Formation Of Alumina Research Articles

Effect of Heat Treatment on the Microstructure and Hardness of Ni-Based Alloy 718 in a Variable Thickness Geometry Deposited by Powder Fed Directed Energy Deposition

Feature addition to existing parts is a trending application for Directed Energy Deposition (DED) and can be used to add complex geometry features to basic forged geometries with the aim to reduce and simplify the number of processing steps as machining and assembling. However, the mechanical properties of as-deposited Inconel 718 fabricated by Powder-fed Directed Energy Deposition (Powder-fed DED) are far lower than the relevant specifications, making it necessary to apply different heat treatment with the purpose of improving deposited material performance. In addition, the effects of heat treatments in both variable thickness deposited geometry and forge substrate have not been studied. In this study, the effect of heat treatment within the Aerospace Materials Specifications (AMS) for cast and wrought Inconel 718 on the microstructure and hardness of both the Ni-Based Alloy 718 deposited geometry and substrate are analyzed in different parts of the geometry. The microstructure of all samples (as-deposited and heat-treated) is analyzed by Scanning Electron Microscope (SEM) and Energy Dispersive Spectrometer (EDS), confirming the formation of aluminum oxides and titanium nitrides and carbonitrides in the deposited structure.

Wear Mechanisms and Notch Formation of Whisker-Reinforced Alumina and Sialon Ceramic Tools during High-Speed Turning of Inconel 718.

Nickel-based alloys, referred to as the most difficult-to-cut materials, pose a great challenge to cutting tool materials due to their excellent high-temperature properties. Ceramic tools have the potential to improve the machinability of these alloys with the advance of toughening mechanisms. In this work, the wear mechanisms of SiC whisker-reinforced alumina and Sialon when high-speed turning Inconel 718 alloy under dry cutting condition were investigated. The results showed that the wear process of Al2O3-SiCw WG300 was dominated by the notch wear, while the flank wear characterized by ridges and grooves perpendicular to the cutting edge was the main wear mode for Sialon SX9. A Ti−enriched belt was found at the boundary of the wear band for both ceramic tools. The SEM inspection and EDS analysis for this belt suggested the trace of diffusion between the workpiece material and tool matrix. As for the notch formation, the periodically adhesive action of the workpiece material at the depth-of-cut line combined with the thermal shock resistance of ceramic tools were considered to account for its formation. In addition, the oxidation of the workpiece material at the depth-of-cut line played a positive role in reducing the adhesive affinity and consequent notch wear.

Mechanical behavior in liquid lead of Al2O3 coated 15-15Ti steel and an Alumina-Forming Austenitic steel designed to mitigate their corrosion

The mechanical behavior in liquid lead of the 15-15Ti steel coated by an alumina layer and an Alumina Forming Austenitic (AFA) steel, with a chemical composition allowing for the formation of alumina at the surface of the steel, has been studied. To investigate the liquid metal embrittlement (LME) sensitivity by liquid lead, tensile tests in air and in liquid lead have been carried out at 400 °C and 500 °C. Then analyses of the cracking and of the fracture surfaces have been performed. No LME sensitivity at the tested conditions was observed for the Al2O3 coated 15-15Ti steel. The AFA steel is not sensitive to LME at 400 °C but suffers from LME at 500 °C. For the AFA steel, the liquid lead promotes at 500 °C an intergranular propagation of surface cracks which suggests a grain boundary wetting-dominated LME mechanism of this steel at 500 °C.

A Facile Green Synthesis and Optical Characterization of Nano Zinc Aluminate Phosphor

This study presents a facile green synthesis of zinc aluminate nanophosphor by the microwave combustion method. Natural Piper nigrum seeds extract was used as a combustion agent. To study the effect of annealing temperature on structural, compositional, and optical properties of nano zinc aluminate, as-synthesized sample was further annealed at temperatures 700⁰C and 900⁰C. The structural and optical characterizations were carried out using XRD, UV-Vis DRS, FTIR, and PL spectroscopy. The XRD and FTIR confirmed the formation of nano zinc aluminate. The calculated average crystallite sizes were between 5-20 nm. The effective energy bandgap of prepared nano zinc aluminate was found to be 4.48 eV, larger than the bulk bandgap of zinc aluminate. Further, the effective band gap was increased with higher annealing temperatures. The PL spectra exhibited intense emission in the UV and blue region with weak emission in the red region.

