High-temperature oxidation process of intermetallic compound Ti-42 at% Al

The oxidation process of two-phase (Ti₃Al and TiAl) intermetallic compound, Ti-42 at% Al, in air at 1073 and 1273 K has been investigated. The oxidation at 1273 K is much faster than that at 1073 K; however, the oxidation kinetics are similar. During heating up, TiO₂ scale forms initially on the compound surface at about 973 K, and then Al₂O₃ scale forms at about 1273 K. For the isothermal heating, TiO₂ scale slowly grows up at 1073 K, while at 1273 K both TiO₂ and Al₂O₃ scales grow up drastically. The outer oxide scale consists of TiO₂ and the inner one consists of a mixture of TiO₂ and Al₂O₃. The volume of Al₂O₃, which forms after TiO₂ formation at the initial stage of oxidation, is larger at an area adjacent to the oxide-compound interface.     

The wettability of titanium diboride by molten aluminum drops

Titanium diboride is widely accepted to be completely wet by liquid aluminum, yet few published wetting studies demonstrate this behavior, and reported contact angles vary widely. Sessile drop substrates from four different sources were selected and their microstructures and chemistries characterized. The results of sessile drop experiments using four techniques to modify oxide film behavior were compared. The Al-TiB₂ interfaces were examined in metallographic sections or after chemical removal of the Al drop. Al wets a material containing 5.5 wt% Ni in vacuum experiments before the hold temperature of 1025^° C is reached. The other TiB₂ substrates are completely wet by Al at 1025^° C, but only after prolonged holds under vacuum. Elimination of boron oxide from the TiB₂ surface leads to a spreading condition. The role of the substrate microstructure (porosity, grain size, roughness, and carbon content) in altering the wetting kinetics is discussed.     

Effect of annealing on microstructure and mechanical properties of bulk nanocrystalline Fe3Al alloy with 5 wt.% Cu prepared by aluminothermic reaction

Microstructure and mechanical properties of bulk nanocrystalline Fe₃Al based alloy with 5 wt.% Cu prepared by aluminothermic reaction before and after annealed at 873, 1073 and 1273 K for 8 h were investigated. Microstructures of the alloy before and after the annealing consisted of a Fe-Al-Cu matrix, a little Al₂O₃ sphere and Fe₃AlC_x fiber phases. The matrix of the alloy before the annealing was composed a nanocrystalline phase with disordered bcc crystal structure and a little amorphous phase. The amorphous phase disappeared after the annealing and Fe₃Al phase with ordered DO₃ structure appeared in the alloy after annealed at 1073 and 1273 K in the matrix of the alloy. Size of the Fe₃AlC_x fiber phase increased with the annealing temperature. The alloy after the annealing had better plasticity, higher yield strength than that of the alloy before the annealing, and the alloy after annealed at 1273 K had the highest yield strength.     

Effect of temperature and surface roughness on the wettability of boron nitride by molten Al

Reactive wetting of hexagonal BN by molten Al at 1073–1273 K was studied using an improved sessile drop method. The temperature and substrate surface roughness have a remarkable effect on the wetting behavior. Reasons for the large discrepancy in the final contact angles reported in the literature were addressed.     

Wetting of MgO and α-Al2O3 by molten Mg: a model of droplet with high vapor pressure

The wetting of MgO and α-Al₂O₃ polycrystalline plates by molten Mg in purified argon was studied between 973 and 1273?K using an improved sessile drop method. The MgO/Mg system is basically non-reactive while Al₂O₃/Mg is reactive. The common features in the non-reactive and reactive systems are the high tendency for oxidation and the high evaporation rate of Mg. It is shown that wetting kinetics in both systems is governed by the evaporation and pinning phenomenon leading to contact angle versus time curves passing through a minimum: the apparent contact angle first decreases and then increases. The intrinsic contact angle should be estimated using the initial value (advancing contact angle) and the maximum value just before the disappearance of the droplet caused by Mg evaporation (receding contact angle).     

