A study of combustion synthesis reaction in the Ti + C/Ti + Al system

Combustion synthesis (SHS) of the Ti + C/Ti + Al system was investigated by using titanium, graphite and aluminum powders as reactants. These powders were thoroughly mixed and pressed into cylindrical compacts, and heated in an argon atmosphere. The effects of the reactant composition and the heating rate were studied. The phase identification and morphology observation of the products were carried out by X-ray diffraction (XRD) and scanning electronic microscopy (SEM), respectively. XRD analysis showed that the products, in addition to the expected TiC/TiAl phases, also contained an appreciable amount of the ternary carbide (i.e., Ti _x AlC). The heating rate was found to strongly affect the extent of the combustion reaction. A possible reaction mechanism based on the experimental results was proposed to describe the whole process of the SHS reaction and the characteristic product morphology. It was considered that the ternary carbides may be formed by the peritectic reaction between TiC and the Ti–Al melt during the cooling stage after combustion.     

Study on Ti fiber reinforced TiAl<Subscript>3</Subscript> composite by infiltration-in situ reaction

This study is concerned with investigation of forming Ti fiber reinforced TiAl₃ composite by infiltration-in situ reaction. The as-cast material was obtained by pressing molten pure Al into a preform which was composed of Ti particles and Ti fibers. Based on the differential scanning calorimetry (DSC) result, in situ reaction samples were obtained by heating as-cast materials to 660, 950, and 1300 °C, and held for 1 h, respectively. The microstructure evolution of in situ reaction samples was analyzed by scanning electron microscope and Energy dispersive X-ray (EDX). In addition, the phase composition of products was inspected by X-ray diffraction (XRD). Experiment results show that TiAl₃ was formed initially, which was the unique product between Ti and Al. While at high temperature, products of Ti fibers and Al were complex, and Ti _x Al_1−x (0.25 < x < 0.75) compounds were formed around Ti fibers. Finally, TiAl₃ decomposed, and oxidation occurred. The mechanism of in situ reaction between Ti and Al in this system was discussed.     

A study of the combustion synthesis of MoSi2 and MoSi2-matrix composites

The combustion synthesis of MoSi₂ from elemental powders was investigated. Higher completion of the reaction was achieved by the use of prior compaction of powders and higher heating rates. The reaction mechanism was studied by performing experiments at a heating rate of 20 K/min. It was established that reaction occurred via capillary motion accompanying the flow of molten silicon to the molybdenum powder particles. During the synthesis of MoSi₂/TiC composites, two ternary (Mo,Ti) Si₂ phases were observed, in contradiction to the thermodynamic predictions of the Mo-Si-Ti-C system. Combustion synthesis of (Mo, W) Si₂ yielded an inhomogeneous product where the composition of the solid solution varied throughout the sample. These observations were discussed in terms of the characteristics of the reaction process.     

Reactive processing of nickel-aluminide intermetallic compounds

NiAl have been fabricated by reactive sintering compacts of ball-milled powder mixtures containing Ni and Al. The reaction mechanism, as well as phase and microstructural development, were investigated by analyzing compacts quenched from different temperatures during reactive hot compaction. It was found that the reaction process was strongly affected by pressure, heating rates, heat loss from the sample to the environment. The application of 50 MPa prior to the reaction resulted in the intermetallic-formation reaction initiating at a temperature (480°C) much lower than that (550°C) when no pressure was applied. At high heating rate (50°C/min), when the heat loss is small, the formation of NiAl occurs rapidly via combustion reaction. On the other hand, if the heat loss is significant as in slow heating rate (10°C/min), the reaction process is controlled by solid-state diffusion. The phase formation sequence for the slow solid-state reaction was determined to be: NiAl₃ Ni₂Al₃ NiAl NiAl (Al-rich) + Ni₃Al NiAl.     

Differential scanning calorimetry and reaction kinetics studies of γ + α2 Ti aluminide

Reaction synthesis method for titanium aluminide processing consists of an exothermic reaction among alloying elements present and primarily between titanium and aluminium particles at specific temperature range. Study of this reaction helps in understanding the process of aluminide formation. Differential scanning calorimetry (DSC) study is the suitable method to study such reactions. In the present work, five different alloy mixtures based on Ti48Al2Cr2Nb0.1B are prepared and DSC study is carried out. Onset temperature, peak temperature and completion temperature of the major exothermic reaction is analyzed at different heating rates. Further, kinetics of the reaction is studied using Johnson–Mehl–Avrami equation. Activation energy and Avrami parameter are calculated and compared with the reported works on binary alloy. It has been observed that exothermic reaction is triggered by melting of aluminium. Boron assists in increasing the enthalpy of reaction by boride formation. Primary reaction product is found to be TiAl₃. Activation energy as well as Avrami parameter is found to have marginal variation due to small change in alloying elements in different alloys and due to heating rates in the same alloy.     

