Diffusion Of Aluminum Research Articles

Local structural effects of Gd3+ ions incorporation in shell of nanostructured silica core – alumina rich shell microspheres

Non-crystalline nanostructured silica core – alumina rich shell microspheres, with Gd ions in the shell were obtained by sol-gel procedure following a synthesis route based on the Stöber method. The samples were characterized by DTA/TGA, XRD, SEM and TEM, EPR, solid-state NMR and XPS techniques. The effect of increasing Gd ions content on the local structure in alumina rich shell and at its interface with silica core was investigated. It was also found that increasing the concentration of the paramagnetic ions leads to a modification of the local magnetic field at sites of neighboring aluminum and silicon ions. Gadolinium ions are distributed in the aluminum sites from alumina rich shell. As result of increasing the gadolinium incorporation at the core-shell interface the diffusion of aluminum in the silica network, as well as the migration of the silicon in the alumina rich shell, are enhanced. Therefore, Si-O-Al bridges are formed at the interface, leading to stable non-crystalline structures, with Gd ions on the external layer of the shell, as proved by XPS, having potential application as contrast agents for magnetic resonance imaging (MRI).

Precipitation of Topologically Closed Packed Phases during the Heat-Treatment of Rhenium Containing Single Crystal Ni-Based Superalloys

Continual development of nickel-based superalloys for single-crystal turbine applications has pushed their operating temperatures higher and higher, most notably through the addition of rhenium. However, this has left them susceptible to the precipitation of topologically closed packed phases (TCPs), which are widely considered detrimental. Whilst these have long been reported as an end-of-life phenomenon in in-service components, they have more recently been observed during the manufacture of turbine blades. Several rhenium-containing alloys (CMSX-4, CMSX-10K, and CMSX-10N) were cast into single-crystal test bars and studied at different times along their solution heat-treatment process to discern if, when, and where these TCPs precipitated. It was seen that all alloys were susceptible to TCPs at some point along the process, with the higher rhenium-containing alloy CMSX-10N being the most prone. They occurred at the earliest stages of the solution process; this was attributed to aluminium diffusion from the segregated interdendritic regions into the dendrite core, causing the concentration of rhenium into the ɣ-matrixes until sufficient potential was achieved for TCP precipitation. As the samples became more homogeneous, fewer TCPs were observed; however, in the case of CMSX-10N, this took longer than the typical 24-h solution time used in industry, leading to components entering service with TCPs still present.

Fabrication of (Ti-Al-Si)/(Ti-C)/Ti – layered alloy by SHS pressing

A metal-carbide-intermetallic material based on combustion products of the layer system (Ti-Al-Si)/(Ti-C)/Ti was for the first time obtained with the help of self-propagating high-temperature synthesis (SHS) combined with pressing. Exothermic synthesis from elementary powders was carried out at a pressure of 10 MPa, and pressing of the hot synthesis product was carried out at a pressure of 100 MPa. It has been shown that SHS pressing contributes to the formation of permanent joints of «metal/carbide/intermetallic» layers. The main features of microstructure formation, phase composition, and strength properties of transition zones at the boundary between reacting SHS compositions, Ti-C and Ti-Al-Si and Ti-metal substrate are investigated. It is shown that during SHS reaction, a homogeneous microstructure of Ti-C and Ti-Al-Si layers with an insignificant content of cracks and pores is formed. The thickness of the transition zone between the layers was at least 15 µm. The main phase formed in the combustion product of Ti-Al-Si layer is, according to the results of X-ray phase analysis, triple phase Ti20Al3Si9, the content of which, calculated by the Rietveld method, was at least 87 wt. %. In addition, the combustion product contains a secondary phase of Ti3Al in the amount of 13 wt. %. The energy dispersion analysis revealed that diffusion of aluminium through the titanium carbide layer into the titanium substrate to a depth of approx. 30 µm is observed. Microhardness value of the combustion product of Ti-Al-Si layer was about 10 GPa. The rectilinear nature of crack propagation in the synthesized combustion product of Ti-Al-Si layer, as well as the Palmquist crack resistance coefficient varying within 5.1-5.7 MPa·m1/2, indicate the fragility of the material.