Revealing the co-doping effects of fluorine and copper on the formation and hydration of cement clinker

Fluorine in fluorite mineralizer and copper were easily doped together into cement clinker during cement kiln co-disposing wastes bearing Cu, yet that co-doping effects on the formation and hydration of cement clinker get few attention. The findings in this work revealed that the co-doping CaF2 (fluorine source) and CuO (copper source) into cement clinker showed a synergistic effect on the burnability, the immobilization rates of fluorine and copper in clinker phased were promoted. Compared with doping CaF2 into cement clinker, Co-doping CaF2 and CuO further enhanced the formation of alite and brownmillerite, while hindered the formation of tricalcium aluminate, and also triggered the right shift of characteristic diffraction peaks for C3S polymorphisms. It co-doping not only changed the microstructures, but also induced the visible defects on the surfaces of clinker mineral grains. In addition, most of fluorine would likely enter into the alite structure by replacing O2– of silicate phases, while most of copper would possibly enter into the brownmillerite structure by replacing Fe3+. Compared with the hydration process of cement clinker doped with CaF2, Co-doping CaF2 and CuO further accelerated the initial hydration process, obviously prolonged the induction period, but promoted the later hydration of co-doped clinker. Although the mechanisms about co-doping on the hydration process of co-doped clinker need more investigation, the above findings could extend the value-added utilization of wastes bearing Cu to reduce the large CO2 emission and energy/resource consumption in cement industry.

Influence of Different Addition Ratios of Fly Ash on Mechanical Properties of ADC10 Aluminum Matrix Composites

Aluminum-fly ash composites are formed by the chemical reaction between fly ash and the high-temperature aluminum-based alloy, which melts to form aluminum oxide as a reinforcing phase, which belongs to a composite of in situ synthetic reinforcing phases. Compared to aluminum-based alloys, composites have superior strength, rigidity, damping capacity, and wear resistance, but lower ductility and toughness. In this study, different fly ash addition ratios (0, 3, 6, 9, 12, and 15 wt%) were added to the ADC10-2Mg alloy melt via stir casting to form the aluminum-fly ash composite under the chemical reaction at 800 °C for 30 h. Subsequently, microstructure observation, density and porosity measurements, and hardness and tensile tests were conducted to analyze the influence of different fly ash weight percentages on the mechanical properties of aluminum-fly ash composites. According to the results, an aluminum-fly ash composite with good dispersibility of fly ash debris can be prepared by stir casting, and the fly ash particles gradually decomposed small debris as they reacted with the aluminum-based alloy at high temperatures during a long-term reaction process. The density of the aluminum–fly ash composite was reduced by adding fly ash, and its hardness and tensile strength were improved as well. However, the porosity increased with the amount of fly ash and the ductility was diminished. For the aluminum-fly ash composite with 6 wt% of fly ash, its density decreased by approximately 2%, the hardness and tensile strength increased by 7% and 49%, respectively, and the ductility decreased by 35%, as compared to those of the ADC10 alloy.

Oxidation mechanism and kinetics of nuclear-grade FeCrAl alloys in the temperature range of 500–1500 °C in steam

The oxidation mechanism and kinetics of two nuclear-grade FeCrAl alloys were investigated in steam up to 1500 °C by transient and isothermal oxidation tests. The slow α-alumina formation kinetics well matched only for the temperature range from 1000 °C to 1300 °C. Below 1000 °C, formation of transient alumina caused faster kinetics. In addition, an excessive Fe-rich oxide formation was observed on the inner surface due to rough surface at 600 °C. Above 1300 °C, convoluted α-alumina was easily spalled and caused faster kinetics. Moreover, the oxide spallation caused Cr and Al depletion and catastrophic oxidation above 1400 °C by the formation of Fe-rich oxide. The catastrophic oxidation caused a liquid phase in Fe-rich oxides, which significantly changed the tube segment geometry.

Comparative Study on Electrical Conductivity of CeO2-Doped AlN Ceramics Sintered by Hot-Pressing and Spark Plasma Sintering.

Aluminum nitride (AlN) ceramics were prepared by both Hot-pressing (HP) and Spark-Plasma-Sintering (SPS) using cerium oxide as the sintering aid. The characterization of AlN raw powder denoted the presence of an amorphous layer that led to the formation of aluminum oxide. During the sintering process, CeO2 introduced as a sintering aid was reduced into Ce2O3. The latter reacted with aluminum oxide to form a transient liquid phase that promotes sintering by both HP and SPS. A reactional path leading to the formation of secondary phases, such as CeAlO3 and CeAl11O18, has been proposed according to the pseudo-binary Al2O3 – Ce2O3. Ceramics obtained from HP and SPS are presented as similar, except for the secondary-phase distribution. The influences of secondary phase composition and distribution on electrical conductivity were evaluated by leakage current measurements. The mechanism of DC conduction and the global conductivity of ceramics were discussed according to the sintering process and the number of secondary phases.

