In-Situ TiB2–TiC–Al2O3 Composite Coating on Aluminum by Laser Surface Modification

To improve the surface hardness of aluminum, in-situ TiB₂–TiC–Al₂O₃ composite coating was deposited on it by pre-placed laser coating process using precursor mixture of (TiO₂ + B₄C) and (TiO₂ + B₄C + Al). Pulsed Nd:YAG laser was used to produce coating track by scanning a laser beam in overlapped condition. Multiple tracks again overlapped to get a wider coating area. Phase constituents and microstructure of the deposited coating were studied by XRD and FESEM analysis. Vickers micro-hardness tester was used to measure micro-hardness of the coating. Results indicate that, in appropriate laser processing condition, coating was obtained with metallurgical bonding to aluminum substrate. XRD and microstructure analysis confirms the formation of TiB₂, TiC, and Al₂O₃ in the coating layer through in-situ reaction of reactant powders. Micro-hardness of the coating was found appreciably higher in comparison to the as-received aluminum substrate, due to presence of hard ceramic particles produced during in-situ reaction and their grain refinement for rapid cooling.     

Mechanically activated combustion synthesis of Ti3AlC2/Al2O3 nanocomposite from TiO2/Al/C powder mixtures

Ti₃AlC₂/Al₂O₃ nanocomposite powder was synthesized by mechanical-activation-assisted combustion synthesis of TiO₂, Al and C powder mixtures. The effect of mechanical activation time of 3TiO₂-5Al-2C powder mixtures, via high energy planetary milling (up to 20?h), on the phase transformation after combustion synthesis was experimentally investigated. X-ray diffraction (XRD) was used to characterize as-milled and thermally treated powder mixtures. The morphology and microstructure of as-fabricated products were also studied by scanning electron microscopy (SEM) and field-emission gun electron microscopy (FESEM). The experimental results showed that mechanical activation via ball-milling increased the initial extra energy of TiO₂-Al-C powder mixtures, which is needed to enhance the reactivity of powder mixture and make it possible to ignite and sustain the combustion reaction to form Ti₃AlC₂/Al₂O₃ nanocomposite. TiC, AlTi and Al₂O₃ intermediate phases were formed when the initial 10?h milled powder mixtures were thermally treated. The desired Ti₃AlC₂/Al₂O₃ nanocomposite was synthesized after thermal treatment of 20?h milled powder and consequent combustion synthesis and FESEM result confirmed that produced powder had nanocrystalline structure.     

Synthesis and characterization of V8C7 nanocrystalline powder by heating milled mixture of V2O5, C and Ca via mechanochemical activation

In this study, nano-crystalline vanadium carbide was synthesized through reduction of V₂O₅ by carbon and Ca using high energy ball milling and subsequent heat treatment. Vanadium pentoxide, calcium and carbon black were placed in a planetary ball mill and sampled after different milling times. The activated powders were synthesized by microwave heating at temperatures 800 °C. XRD and FESEM were used for characterization of synthesized powder. On the basis of obtained results, the synthesized V₈C₇ crystallites were in the scale of nanometers and the lattice parameter had some deviation from the standard value. Furthermore, investigations showed that at higher milling time, the amorphization degree of V₄C₃ phase increased, while the degree of crystallite decreased.     

Synthesis of aluminum nitride powder by carbothermal reduction of a combustion synthesis precursor

A new homogeneous mixing precursor of alumina and carbon (Al₂O₃ + C) was prepared by low temperature combustion synthesis (LCS) with aluminum nitrate (Al(NO₃)₃·9H₂O), urea (CO(NH₂)₂), and glucose (C₆H₁₂O₆·H₂O) as starting materials. The precursor prepared was a foamy and porous mass due to the large amount of gases liberated in the combustion reaction and composed of nanosize particles. XRD analysis showed that this precursor consisted of amorphous alumina and carbon. The amorphous alumina in the precursor first transformed into γ-Al₂O₃, and then γ-Al₂O₃ was directly nitrided to yield AlN during the calcination process. The reaction temperature needed for a complete conversion for the precursor was about 1400 °C, which is much lower than that when using alumina and carbon black as starting materials. The synthesized AlN powder was composed of very fine particles and had good dispersability.     