Experimental and clinical study of the concentrations of chemical elements in oral fluid from prostheses and implants

BACKGROUND: Electrochemical reactions can occur in the mouth of patients with prostheses made of metal alloys. The phenomena of galvanism reflect the hidden process of corrosion of structural alloys in oral fluid, which makes it necessary to analyze the chemical contents of prostheses under clinical conditions in patients with implants and experimental conditions.
 AIM: Comparative analysis of the trace element content in the oral fluid from prostheses in implants in the clinic and an experiment.
 MATERIALS AND METHODS: National Research Technological University MISIS analyzed artificial saliva after prostheses were in contact with grade 4 and grade 5 implants for 3 months. Titanium, cobalt, chromium, aluminum, vanadium, molybdenum, tungsten, manganese, nickel, and iron were determined by spectrometry. The same chemical elements were analyzed using an identical spectrometric method in 32 patients with fixed dentures on implants.
 RESULTS: Under the experimental conditions, nine trace elements were found in the container around one grade 4 implant with a crown, including titanium (0.4 g/l), cobalt (3.8 g/l), chromium (0.5 g/l), aluminum (2.6 g/l), vanadium (0.6 g/l), molybdenum (1.5 g/l), tungsten (3.0 g/l), manganese (0.9 g/l), nickel (0.5 g/l), and iron (0.6 g/l). The grade 5 implant was characterized by a greater degree of diffusion of aluminum, nickel, and manganese into artificial saliva. Mass spectrometry of the oral fluid in patients with dental implants revealed more trace elements than experimental data. The difference in the concentration of microelements from the prostheses under clinical conditions and in the experiment ranged from 0 to 128 times the minimum.
 CONCLUSION: Fixing experimental prostheses on implants for 3 months in artificial saliva was accompanied by the diffusion of microelements from the prostheses and implants into the environment, particularly cobalt, aluminum, and vanadium. Prostheses on implants with a lower titanium content are characterized by a greater degree of diffusion of aluminum, cobalt, nickel, and manganese. More significant diffusion of microelements occurred from the prostheses into the saliva under clinical conditions, according to the spectrometric data.

Transient tin mineralization from cooling of magmatic fluids in a long-lived system

AbstractFertility and longevity of hydrothermal systems are key parameters required to improve our ability in predicting new deposits and directly extracting metals from metalliferous fluids. Reconstructing evolutional trajectories of metalliferous fluids with high temporal resolution is critical for pushing our understanding forward, but this is inevitably challenging because traditional approaches for obtaining this information either have poor temporal resolution and/or bear considerable uncertainties. We present a novel approach (translating texture-controlled information to temporal patterns) to reconstruct the thermal and isotopic history of the Weilasituo vein-type tin deposit (Inner Mongolia, China) at the millennial scale. In situ oxygen isotope thermometry of paragenetically constrained quartz and cassiterite reveals that tin deposition was accompanied by gradual cooling of pure magmatic fluids from ~500 °C to ~390 °C at lithostatic conditions, while fluid mixing and/or water-rock interaction were not required. The system then transitioned to hydrostatic conditions and permitted penetration of meteoric water and further cooling. Aluminum diffusion in quartz chronometry yields time scales of ~50 k.y., ~5 k.y., and ~200 k.y. for pre-, syn-, and post-ore stages, respectively. Our results highlight that the magmatic-hydrothermal system did not form ore minerals for most of its lifetime, with mineralization occurring only briefly (i.e., <5% of its lifetime). Hence, the rates and efficiency of ore formation may need significant revision. For magmatic-hydrothermal systems with felsic magmas being stored at high crystallinity after extensive volatile exsolution, the efficiency of scavenging metals from melts to fluids critically controls their fertility. To directly extract metals from metalliferous fluids, the key is targeting systems with a high degree of magma crystallization (e.g., higher metal contents in fluids) in warm crust (e.g., able to sustain long-lived systems).

