Nickel-free 0Cr18AlSi FHSS exhibited good oxidation resistance at 800°C and 900°C due to dense, continuous, and well-adhered multicomponent oxide films containing Al2O3, Cr2O3, MnCr2O4, and a small amount of MnFe2O4. The oxidation resistance of 0Cr18AlSi FHSS declined at 1000°C, mainly due to the formation of nonprotective Fe2O3 and severe internal oxidation of aluminum.

To achieve higher engine combustion efficiency while reducing emissions, it is necessary to address the challenges posed by elevated operating temperatures. High Entropy Alloys (HEAs) have emerged as promising materials for this purpose, offering exceptional properties at high temperatures, including synergistic effects and excellent resistance to oxidation and corrosion. In this study, a FeCoNiCrAl HEA was investigated as a bond coat material due to its excellent balance of strength and ductility, coupled with outstanding oxidation resistance. It was deposited using HVAF M3 and i7 guns equipped with different nozzles/powder injectors and pressures. Notably, this research marks the first study of the i7 gun globally for the HEA bond coat, coupled with the optimization of HVAF parameters for both i7 and M3 guns. Characterization of both powder and as-sprayed samples was carried out using X-ray Diffraction (XRD), Energy-dispersive X-ray spectroscopy (EDS), and Field Emission Scanning Electron Microscopy (FESEM) techniques. The results revealed the formation of a dense and homogeneous microstructure. Additionally, isothermal oxidation tests were conducted to analyze the behavior of the thermally grown oxide. After 50 hours at 1000 °C, a dense, uniform, and thin alumina TGO layer was observed to have formed. These tests revealed that FeCoNiCrAl HEA exhibits significant potential to enhance oxidation resistance at high temperatures.