Wetting of (0001) α-alumina single crystals by molten Mg–Al alloys in the presence of evaporation

The wetting of (0001) α-alumina single crystals by Mg–Al alloys over a wide composition range at 1073?K was investigated using an improved sessile drop method in a flowing argon atmosphere. The initial contact angles are between 103° and 84°, almost linearly decreasing with increasing nominal Mg concentration, suggesting that the addition of Mg to Al improves the initial wettability. According to the evolution of contact angle and contact diameter, representative stages were identified to characterize the complex wetting behavior in the presence of evaporation. The wetting kinetics was dependent on the nominal Mg concentration in the alloy. Two patterns of “stick–slip” behavior were observed in the wetting process and interpreted by combining the effects of interfacial reaction and evaporation of magnesium. In addition, the dependence of the interfacial reaction on the Mg–Al alloy concentration was thermodynamically analyzed. The dominant reaction product at 1073?K should be MgO when x _Mg?>?9?mol%, while MgAl₂O₄ when x _Mg?<?9?mol%. However, because of the continuous consumption of Mg due to the evaporation and reaction, its concentration in the alloy progressively decreased with time. As a result, MgO formed usually earlier while MgAl₂O₄ later even for the alloys with higher than 9?mol% Mg.     

Effect of Nb?+?Ti coating on the wetting behavior, interfacial microstructure, and mechanical properties of Al/Al2O3 joints

The subject of the work was to study the effect of Nb + Ti thin film deposited by PVD method on alumina substrates on the wetting behavior, bond strength properties, and structure of interface in the Al/Al₂O₃ joints. Applying the sessile drop method, the wetting behavior of molten Al (99,999%) on coated alumina substrates was studied in the temperature range between 953 and 1373 K under a vacuum of 0.2 mPa for 30 min of contact. The sessile drop samples were used to examine the interface structure, shear strength, and interfacial fracture toughness under the concentrated load. The introduction of the thin Nb + Ti film layer of 900 nm thickness: (1) greatly improves the wettability of alumina by molten Al at above 1223 K and the shear strength of Al/Al₂O₃ joints produced at 1223 K, (2) has positive effect on structure transformation in the interface and leads to fabrication of reliable metal–ceramic joints. Microstructural investigations of the interface indicated that the precipitates of Nb and Ti-rich intermetallic phases were formed at the Al/Al₂O₃ interface, which influenced strengthening of these joints. Hence a conclusion can be drawn that the interface structure influences the durability increase in Al/Al₂O₃ joints.     

Reactive hot pressing of Al2O3-Ni composites

Reactive hot pressing has been used to form Al₂O₃-Ni composites from Al and NiO. The effect of attrition milling on the precursor powder and subsequent composite formation was examined. The surface area, phase assemblage, reaction temperature, and morphology of precursor powders were characterized as a function of milling time, which ranged from 0 (unmilled) to 480 min (8 hrs). During milling, particle surface area increased from less than 1 to more than 11 m²/g as the size of the Al and NiO particles decreased. At the same time, the temperature at which Al and NiO reacted to form Al₂O₃ and Ni decreased from more than 1000°C to around 600°C. Formation of Al₂O₃ or Ni during milling was not detected, regardless of time. Precursor milling time also affected the morphology and phase assemblage of composites produced by reactive hot pressing. Composites formed from unmilled powders contained a small amount of unreacted NiO and had a Ni ligament size greater than 10 m. The composite forming reaction went to completion when powders milled for one hour or more were hot pressed. Based on microstructural evidence and analogy to similar reactions, it appears that the composite forming reaction proceeds by Al diffusing into and reacting with NiO.     

Surface and grain boundary wetting of Fe based solids by molten Pb and Pb-Bi eutectic

Surface and grain boundary wetting and penetration of pure Fe and a martensitic steel by Pb and Pb-Bi eutectic alloys were studied by the sessile drop and dispensed drop techniques at different temperatures (400–900^°C) and times (up to 30 h). By using two different atmospheres—high vacuum and a He-H₂ gas—and different heat treatments, wetting was studied for both oxidized and deoxidized solid substrates.     