Karakteristik Sifat Mekanik Hasil Pengelasan Gesek Aluminium dengan Tembaga Menggunakan Variasi Kecepatan Putar dan Kekasaran Permukaan Kontak

Friction welding is a type of welding that is used for a variety of materials. Friction welding is a method of joining two materials by swiping them together until they reach some of the melting points of the materials to be joined, then applying constant pressure. Aluminum and copper are the materials to be joined in this study. Using rotational speed and contact surface roughness variations. The purpose of this research is to determine the mechanical properties of friction welding results between aluminum and copper. The speed settings are 1230, 1500, and 2500 rpm. Use sandpaper grades #100, #800, and #1500 to achieve different levels of surface roughness. According to the results of this study, the rotational speed with the highest tensile strength and hardness values is 1230 rpm, namely 59.6 MPa and 54.6 HVN (Al), 112.1 HVN (Al) (Cu). In terms of surface roughness variations, the surface roughness using grade #1500 sandpaper has the highest value, namely 54.18 HVN (Al), 112.1 HVN (Cu). And #100 sandpaper with a value of 52.48 Mpa. The results of the microstructure test in the weld joint area at a magnification of 1000x indicated that the friction welding of aluminum and copper was successful, as evidenced by the diffusion of aluminum and copper in this area.

Defect Formation in Aluminum-coated Steel by Heat Treatment

A carbon tool steel plate was coated with an aluminum foil with a thickness of 100 μm by explosive welding. This aluminum-coated steel was heat-treated in the temperature range of 973−1273 K for up to 3.6 ks in the atmosphere to form a Fe-Al alloy layer on the steel surface. The alloy layer was basically composed of Fe2Al5, two kinds of FeAl (Al-rich FeAl and Fe-rich FeAl), Fe3Al containing carbon (Fe3Al(C)) and ferrite stabilized by aluminum diffusion (α-Fe(Al)). FeAl2 was also detected together with Fe2Al5 at more than 1223 K. At 1273 K, island-shaped Fe3Al(C) was produced in α-Fe(Al), and the Fe-rich FeAl/α-Fe(Al) interface became indistinct. In addition, defects like a crack and a void were observed in the vicinity of Fe-rich FeAl. The purpose of the present study is to investigate the reason for the defect formation. Various heat treatments were conducted for the aluminum-coated steel, and it was found that a temperature-rising step was responsible for the defect formation. This consideration got support from the results of two-step heat treatment, which was a heating process at 1273 K immediately after holding for 600 s near the eutectoid temperature in the Fe-C binary system.

Parameters to be considered for the development of highly photoactive TiO2 layers on aluminium substrates by RF magnetron sputtering for treating polluted air

In this study, photoactive titanium dioxide films deposited on aluminium substrates using Radio-Frecquency (RF) magnetron sputtering have been developed, with the goal of advancing this technology for indoor air treatment. With this goal in mind, the influence of deposition parameters, namely post annealed heat treatment, working pressure and deposition time, on the physico-chemical properties of the layers has been studied. Photocatalytic efficiency for trichloroethylene degradation in gas phase, and self-cleaning performance of the developed materials have been evaluated. Samples have been thoroughly examined using different techniques (SAXRD, UV-Vis, SEM-EDX, contact angle, profilometry, hardness, among others) in order to determine the structural-photocatalytic relationship. The findings indicate that high-temperature annealing promotes the diffusion of alumina, copper and iron ions towards the surface of the samples resulting in the formation of aluminium alloys. These alloyed species can act as recombination centres adversely affecting the photocatalytic activity. The sputtering pressure also affects the nature of the titanium phases formed during the sputtering. Increasing the thickness of the TiO2 layer leads to rougher surfaces with TiO2-anatase as the predominant crystal phase. Annealing temperatures of 350 °C, argon pressures between 0.8 and 1.0 Pa, and TiO2 layer thickness of ca. 110 nm are the optimal conditions to prepare well-adhered TiO2 layers with high photocatalytic performance. UV-A photoinduced superhydrophilicity phenomena was observed for all TiO2/Al materials.