Synthesis of Al2O3-WC composite powder by SHS process

Al₂O₃-WC composite powder was synthesized by self-propagating high-temperature synthesis using Al powder as a reducing agent. WC, W₂C and Al₂O₃ were concurrently formed in WO₃-Al-C system. It was found that the complete reaction was achieved with excessive addition of carbon and appropriate processing parameters such as degree of dilution, particle size of aluminum, pellet compaction pressure and carbon source. The final product which was leached by 50% 1 : 4 HNO₃ + HF diluted solution was consisted of Al₂O₃-55wt%WC having 2–3 m of mean particle size.     

Characterization and thermal properties of carbon-coated aluminum nanopowders prepared by laser-induction complex heating in methane

Carbon-coated aluminum (Al) nanopowders were successfully synthesized by laser-induction complex heating in methane. The characterization of the samples was carried out using X-ray diffraction (XRD), transmission electronic microscopy (TEM) and high resolution transmission electronic microscopy (HRTEM). It was found that the nanoparticles show a spherical morphology with the size ranging from 28 to 50 nm and are covered with 3–4 graphitic layers. Thermal analysis using differential scanning calorimeter (DSC) and thermal gravimeter (TG) revealed that the as-prepared powders exhibit a lower oxidation onset and peak temperature, and a higher enthalpy change, as compared with those of Al₂O₃-passivated Al nanopowders. It was also found that the mass gain for the first oxidation stage of carbon-coated Al nanopowders is 1.5 times higher than that of Al₂O₃-passivated Al nanopowders. These demonstrate that the carbon coating is an effective route to increase the reactivity of Al.     

Mechanochemical carboaluminothermic reduction of rutile to produce TiC–Al2O3 nanocomposite

This investigation aims to produce TiC–Al₂O₃ nanocomposite by reducing rutile with aluminum and graphite powder via a mechanochemical process. The effect of milling time on this process was investigated. The characterization of phase formation was carried out by XRD and SEM. Results showed that after a 10 h milling, the combustion reaction between Al, TiO₂ and C was started and promoted by a self-propagation high temperature synthesis. Extending the milling time to 20 h, the reaction was completed. The XRD study illustrated after a 20 h milling, the width of TiC and Al₂O₃ peaks increased while the crystallite sizes of these phases decreased to less than 28 nm. After annealing at 800 °C for 1 h in a tube furnace, TiC and Al₂O₃ crystallite sizes remained constant. However, raising the annealing temperature to 1200 °C caused TiC and Al₂O₃ crystallite size to increase to 49 nm and 63 nm, respectively. No new phase was detected after the heat treatment of the synthesised TiC–Al₂O₃ nanocomposite.     

Fabrication of Ti2AIC by hot pressing of Ti, TiC, Al and active carbon powder mixtures

Polycrystalline Ti₂AlC samples were synthesized by hot pressing of Ti, Al, TiC and active carbon powder mixtures. X-ray diffraction (XRD) and scanning electron microscope (SEM) were used for phase identification and microstructure evaluation. No other phase except Ti₂AlC was detected in samples synthesized by hot pressing of the 0.5TiC/1.5Ti/1.0Al/0.5C powder mixtures at 1400°C for 1 and 3 h under a pressure of 30 MPa. The densities of these two samples were 96.1 and 98% of the theoretical value of pure Ti₂AlC, respectively. The reason that the densities of these two samples were lower than the theoretical density of pure Ti₂AlC is that pore existed in these two samples. At lower temperature of 1300°C, the speed of the reaction forming Ti₂AlC was slow. While at higher temperature of 1500°C, Ti₂AlC transformed to Ti₃AlC₂. So these two temperatures are not suitable for the fabrication of Ti₂AlC.     