Formation and characterisation of porous anodized aluminum oxide, synthesized electrochemically in the presence of graphene oxide

Formation and characterisation of porous anodized aluminum oxide, synthesized electrochemically in the presence of graphene oxide

Microstructural evolution of ceramic nanocomposites coated on 7075 Al alloy by plasma electrolytic oxidation

Alumina-zirconia nanocomposite coatings were prepared on 7075 Al alloy by plasma electrolytic oxidation (PEO) at current densities 0.1–0.3 A/cm2 in a Zr-containing electrolyte under DC galvanostatic mode. Coatings were investigated by X-ray photoelectron spectroscopy (XPS), X-ray and electron diffraction, and high-resolution transmission electron microscopy (HRTEM). The coatings developed at current density higher than 0.2 A/cm2 consist of γ-, t- and α-alumina and tetragonal zirconia and were composed of four sub-layers: (i) interface nanocomposite layer close to the substrate with a dense structure of heterogeneously distributed α-Al2O3 and t-Al2O3 nanocrystals (2–5 nm in size) embedded in an amorphous matrix, followed by (ii) a pure amorphous Al2O3 sublayer, (iii) a dense zirconia encapsulated alumina sublayer and (iv) a top amorphous sublayer with dispersed globular alumina and zirconia nanoparticles. The zirconia encapsulated alumina sublayer is composed of large t-Al2O3 and γ-Al2O3 grains bordered by fine t-ZrO2 nanoparticles. The t-ZrO2 phase was nucleated in an atomically coherent manner at Al2O3 grain boundaries. The microstructure evolution in the coating revealed alumina formation at the early stages of the process while the oxidation, crystallization and incorporation of zirconia occurred at the last stages of the PEO process. The phenomenology of the PEO Al2O3/ZrO2 nanocomposite coating formation is presented to account for the present experimental observations.

Isothermal short‐term oxidation behavior of MAR‐M246 nickel‐based superalloy at 800°C and 1000°C

AbstractSuperalloys are widely employed at high temperatures for structural applications. Hence, knowledge about the oxidation of these materials is essential. However, the literature is scanty when it comes to some families of superalloys. The purpose of this study was therefore to analyze the MAR‐M246 polycrystalline alloy in isothermal short‐term tests at 800°C and 1000°C for up to 240 h. Thermodynamic simulations were performed to evaluate the material's phase stability as a function of temperature and to assess the expected phases in response to oxygen pressure. The oxidized samples were characterized by SEM‐EDS and DRX, which revealed a tendency for scaling of oxidized material, particularly at temperatures of 1000°C. Nevertheless, protective layers of Cr2O3 and Al2O3 oxides were formed, which enabled the formation of fairly thin oxide layers, in addition to NiO and complex oxides. The region of the metallic substrate close to the oxide layer underwent aluminum depletion, causing the gamma‐prime phase to disappear, as well as formation of aluminum oxides and titanium nitrides. Last, a good correlation was found between the thermodynamic simulations and the oxides that were formed.

Plasma electrolytic oxidation coatings on additively manufactured aluminum–silicon alloys with superior tribological performance

Due to the increasing demand for lightweight structures with complex geometries, additively manufactured (AM) aluminum–silicon (AlSi) alloys have aroused considerable attention. However, their insufficient hardness and poor tribological performance remain a critical issue for several applications requiring high contact pressures and wear resistance. Herein, current-controlled plasma electrolytic oxidation (PEO) coatings were developed on AM AlSi alloys in alkali electrolytes. To obtain PEO coatings with excellent mechanical properties, the electrolyte system was modulated and optimized by adjusting the concentration of the silicate and hexametaphosphate. The corrosion behavior of AM AlSi alloys in various electrolytes were evaluated via potentiodynamic polarization tests. The influence of silicate and hexametaphosphate on the structure, composition, thickness, roughness, and hardness of the prepared PEO coatings were determined using scanning electron microscopy, X-ray diffraction, energy-dispersive X-ray spectroscopy system, microprofilometer, and microhardness tester. The tribological behavior of the PEO coatings were tested using a ball-disc-tribometer without lubrication. Silicate benefits the PEO process under the constant current mode; however, a mass of mullite and noncrystalline materials form. Hexametaphosphate adversely affects the formation of the initial passive film on AM AlSi alloys, and many defects form on the obtained coating. But it can decrease the roughness of the coating and accelerate the dissolution of the matrix, promoting the formation of alumina and inhibiting the formation of mullite and SiO2. The PEO coatings, prepared in the mixed electrolytes (with a concentration ratio of silicate and hexametaphosphate of ~0.7–1), have the fewest defects and highest hardness (~1300 HV), and possess superior tribological performance; the wear rates of the coatings were only ~1/150th that of the AM AlSi10Mg substrate. Both the synergy of silicate and hexametaphosphate and the modified growth mechanisms of PEO coatings on AM AlSi alloys have been proposed.