Effects of aluminum diffusion on the oxide of the FeCrAl alloys surface: A first–principles study

The oxide structure on a FeCrAl alloy containing a mixture of Al 2 O 3 and Cr 2 O 3 or a layer of Al 2 O 3 was observed in experiments. Given that its atomistic mechanisms remain unclear, the formation mechanism of the layer structure of FeCrAl/(Cr 2 O 3, Al 2 O 3 )/Al 2 O 3 was explored based on Al atom diffusion using first–principles calculation. The energy of Al diffusion and the work of separation of the FeCrAl/Cr 2 O 3 interface show that Al atoms tend to diffuse into Cr 2 O 3 and enhance the bonding strength of the interface when the concentration of Al in the Cr atoms layer of Cr 2 O 3 is lower than or equal to 50%. The separation work of the interface is 2.43 J/m 2 and 3.46 J/m 2 for no Al and Al diffuse, respectively. Electronic structure analysis shows that more electrons of Al atoms participate in the formation of Al–O bonds and that reduces the system energy when Al atoms diffuse into Cr 2 O 3 . Hence, the Al atom tends to diffuse into Cr 2 O 3 , and the structure of FeCrAl/(Cr 2 O 3 , Al 2 O 3 ) is formed. When the concentration of Al atoms in the Cr atoms layer of Cr 2 O 3 is more than 50%, Al atoms diffuse close to the surface and the FeCrAl/(Cr 2 O 3 , Al 2 O 3 )/Al 2 O 3 structure may be formed. The crystal orbital Hamilton population (COHP) of Al–O bonds indicates that Al diffusing close to the surface of Cr 2 O 3 can reduce local lattice distortion and restore the strength of the Al–O bond. The average bond length of the Al–O bond changes from 1.923 Å to 1.945 Å, and then returns to 1.926 Å, and the –COHP of the Al–O bond changes from 4.483 to 4.740.

Influence of High Magnetic Field-Thermal Coupling Processing on Diffusion Bonding Properties and Element Diffusion of 1420 Al-Li Alloy

The high uniform magnetic field combined with pre-deformation and vacuum heat treatment processing is designed for improving the diffusion bonding properties of the 1420 Al-Li alloy. Serial magneto-thermal coupling treatment experiments of 1420 Al-Li alloy and the pure aluminium diffusion couple, together with the gallium interlayer, is carried out in a superconducting high magnetic field device. Various parameter combinations are used to produce different samples on which interface organization and connection performance are studied. Electron microscopic analysis reveals various interface topographies and fracture morphologies after shear strength tests. Influence of diffusion bonding temperature on element diffusion under a high uniform magnetic field is investigated. The diffusion activation energy of Mg element in pure aluminum under a 12 T magnetic field is calculated in this paper. It is found that the bonding quality and bonding performance of the interface are improved greatly after heat treatment with a strong magnetic field, and the bonding temperature is an important factor affecting the interface bonding and bonding strength. The diffusion coefficient of the Mg element in the 1420 Al-Li alloy to L2 pure aluminum increases with the increase of diffusion bonding temperature. Reducing the activation energy of elemental diffusion is beneficial to atomic diffusion.

The Microstructure Formation of a Protective Oxide-Scale Layer on Small-Diameter FeCrAl Fibers

FeCrAl fibers, at high temperatures, form a protective oxide-scale layer dominated by aluminum oxide on the surface to prevent further oxidation of the base metal alloy. This study investigates the effects of heat treatment on the microstructure formation of the oxide-scale layer on small-diameter FeCrAl fibers, 12 and 17 µm, produced using a bundle drawing process. The morphology examination and chemical analyses of the small-diameter fibers exhibit the microstructure and chemical compositions of the surface and cross-section areas, revealing a distinctive interface layer with a high aluminum concentration between the base metal and the oxide-scale layer. Furthermore, thermogravimetric analysis results show that the 12 µm fibers have about a 60% higher oxidation rate than the 17 µm fibers-caused by the high outward diffusion of aluminum to the surface of the fibers due to their high surface-area-to-weight ratio. Consequently, the high growth rate of the nonuniform oxide-scale layer and the limited aluminum reservoir of the 12 and 17 µm diameter fibers lead to faster depletion of aluminum from the base metal alloy-limiting the lifetime and durability of the smaller-diameter fibers in high-temperature applications.