Thermal stability of Al/Al11 Ce3 and Al/Al11 La3 /Al3 Ni eutectics obtained by Bridgman growth

Abstract

The effects of extended treatment at 500°C on microstructure and microhardness of Al/Al₁₁ Ce₃ and Al/Al₁₁ La₃ /Al₃ Ni eutectics in Al–12·1Ce and Al–11·3La–5·4Ni (wt-%) alloys Bridgman solidified at 0·1 mm s^-1 are reported. Coarsening of Al/Al₁₁ Ce₃ occurs more rapidly in some eutectic cells than others such that areas coarsening at a lower rate still form 10% of the microstructure even after 3024 h. It was found that Al/Al₁₁ La₃ /Al₃ Ni showed accelerated coarsening at eutectic cell boundaries but association between Al₁₁ La₃ and Al₃ Ni still remained within cells and at cell boundaries even after 3024 h. Associated decreases in hardness for Al/Al₁₁ Ce₃ were similar to fibrous Al/Al₆ Fe of similar initial spacing subjected to the same heat treatment. The 30% higher initial hardness of Al/Al₁₁ La₃ /Al₃ Ni largely persisted even after 3024 h at 500°C.     

Oxidation and reduction behavior of Ni and NiO layers sputter deposited onto yttrium-stabilized zirconia single crystals

Thin single-crystal yttrium-stabilized zirconia (YSZ) substrate was prepared by indentation fracture and mechanical polishing. The specimen was analyzed in detail by transmission electron microscopy (TEM). The (110) edge surface was faceted, in contrast to the smooth (001) edge surface, and the facet surfaces were identified as {111}-type planes. Good cross-sectional TEM specimens comprised of crystalline Ni and NiO layers deposited on YSZ edge surface could be prepared by sputtering of a Ni support grid using Ar⁺ ion milling and subsequent re-deposiotion on the smooth (001) fracture surface of the YSZ specimen. The epitaxial growth of a pure Ni layer on the YSZ edge planes occurred during ion milling in vacuum. However, subsequent ion milling of the specimen after exposure in air for several minutes resulted in the formation of a NiO layer on top of the first Ni layer. Reduction of the NiO layer was confirmed by electron energy-loss spectroscopy after annealing at 973 K in a vacuum of 1.2 × 10⁻⁵ Pa. This Ni layer was re-oxidized upon annealing in air at 1073 K for 1 h. The deposition behavior of the Ni and NiO layers was discussed on the basis of the surface oxidation of Ni layer.     

Structural,Morphological, Optical and Magnetic Properties of Al-Doped CoFe<Subscript>2</Subscript><Emphasis Type="Bold">O</Emphasis><Subscript>4</Subscript> Nanoparticles Prepared by Sol–Gel Auto-Combustion Method

CoFe_2?x Al _x O₄ (x = 0.0,0.5,1.0, and 1.5) ferrite nanoparticles have been synthesized by the sol–gel auto-combustion method. The effect of non-magnetic Al content on their structural, morphological, optical, and magnetic properties was also investigated. X-ray diffraction (XRD) diffraction analysis was applied and indicated that the synthesized nanopowders of samples with x<1.5 and calcined at 800 ^°C have single-phase spinel structure. It has shown also by increasing Al content, the particle size, lattice parameter, unit cell volume, coercivity, anisotropy constant, and magnetization decrease, while the energy band gap increases. The size of particles was measured by TEM being in the range of 65–75 nm (for x = 0.0) and 9–10 nm (for x = 1.0). For sample with x = 1.5, the minimum calcination temperature for obtaining a single-phase spinel structure was 1000 ^°C. By increasing the calcination temperature from 1000 to 1100 ^°C, the mean crystallite size and crystallinity increase, while the lattice parameter, coercivity, anisotropy constant, and magnetization decrease. The average grain size evaluated by SEM analysis was found to be \(\tilde 91\) and 166 nm for samples calcined at 1000 and 1100 ^°C, respectively.     