Influence of Catalyst Reduction Temperature on Autogenous Glycerol Hydrogenolysis over NiAl Catalyst

AbstractAutogenous glycerol hydrogenolysis to 1,2‐propanediol by aqueous phase reforming (APR) was investigated over supported nickel catalysts. Effect of reduction temperature on physico‐chemical properties of catalysts played a significant role in tuning conversion and product selectivities. The formation of nickel aluminate (NiAl2O4) spinel phase during catalyst reduction led to rearrangement of Ni species to obtain small and stable Ni particles. The catalyst activation temperature alters the extent of reduction of multivalent Ni species (Ni0, Ni+2/+3) which facilitated glycerol dehydration and hydrogenation while suppressing C−C cleavage and thus avoiding undesirable side products. Additionally, presence of moderate BrØnsted/Lewis acid ratio of the catalyst promoted higher 1,2‐PDO selectivity. In‐situ glycerol hydrogenolysis involves glycerol dehydration to acetol with simultaneous reforming to H2 and CO2 and this hydrogen converts acetol to 1,2‐PDO.

New insights on the effect of the hydrolysing agents in the synthesis of pristine fibrous silica zeolite X for enhancement of side chain methylation of toluene: Theoretical and experimental studies

This article aims to clarify the role of hydrolysing agent in the synthesis of a pristine fibrous silica zeolite X (FZX) catalyst using DFT simulation, thermodynamics, and experimental data. FZX was fabricated using various hydrolysing agents such as urea (FZXU), formamide (FZXF) and N, N methylenediacrylamide (FZXN), to perform the side chain methylation of toluene process. It was found that disparity of hydrolysing agents resulted in different degree of silica hydrolysation which eventually altered the each FZX catalyst properties. Based on the chemical reactivity and visualized charge distributions as revealed by the DFT simulations, it was discovered that most electronegative regions located around negatively charged oxygen in hydrolysing agents structure exhibited a higher degree of silica hydrolysation resulted in excellent catalyst behaviour. Higher density of dendrimeric silica fiber in FZXN reduced the coke formation and induced the formation of extra-framework alumina (EFAl) species. The hydrolysing agents also affect the basicity of the decorated FZX catalyst and followed the sequence FZXN > FZXU > FZXF. The FZXN exhibited higher side chain methylation of toluene catalytic activity of 57 % toluene conversion and 49.6 % of styrene and ethylbenzene yield. Furthermore, a thermodynamic analysis was carried out, and it was discovered that the Boudouard reaction was the primary source of carbon deposition during the side chain methylation of toluene.

Synthesis of self-regenerating NiAl-Al2O3 composite coatings

The aluminothermic reactions involved either in the mixtures or the stack of NiO and Al powders were elucidated in a previous work (Troncy et al., 2020) [1]. The present work now investigates the formation of potential self-regenerating coatings using the aluminothermic reaction between nickel oxide and aluminum. The use of this exothermic reaction allows the simultaneous formation of both alumina and nickel aluminides. The coatings were obtained by first pre-oxidizing the nickel substrate, then spraying an aluminum slurry before applying an adequate heat treatment. The most promising self-regenerating coatings display a first composite layer, made of an intermetallic compound (Al3Ni or NiAl) and alumina, and a second layer with a higher concentration of oxide. The first layer is expected to play the role of an Al reservoir coating and the second one that of a diffusion barrier to slow down the Al flux to maintain the Al reservoir in the coating. The effects of preoxidation time and of temperature on the resulting coating microstructures are discussed as a function of time.