Primary Production of Aluminium and Its Alloys in Molten Salts with Oxygen Evolving Anodes: Overview

Carbon dioxide and other greenhouse gases are emitted during the production of primary aluminium through the Hall-Heroult process. Moreover, the process requires specific energy consumption of as high as 14.685 kWh/tonne of Al. There is an urgency to replace the existing process with a more environmentally and economically friendly process. One such process involves the use of inert anodes instead of traditional carbon anodes, where Al2O3 → 2Al + 1.5O2 occurs instead of 1.5C + Al2O3 → 2Al + 1.5CO2. The life span of inert anodes can be improved by using them at low operating temperatures, however, alumina solubility and dissolution rate decrease at low operating temperatures. Instead of using a traditional NaF-AlF3 molten solution, NaF-KF-AlF3 or KF-AlF3 could be used as low-temperature electrolytes. Introducing KF salt into the NaF-AlF3 mixture increases the alumina solubility and reduces the liquidus temperature of the molten salt. Another factor that affects the liquidus temperature of the mixture and alumina solubility in the electrolyte is the cryolite ratio , where M = Na or K (or both). It should be noted that the cells with AlF3-rich NaF electrolytes suffer with problems such as high cell voltage, low aluminium purity and unstable frozen side ledges. The addition of CaF2, MgF or LiF to the electrolyte mixture could improve the physical-chemical properties of the electrolyte. For instance, the addition of CaF2 to KF-AlF3 molten salts improves the electrical conductivity of the electrolyte. Specific energy consumption of the cell can be significantly reduced by using a vertical electrode cell with reduced anode-cathode distance (ACD). However, the major problem with low ACD is the contamination of reduced aluminium at cathode by anode products. This problem can be solved by using suspension electrolytes where the electrolyte contains excess about of Al2O3 in the electrolyte which obstructs the transportation of anode products into the aluminium diffusion layer on the cathode, thus maintaining high aluminium purity. In this paper, we summarise the developments obtained in designing the inert anode cells. We mainly focus on three main components: Inert anodes, wettable cathodes, and low-temperature electrolytes. The influence of additives on the properties of electrolytes will be discussed in detail. We also discuss the energy consumption, environmental impact and economic aspects of producing aluminium using vertical electrode cells.

Hydrogen delaying the formation of Guinier-Preston zones in aluminium alloys

The consequences of hydrogen on the formation and growth of GP zones during natural ageing in an Al-5Cu alloy is investigated experimentally down to the atomic scale and numerically using ab initio calculations. As observed through scanning/transmission electron microscopy, the hardening kinetic is delayed due to a slower growth of GP zones during natural ageing when hydrogen is incorporated. According to ab initio calculations, the delayed hardening results from hydrogen trapped in vacancies that reduces significantly the diffusion coefficient of copper and the self diffusion of aluminium. This reduction of the diffusion coefficient is either due to hydrogen being in the path of the atom that exchanges with the vacancy (and increasing the energy barrier) or due to hydrogen being dissociated from the vacancy (and leading to a less stable state).

Microstructure and mechanical response of dissimilar joint of ferritic interstitial-free steel to austenitic low-density steel produced by diffusion bonding