Perfect wettability of carbon by liquid aluminum achieved by a multifunctional flux

The wettability of carbon (graphite and glassy carbon) by liquid aluminum was studied. A special molten salt (flux) system was developed under which perfect wettability (a zero contact angle) of liquid aluminum was achieved on carbon surfaces. The principal component of the flux is K₂TiF₆ dissolved in a molten alkali chloride. K₂TiF₆ is a multifunctional flux component as it performs the following tasks: (i) dissolves the oxide layer covering liquid aluminum, (ii) through an exchange reaction with liquid aluminum it ensures the necessary amount of Ti dissolved in liquid Al, which is needed to cover the Al/C interface by TiC. As TiC is a metallic carbide, it is perfectly wetted by liquid Al–Ti alloys. In this paper, the conditions of perfect wettability of carbon by liquid Al under MCl–K₂TiF₆ molten salts (fluxes) are found as function of: (i) the basic component of the flux (MCl = LiCl, or NaCl–KCl or CsCl), (ii) K₂TiF₆ content of the flux, (iii) temperature, (iv) flux:Al weight ratio, (v) specific surface area of Al, and (vi) specific surface area of carbon. A simplified theoretical equation is derived to reproduce the experimental data.     

Structural characterization of a mechanically milled carbon nanotube/aluminum mixture

The structural evolution of carbon nanotubes (CNTs) during mechanical milling was investigated using SEM, TEM, XRD, XPS and Raman spectroscopy. The study showed that milling of the CNTs alone introduces defects but preserves the tubular structure. When milling the CNTs with aluminum (Al) powder in order to produce a composite, Raman spectroscopy has shown that most of the nanotubes are destroyed. During sintering of the CNT/Al milled mixture, the carbon atoms available from the destruction of the nanotubes react with the Al to form aluminum carbide (Al₄C₃). The effect of milling on the Al matrix was also studied.     

Production of Fe3Al–TiC nanocomposite by mechanical alloying of FeTi2–Al–C powders

Abstract

The aim of the present work was to produce Fe₃Al/TiC nanocomposite by mechanical alloying of the FeTi₂30Al10C60 (in at-%) powder mixture. The morphology and the phase transformations in the powder during milling were examined as a function of milling time. The phase constituents of the product were evaluated by X-ray diffraction (XRD). The morphological evolution during mechanical alloying was analysed using scanning electron microscopy (SEM). The results obtained show that high energy ball milling, as performed in the present work, leads to the formation of a bcc phase identified as Fe(Al) solid solution and an fcc phase identified as TiC and that both phases are nanocrystalline. Subsequently, the milled powders were sintered at 873 K. The XRD investigations of the powders revealed that after sintering, the material remained nanocrystalline and that there were no phase changes, except for the ordering of Fe(Al), i.e. formation of Fe₃Al intermetallic compound, during the sintering process.     

Interactions between Y2O3–Al mixture studied by solid-state reaction method

Interactions between Y₂O₃–Al mixture studied by solid-state reaction method were investigated in present paper. Interactions between Y₂O₃–Al mixture was characterized by differential thermal and thermogravimetric analyses and X-ray diffraction, Y₂O₃–Al mixture and yttrium aluminum garnet (YAG) powder as final reaction product were characterized by scanning electron microscopy. The results show Al is isolated with Y₂O₃ by aluminum oxide layer in air, and no opportunity of directional reaction between Y₂O₃–Al systems. With temperature increasing to ∼569 °C, aluminum partly turned into transitional aluminas, Y₂O₃ reacts with transitional aluminas instead of aluminum to form yttrium aluminum monoclinic (YAM) and yttrium aluminum perovkite (YAP) phases after calcination at 600 °C, 800 °C separately, and pure YAG powder is obtained after calcination at 1200 °C. From the point of view of reaction temperature, the reaction between Y₂O₃ and transitional aluminas is easier than that of Y₂O₃ and Al or α-Al₂O₃.     