Critical assessment of rapid methods to qualify supplementary cementitious materials for use in concrete

In this study, several tests for supplementary cementitious materials (SCMs) were evaluated to find the best methods to rapidly screen out inert materials, measure overall reactivity, and differentiate between pozzolanicity and latent hydraulicity. The R3 matrix and lime reactivity tests were found to be the most effective at quickly screening out inert materials. However, slow-reacting materials may appear to have low reactivity in both tests and extending the test duration better depicts material reactivity. Additionally, SCMs with higher alumina content perform better in the R3 tests due to the higher heat release and more bound water associated with the formation of calcium aluminate hydrates compared to calcium silicate hydrates, creating reactivity bias when compared to SCMs with lower alumina content. Measuring the calcium hydroxide content of R3 pastes, through thermogravimetric analysis or single point mass loss, can also differentiate between pozzolanic and hydraulic materials.

Fabrication of an al2o3-1% ti composite with some characteristics of a biomaterial

An Al2O3-1 wt.% Ti composite was prepared by powder techniques with the intention of analyzing its mechanical and chemical properties for its possible application as a biomaterial. Alumina was synthesized using the reaction bonding aluminum oxide (RBAO) process. The powders resulting from the milling stage present sizes minor than 1.5 microns. With the help of X-ray diffraction and differential thermal analysis, it was determined that aluminum oxidizes in both solid and liquid states during the RBAO process. It was also found that the alumina formation reaction in this process is completed at 1,100oC. From the measurements of mechanical properties (HV, KIC, E) in the Al2O3-1 wt.% Ti composite, it was determined that these properties are better than the same properties of compact bone. Electrochemical impedance spectroscopy, indicates that additions of 1 wt.% Ti on Al2O3 enhance its corrosion resistance. The bioactivation of a Al2O3-1 wt.% Ti composite was successful using a biomimetic method, because after 21 days, hydroxyapatite begins to proliferate on the surface of the substrate. With all these results it can be commented that it is feasible to use Al2O3-1 wt.% Ti composite in the elaboration of synthetic bone for its application as a biomaterial.

THERMODYNAMIC ASPECTS OF OXIDES REDUCING BY ALUMINUM AND TITANIUM AT THE THERMIT WELDING OF RAILS

During aluminothermic welding of rails, formation of aluminum oxides can take place, which become centers of residual stresses forming resulting in cracks formation followed by fracture and destruction of the welding joint. It was proposed to add titanium scrap in the form of preliminary made powders to the welding thermites composition. At the replacement of aluminum by titanium, titanium oxides formation takes place providing higher resistance against cracks formation. Comparative estimation of thermodynamic properties [∆rH°(T), ∆rG°(T)] of thermite reactions with aluminum and titanium in the systems metal oxide – aluminum and metal oxide – titanium under standard conditions presented. As reaction components for calculation of thermodynamic properties, oxides of iron, manganese, silicon and chrome were used. Thermodynamic analysis of 6 reactions with aluminum and 30 reactions with titanium were carried out. It was established that all the reactions under consideration are thermodynamically probable. However, reactions of both aluminum and titanium with iron oxides had the highest probability and correspondently the highest heat generation. Reducing of silicon oxide had the lowest probability and correspondently the lowest heat generation. The reaction with oxides of manganese and chrome had an intermediate position. By the reducing ability titanium is not worse than aluminum and in reactions with iron oxide even exceeds it, particularly in the temperature range over 2000 K. Enthalpies of reducing reactions of iron oxide by titanium is in the same area of values as the enthalpies of reaction of reducing by aluminum. A thermodynamic analysis showed that application of titanium as a reducing agent at the thermit welding is quite acceptable along with aluminum.

Formation of Magnesium and Aluminum Oxides from Water and Metal Hydrides: Creation of the Smallest Ruby

Formation of Magnesium and Aluminum Oxides from Water and Metal Hydrides: Creation of the Smallest Ruby

Revisiting the Structural Evolution of Hydrotalcite‐Derived Mixed Metal Oxides upon Alkali Metal Doping and Its Impact on Base Catalysis

AbstractThe structural evolution of classic coprecipitation‐derived Mg−Al mixed oxide (MMO) upon doping of different alkali metals in a wide level (1–20 wt.%) was revisited. The pristine lateral MMO of aggregates into particulates, accompanied with the dramatic loss of the surface areas and the formation of aluminates. These phenomena become severer with the decrease of the atomic radius. The formation of NaAlO2 likely occurs via the gradual dealumination of the MMO, which is highly dependent on both the dopant content and the activation temperature. This leads to ultimately a new ensemble with MgO as the core decorated by NaAlO2 in the outer surfaces at high Na doping level and above 973 K. When evaluated in the base‐catalyzed transesterification and acylation model reactions, the Na‐doped MMO show enhanced performance and a plateau at increasing doping that might be explained by an interplay between the number and strength of strong basicity.