Lightweight Fe–Mn–Al–C steels, also referred as low-density steels (LDS), have gained significant importance in the recent past owing to their reduced density, high strength and ductility, along with good oxidation resistance at elevated temperatures. The present study is aimed at exploring the feasibility of solid-state joining of LDS to the other prominent ferrous alloys, for example, interstitial-free steel (IFS), in order to extend their applications. In the present work, diffusion bonding has been attempted to produce IFS-to-LDS joints with and without the application of nickel (Ni) interlayer. Diffusion bonding is performed at two different temperatures of 850 °C and 950 °C for 30 min at the bonding pressures of 10, 20 and 30 MPa under high vacuum conditions. The interfacial microstructures of the joints are examined using an optical microscope and scanning electron microscope in conjunction with energy dispersive spectroscopy and electron backscattered diffraction. Microhardness and nanoindentation measurements across the interface and shear tests are performed to evaluate the interfacial bond strength of the joints. The joint produced with the application of Ni interlayer at 950 °C under 30 MPa bond pressure exhibited superior shear properties with a maximum shear strength of ∼340 MPa. The formation of stable oxides resulting, especially by diffusion of aluminum from LDS to the faying surfaces is found to be detrimental to establish a good interface at lower bonding parameters. The application of higher bonding pressure and Ni interlayer along with enhanced diffusional activities from LDS to IFS within the austenite phase are found to be effective in achieving a sound joint. The metallurgical aspects involved in the formation of the reaction products and subsequently, the influence of the evolved interface microstructure on the mechanical response of the joints are discussed in-detail.

Development of Cr and Al Pack Cementation Coatings on Co-Based γ/γ′ Superalloys

Co-based superalloys have been developed as candidate materials to replace Ni-based superalloys in hot sections of turbine engines, however, their oxidation resistance is limited. Therefore, in this work chromium and aluminum diffusion coatings were developed via the pack cementation process for novel γ/γ′-strengthened superalloys of the Co–Ni–Al–W system. This alloy system also offers the opportunity to vary the cobalt-to-nickel ratio and the tungsten content systematically to investigate their influence on the coating process. The coating process was shown to successfully enrich the surfaces of such alloys either in aluminum or chromium. For Al, the result was the formation of (Co, Ni)Al on top of an interdiffusion zone, while for chromium a Cr-rich layer in solid solution with the substrate along with the formation of σ-phase were observed. The coating formation mechanisms are discussed for both coatings in the light of activities and phase formation.

Failure Mechanism of Aluminum Diffusion in Low-Voltage Trench MOSFET With High Cell Density

In this paper, a low-voltage trench metal oxide semiconductor field effect transistor (MOSFET) with high cell density is researched on the process design. The experimental device pitch is reduced to 0.5 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${\mu }\text{m}$ </tex-math></inline-formula> by three-dimensional (3-D) design of P plus region and self-aligned process, while the contact between silicon and metal is convex rather than the concave in conventional trench MOSFET. BV failure was founded because of the particularity of this structure and the failure is caused by the diffusion of aluminum (Al) into the drift region during the final alloy step in the metal forming process. This paper reveals the mechanism of Al / Silicon (Si) double diffusion and presents the refined surface metal film, which can suppress reverse diffusion and forward diffusion simultaneously. The risk of failure is reduced and functional devices are obtained by the application of the proposed surface metal film. Furthermore, it is recommended to use TiN layers in a total 50 nm as the barrier layer in the metallization process to provide a certain diffusion length and the fabricated device achieves a low specific on-resistance of 3.58 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text{m}\Omega \cdot $ </tex-math></inline-formula> mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> with a corresponding breakdown voltage of 26.2 V eventually.

High temperature stability and transport characteristics of hydrogen in alumina via multiscale computation

The impact of hydrogen charge states on the stability and transport characteristics of hydrogen interstitials in alumina polymorphs is evaluated by multiscale computational methods including density functional theory (DFT), ab initio molecular dynamics (AIMD) and machine learned force fields. Thermodynamic calculations show that the protonic Hi+1 interstitial is the most stable defect species for most values of the electronic bandgap in both α and amorphous alumina (Al2O3). Further, active learned Gaussian approximation potentials (GAP) were developed using AIMD data to study temperature dependent long time proton diffusion in alumina. Diffusivity calculations from GAP-MD simulations are found to be comparable with of the AIMD data, while being ∼340 times faster and scalable to larger systems. Comparisons with diffusivity values for other interstitial charge states (Hi0 and Hi−1) and published experimental literature indicate that Hi+1 diffusion is the likely mechanism of hydrogen transport. A good agreement is obtained between Hi+1 diffusivity calculated in α-Al2O3 from DFT: 5.05 × 10−3 exp(-0.81 eV/kB/T) cm2/s and reported experiment: 9.7 ×10−4 exp(-0.83 eV/kB/T) cm2/s. Computationally and experimentally calculated energy barriers (0.81 and 0.83 eV respectively) only differ by 2.5%. Similarly, the pre-exponential diffusion coefficients only differ by 0.5 orders of magnitude. Moreover, the diffusivity of Hi+1 in amorphous Al2O3 in the 1000–2000 K range is calculated to be 2.53 × 10−2 exp(-0.89 eV/k B/T), just one order of magnitude higher than the corresponding value in α-Al2O3. This suggests that local structural disorder does not significantly affect the energy landscape and diffusion behavior of Hi+1 in Al2O3. Overall, these results show promise for the application of alumina polymorphs as hydrogen permeation barriers.