Microstructure and its influence on refining performance of AlTiC master alloys

Abstract

The microstructures of four different AlTiC master alloys produced through a new method involving the reaction of pure Ti and carbon in an Al melt have been examined using XRD, SEM, EPMA, and TEM, and their refining efficiencies tested. Individual TiC particles were found to be single crystals with polyhedral or spherical morphologies. In addition to the aluminium matrix, Al - 5Ti - 0.3C (wt-%) refiners contain TiC and TiAl₃ phases, whereas Al - 8.4Ti - 1.8C contains only TiC. Three types of agglomerating behaviour of TiC particles, namely discrete particles, homogeneously distributed clusters, and compact blocks along grain boundaries, were found in master alloys produced at different temperatures, and these further influence the refining performances. Ti was found to play an important role in the refinement of Al by AlTiC refiners; observation of grain centres showed that TiC particles are located at the grain centre and the excess Ti segregates to them and forms a roselike structure around them. It is clear that the refinement of Al by AlTiC results from the combined action of TiC and Ti, and it is possible that AlTiC and AlTiB share some common rules in refining Al. Possible refinement mechanisms have been discussed based on microstructure observations and previous studies.     

Magnetic and microwave-absorbing properties of SrAl<Subscript>4</Subscript>Fe<Subscript>8</Subscript>O<Subscript>19</Subscript> powders synthesized by coprecipitation and citric- combustion methods

Al-substituted M-type hexaferrite is a highly anisotropic ferromagnetic material. In the present study, the coprecipitation and the citric-combustion methods of synthesis for SrAl₄Fe₈O₁₉ powders were explored and their microstructure, magnetic properties, and microwave absorptivity examined. X-ray diffraction (XRD), scanning electron microscopy (SEM), a vibrating sample magnetometer, and a vector network analyser were used to characterize the powders. The XRD analyses indicated that the pure SrAl₄Fe₈O₁₉ powder was synthesized at 900°C and 1000°C for 3 h by coprecipitation, but only at 1000°C for the citric-combustion processes. The SEM analysis revealed that the coprecipitation process yielded a powder with a smaller particle size, near single-domain structure, uniform grain morphology, and smaller shape anisotropy than the citric-combustion process. The synthesis technique also significantly affected the magnetic properties and microwave-absorptivity. Conversely, calcining temperature and calcining time had less of an effect. The grain size was found to be a key factor affecting the property of the powder. The powders synthesized by coprecipitation method at calcining temperature of 900°C exhibited the largest magnetization, largest coercivity, and best microwave absorptivity.     

Combustion reaction of Ti–Al–C–N system

The combustion reaction of Ti–Al–C–N system was investigated by using Ti powders and one CNx precursor powder as reactant powder blends. The reactant powder blends ratio was adjusted to obtain different materials. The phase composition of the samples was investigated by X-ray diffraction (XRD). The microstructure of the samples was observed by scanning electron microscopy (SEM). The result showed that Ti₂Al(C, N)–TiAlx, AlN–Ti(C, N) and Ti₃Al(C, N)₂–TiC composites can be fabricated by changing the reactant powder blends ratio.     

Microwave Synthesis of Yttrium Aluminum Iron Garnet Powder

The solid solution of yttrium aluminum garnet-yttrium iron garnet [YAIG, Y₃(Al,Fe)₅O₁₂] was synthesized from the component oxides by 28 GHz-microwave irradiation. The compositions of the resulting garnets were not the same as the nominal compositions. This difference could be explained by selective coupling of chemical species with microwaves, predictable from the temperature–time profiles of each raw material under microwave irradiation. The diffusion process under microwave irradiation was discussed.     