Preparation of PbGeO<Subscript>3</Subscript> by the oxidation of Pb–Ge melts and its properties

This paper presents experimental data on the oxidation kinetics of a liquid 65 at % Pb + 35 at % Ge alloy in air. Lead metagermanate, PbGeO₃, in a glassy state has been prepared by high-temperature oxidation of the melt at 1273 K. Interaction of molten PbGeO₃ with crucible materials (SnO₂, Au, Pd, and Pt) has been studied by the sessile drop method. Liquid lead germanate has been shown to be highly reactive with substrates from these materials. The heat capacity of the glassy lead metagermanate obtained has been determined by differential scanning calorimetry in the temperature range 300–800 K. In the range 330–630 K, the C_p(T) data for glassy PbGeO₃ can be represented using the Maier–Kelley equation. Above 630 K, there is a characteristic peak due to glass crystallization processes.     

Microscopic phase-field study on atomic site occupation probability in L12-Ni3(Al1 − xFex) structure

The effect of Fe content on atomic site occupation in L1₂-Ni₃(Al_1 − xFe_x) structure at 1073 K was studied by using microscopic phase-field model. The results show that with increasing Fe, Fe atoms prefer to occupy the Al sites (β sites), and the site occupation probability (SOP) gradually increases, while the SOP of Al atoms on β sites decreases accordingly. Meanwhile, a small amount of Ni atoms also occupy β sites which is the so-called anti-site behavior; reversely, such behavior happens as a trace amount of Al and Fe atoms occupies α sites. Unlike Ni atoms, the anti-site behavior of Al and Fe atoms is strengthened with the increase of Fe content. Further, the precipitation mechanism, the size and the ordering degree of L1₂ structure also present obvious changes with the addition of Fe.     

In situ monitoring of NiO–Al2O3 nanoparticles synthesis by thermo-Raman spectroscopy

Simultaneous thermogravimetric analysis and thermo-Raman spectroscopy (TRS) measurements for in situ monitoring of wet chemical reaction of Ni(OH)₂·4H₂O and Al(OH)₃ forming NiO–Al₂O₃ nanoparticles is studied and compared with the solid-state reaction. Herein, a different approach of synthesis and monitoring of NiO–Al₂O₃ by TRS is presented, in which, in situ thermo-Raman spectra are recorded at every degree interval from 25 to 800 °C to understand the structural and compositional changes in NiO–Al₂O₃ as a function of temperature. Slow controlled heating of the sample as in TRS, enables better control over morphology and particle size distribution (～10–20 nm diameter). The X-ray diffraction (XRD) shows that smaller particle size is obtained using wet chemical reaction than the solid-state reaction (～25 nm diameter). TRS studies also reveal that, the bulk NiAl₂O₄ forms at temperatures above 800 °C, although, the onset of formation is around 600 °C. Condensation of Al(OH)₃ forming Al₂O₃ is also monitored, wherein, presence of hydrocarbon is found to contribute to the observed fluorescence background. Based on the TRS and complementary characterizations using XRD, scanning electron microscopy, transmission electron microscopy, and energy-dispersive X-ray analysis, the formation of supported NiO–Al₂O₃ is discussed.     

Low temperature aluminisation of alloy steels by pack cementation process

Abstract

The pack aluminisation process is normally applied at temperatures >973 K at which the mechanical properties of alloy steels would degrade. Thus, the present study was undertaken to apply this process to aluminising the alloy steels at temperatures <973 K in order to increase their high temperature oxidation resistance while maintaining their microstructure and hence mechanical strength and creep resistance. A type of commercial alloy steel P92 (9Cr–1Mo) was used for the present study. Pack powder mixtures consisting of Al, AlCl₃ (anhydrous) or NH₄Cl and Al₂O₃ were used to carry out the process. The aluminising temperature was varied from 773 to 973 K, pack Al content from 1 to 30 wt-% and aluminising time from 1 to 16 h to investigate their effects on the coating growth kinetics in the AlCl₃ activated packs. It was observed that all the coatings formed in the AlCl₃ activated packs were of a single layer structure with Fe₂Al₅ as the main coating phase. It was established that the interrelationship between the thickness h (in μm) of this coating layer and aluminising temperature T (in K), time t (in h) and pack Al content W (in wt-%) can be described by h=83005·9W^1/2t^1/2e^?73330/(RT). In the NH₄Cl activated packs, it was found that coating formation and dissolution took place simultaneously at 923 K and stable growth of a coating layer was only possible when the pack Al content was sufficiently high. However, the coatings formed in these packs had highly uneven regions.     