Thermal stability and selective nitridation of Cr2AlC in nitrogen at elevated temperatures

The thermal stability and selective nitridation of Cr2AlC under a high–temperature nitrogen atmosphere were studied. Cr2AlC began to decompose beyond 1073 K in N2, and the activation energy was estimated to be 108.93 kJ·mol−1. Initially, the selective nitridation led to the generation of an AlN layer, which shielded the underlying Cr2AlC and the process was dominated by a surface reaction. The final products were AlN, Cr7C3 and Cr3C2, and the weaker Cr–Al bonds in Cr2AlC facilitated the rapid diffusion of aluminum from the interior outwards. As the reaction proceeded, micropores were observed on the grain surface, as well as a loose structure, which facilitated the diffusion of N2 and thus accelerated the reaction. Finally, the intensive reaction involving Cr2AlC and N2 could be attributed to the gas diffusion channels caused by improved temperature and continuous escape of vaporized aluminum.

Influence of alumina addition on steam corrosion behaviour of ytterbium disilicates for environmental barrier coating applications

The sintering and steam corrosion behaviour of Yb2Si2O7 with and without the addition of Al2O3 has been studied on compacted specimens. While, the addition of Al2O3 to Yb2Si2O7 enhanced the sintering of Yb2Si2O7 significantly, it also promoted the formation of a thin (80–100 nm), dense and protective Yb3Al5O12 layer at the Yb2Si2O7 surface when exposed to steam. TEM analysis confirmed that the Yb3Al5O12 layer is formed in steam environment via surface diffusion of aluminium from residual Al2O3 present on the surface of Al2O3/Yb2Si2O7 composite. These findings demonstrate the benefit of Al2O3 addition to protect environmental barrier coatings from steam corrosion.

Garnet crystallization mechanisms and localized polymetamorphism in the southwestern Meguma Terrane, Nova Scotia, Canada

AbstractWe present data on the pressure and temperature (P–T) conditions experienced by metamorphic rocks of the Meguma Terrane, Nova Scotia, Canada, also utilizing three‐dimensional microstructural data on one sample to better constrain the mechanisms that controlled garnet crystallization. Inverse and forward thermodynamic modelling place peak P–T conditions in the southwestern Meguma Terrane at ~650°C and 4.5 kbar. Interpretation of these results with petrographic observations and previous P–T constraints across the terrane suggests that amphibolite facies metamorphism occurred during the Devonian Neoacadian orogeny (406–388 Ma). Integration of quantitative 3D textural data with an estimated metamorphic heating rate of &lt;5°C/Myr is consistent with amphibolite facies metamorphism resulting from tectonic loading during the Neoacadian orogeny, though the exact nature of the orogeny is still not well understood. Further, the intrusion of granitic plutons into the Meguma metasediments at 373 Ma likely locally drove metamorphic recrystallization (polymetamorphism). The 3D size, shape, and location of garnet crystals in one sample reveal that the rate‐limiting step for garnet crystallization was likely the diffusion of aluminium through the intergranular matrix at length scales less than the mean nearest neighbour distance between garnet crystals. Nucleation was aided by epitaxial overgrowth onto a muscovite substrate, though it appears there may have been a decoupling between minerals providing a substrate and those providing nutrients during garnet growth.