Study of the oxidation and sintering behaviours of Al-Al2O3 powder mixture by using a home-style microwave oven

By using a home-style microwave oven, Al-Al₂O₃ powder mixture could be heated and Al powder oxidized to convert the Al-Al₂O₃ powder mixture into an Al₂O₃ body and, subsequently, this Al₂O₃ body was sintered. The differences of powder characteristics which resulted from differently processed raw materials affected the oxidation behaviours, and these effects were more serious when a microwave oven was used than when a conventional furnace was used. Heating and oxidation mechanisms are discussed. Also the simultaneous oxidation and sintering of an Al-Al₂O₃ powder mixture were performed within 1 h by using microwave hybrid heating (MHH), which greatly reduced processing time and energy.     

Fabrication and wear properties of Al2O3-SiC ceramic coatings using aluminum phosphate as binder

Refractory and wear-resistant Al₂O₃-SiC ceramic coatings have been fabricated on A₃ steel using abrasive ceramics (Al₂O₃, SiC), aluminum phosphate binder (inorganic binder), and aluminate (Al₂O₃ · CaO) as starting materials. The Powder X-ray diffraction (XRD) and scanning electron microscopy (SEM) techniques are used to investigate the chemical compositions of the in-house synthesized aluminum phosphate binder and the morphologies of the fabricated ceramic coatings after abrasion test. The XRD results indicate that monoaluminium phosphate (Al(H₂PO₄)₃) is the most effective binding phase in aluminum phosphate binder, and that aluminum phosphate binder at high temperatures is a mixture of several phases. It is also found that the addition amount of the stabilizer (oxalic acid) has remarkable effect on the storage life of aluminum phosphate binder. The wear test results show that the wear resistance of the A₃ steel covered with Al₂O₃-SiC ceramic coatings is about two times higher than that of the uncoated A₃ steel. The results also indicate that the wear properties of Al₂O₃-SiC ceramic coatings are dependent on fabrication conditions, such as the weight ratio of ceramics to the binder (RCB), the particle size distribution of ceramics, the density of the aluminum phosphate binder, and the Al/P atomic ratio in the aluminum phosphate binder. The optimal fabrication conditions for achieving good wear resistance of Al₂O₃-SiC ceramic coatings are suggested based on the above results.     

Reaction mechanism in SiO2-MnO2-Fe-Al system for producing ferrosilicomanganese powder

In this research, the ferrosilicomanganese powder was synthesized via mechanical alloying and SHS methods. Silicon oxide, manganese oxide, iron and aluminum powders were used as starting materials. SHS process was initiated by oxyacetylene flame. The activated and inactivated powder mixtures were used for producing ferrosilicomanganese grade 26%Si-53%Mn-21%Fe. The powder mixtures were characterized by X-ray diffractometry, scanning electron microscopy and energy dispersive X-ray spectroscopy. Thermodynamic analysis showed that aluminothermic reduction of MnO₂ is more thermodynamically favored as compared with aluminothermic reduction of SiO₂. Adiabatic temperature of MnO₂ and SiO₂ reduction by Al was calculated about 2946 K. It was found that no reaction took place during mechanical alloying. After annealing and SHS processes, first MnO₂, then SiO₂ oxides were reduced by Al and ferrosilicomanganese and Al₂O₃ phases were produced. Due to the high adiabatic temperature, all products were formed in liquid state, leading to produce sintered ferrosilicomanganese and isolated Al₂O₃ particles.     

微波碳热还原法制备氮化铝粉末的工艺研究

采用微波碳热还原法制备了氮化铝粉末,研究了铝源、碳源和添加剂对制备氮化铝粉末的影响. 通过对所合成的产物进行XRD检测分析表明,氢氧化铝和乙炔黑是最合适的铝源和碳源、单质添加剂的氮化催化效果最明显. 以氢氧化铝和乙炔黑为原料,加入单质添加剂,在氮气气氛下反应温度为1300℃、反应时间为1h时能获得完全氮化的氮化铝粉末,可见微波碳热还原工艺能够大大降低碳热还原法制备氮化铝粉末的反应温度,并缩短反应时间.