Solid-state bonding of alumina to austenitic stainless steel

A low temperature and low pressure bonding process for alumina and 316L austenitic stainless steel has been developed using a titanium/molybdenum laminated interlayer. The intermetallic compounds of Ti₃Al (or Ti₂/Al) and TiAl were formed at the alumina/titanium interface on bonding at above 1273 K. The activation energy of the layer growth was about 142 kJ mol^–1. The construction of Al₂O₃/Ti/Mo/steel gave the most stable joints. The highest tensile strength was above 60 MPa with a titanium 0.4 to 0.6mm thick/molybdenum 0.4 to 0.5 mm thick interlayer on bonding at 1273 K for 3 h under pressure of 12 MPa.     

Phase transformation and hardness of the Ni–P–Al ternary coatings under thermal annealing

Ternary Ni–P–Al coatings were fabricated by the dual-gun rf magnetron sputtering technique. The as-deposited Ni–P–Al coatings exhibited a major Ni nanocrystalline phase (with a (111) texture) with Al and P co-deposited. After 400 °C heat treatment, various Ni_xP_y compounds, including Ni₁₂P₅, Ni₅P₂, and Ni₃P, formed within the recrystallized Ni matrix. Accordingly, the hardness of the coating increased to 10 GPa due to the Ni_xP_y precipitation. For heat treatment temperatures higher than 450 °C, Ni_pAl_q hard phases were observed in the ternary Ni–P–Al coating. A further increase in coating hardness from 10 to 12 GPa was revealed. The hardening of the annealed Ni–P–Al coatings was attributed to precipitation of Ni–P and Ni–Al compounds formed around 400 and 500 °C, respectively. A two-stage hardening in Ni–P–Al coating by Ni_xP_y and Ni_pAl_q precipitation through heat treatment was then demonstrated. Through surface analysis, the increase in surface roughness for the Ni–P–Al coating due to the formation of Ni_xP_y and Ni_pAl_q compounds was revealed.     

One-pot solvothermal synthesis of size-controlled NiO nanoparticles

In this work, NiO nanoparticles with diameter of ～4.5?nm for application as electrode material in supercapacitor were directly synthesized by an one-pot solvothermal strategy using N,N-dimethylformamide (DMF) as solvent and hexadecyl trimethyl ammonium bromide (CTAB) as additive. The experiment results indicated that CTAB is the key for the phase formation of NiO. Ni(OH)₂ phase was formed in the absence of CTAB, while NiO phase was produced in the presence of CTAB as an additive. As compared with the NiO prepared by calcination on the layer-structured Ni(OH)₂ intermediate produced in the absence of CTAB, the NiO nanoparticles prepared by this strategy had smaller particle size. Due to large specific surface area, small size and residual Ni(OH)₂, these NiO nanoparticles presented high electrochemical performance. They had high specific capacitance of 1128?F?g^?1 at low current density of 3.1?A?g^?1 and 625.0?F?g^?1 at high current density of 62.5?A?g^?1.     

The use of aluminium-enriched layers on Hastelloy X against high temperature carburization in high temperature gas-cooled reactor helium

The possibility of protecting nickel-based alloys against carburization using NiAl layers is discussed. NiAl layers were produced using the chemical vapour deposition reaction: $\text{AlCl}{}_3\text{+}\text{3}\text{2}\text{H}{}_2\text{+}\text{Ni-based alloy}\text{→}\text{NiAl}\text{+3}\text{HCl}$ The kinetics of growth of an NiAl layer on nickel and the nickel-based alloys Hastelloy X, IN-738LC and IN-617 in the temperature range 1173–1373 K at total pressures ranging from 100 to 800 mbar are described.No carbon uptake occurs in the layer or in the bulk alloy, because of the formation of an α-Al₂O₃ oxide layer in CH₄H₂ gas mixtures with carbon activities from 0.2 to 0.8 and at temperatures of up to 1273 K. Al₂O₃ formation is caused by the presence of oxygen as an impurity in the CH₄H₂ gas mixtures.