Synthesis of nitrogen ceramic povvders by carbothermal reduction and nitridation Part 3 Aluminium nitride

Abstract

It has been found that the reaction rate between alumina and carbon in nitrogen can be considerably increased by using Al(OH)₃–C mixtures instead of α Al₂O₃–C mixtures. The C/Al(OH)₃ ratio in the starting mixtures should be greater than the stoichiometric ratio to complete the reaction. It was observed that alumina is reduced and nitrided to form aluminium nitride via spinel type aluminium oxynitride which is an intermediate compound. The overall reaction rate increased with decreasing Al(OH)₃ particle size and increasing nitrogen flowrate. It is thought that the faster reaction rates obtained from Al(OH)₃–C mixtures were derivedfrom the high specific surface area of transition aluminas, such as γ Al₂O₃, which formed during heating of the samples to the reaction temperature. The particle morphologies of the Al(OH)₃ powders did not change during the reduction and nitridation reaction, an important attribute to the process.

MST/1346c     

The role of the Al2O3 passivation shell surrounding nano-Al particles in the combustion synthesis of NiAl

The self-propagating combustion behaviors of Nickel (Ni) and Aluminum (Al) thermites were studied as a function of bimodal Al particle size distributions. In particular, the low melting temperature of nano-scale Al particles coupled with the low concentrations of Al₂O₃ in micron-scale Al particles were exploited in order to optimize the macroscopic properties of the final alloy. Bimodal Al size distributions ranging from 0 to 50 wt% nano-Al combined with 50 wt% Ni were studied. Laser ignition experiments were performed on pressed pellets to determine flame propagation behavior and product microstructural features as a function of Al particle size. A new imaging technique is also presented that allows visualization of the surface reaction through highly luminescent flames and more accurate evaluation of burn rates. The wear behavior of the product alloy was measured and reported. Results show that composites composed of more micron-scale than nano-scale Al particles absorb more laser energy prior to flame propagation and experience an effective preheating. When 10–30 wt% nano Al is combined with micron Al and Ni, the wear resistance of the product alloy is optimized. Electron micrographs of the alloys suggest these properties may be attributed to whisker formations that behave as binding strings improving the overall abrasion resistance of the composite.     

A comparison of the thermal decomposition mechanism of wurtzite AlN and zinc blende AlN

The sublimation route is one of the primary and most significant methods for the synthesis of an aluminum nitride (AlN) single crystal. Its long synthesis time and high reaction temperature, however, limit the production of its commercial product. In this work, we applied HSC Chemistry 6 software, ab initio molecular dynamics, and X-ray diffraction to investigate the thermal decomposition of AlN. We calculated the decomposition temperatures of AlN under vacuum and simulated the decomposition mechanism of AlN by the ab initio molecular dynamics method. According to the thermodynamic calculations, the decomposition temperature of AlN decreased following a decrease in the system pressure. The ab initio molecular dynamics results indicated that wurtzite-type AlN (w-AlN) was decomposed by the layer-by-layer mechanism and followed a decomposition reaction equation of AlN?→?Al(g)?+?0.29N₂(g)?+?0.42N(g), which originated from the inequality sp³ hybridization. The zinc-type AlN (z-AlN) decomposed from the surface to interior of the structure because of the equality of the sp³ hybridization, and the z-AlN decomposition reaction equation followed AlN?→?Al(g)?+?0.5N₂(g). The AlN decomposition experiments further verified that Al(g) was the product of the wurtzite-type AlN thermal decomposition. This work can provide valuable information for the preparation of the AlN single crystal.     

Kinetics of a non-catalytic gas-solid chemical reaction under SHS-like conditions

Nitration of metallic tantalum under self-propagating high-temperature synthesis (SHS) conditions (i.e. rapid heating rates 2000–3000 Ks^–1, and short heating periods 2–100 s) has been studied. Phase analysis and microstructural characterization have been performed. A two-phase mixture region of solid solution TaN_x and tantalum subnitride, namely Ta₂N, was observed in the nitrided tantalum at temperatures of 1600–2600 K and a partial nitrogen pressure of 0.1–0.8 MPa. It was also found that tantalum subnitride phase formed before the formation of Ta-N solid solution throughout the sample during nitration. A theoretical study was conducted in order to determine the intrinsic reaction kinetics of a typical combustion synthesis reaction. A new model was developed to explain all experimental observations. The computer simulation results are in good agreement with experimental data. The results indicate that at sufficiently large heating rates the product layer does not necessarily act as a diffusion barrier that prevents further uptake of nitrogen. The results are helpful in developing an understanding of the mechanism of powder-based SHS.     

Improvement of the temperature resistance of aluminium-matrix composites using an acid phosphate binder

The use of phosphate binders instead of the widely used silica binder resulted in improved temperature resistance, increased tensile strength and decreased coefficient of thermal expansion. The effects were largest for the phosphate binder which contained the largest amount of phosphoric acid (P/Al atom ratio = 24 in the liquid binder). These effects were probably due to the protection of the SiC whiskers by the binder phases (aluminium metaphosphate or aluminium orthophosphate), the binder-SiC reaction product (SiP₂O₇) and the binder-aluminium reaction product (AIP) from further reaction between the SiC and aluminium. The tensile strength of the composite containing the SiC whisker preform made with the phosphate binder (P/Al atom ratio = 6 or 24 in the liquid binder) was increased after heating at up to 600 °C for 240 h. The silicon phosphate (SiP₂O₇) acted as an in situ binder and was primarily responsible for increasing the compressive strength of the preform and increasing the temperature resistance of the composite. The carbon fibre composite containing the preform made by using the phosphate binder (P/Al atom ratio = 24 in the liquid binder) with either water or acetone as the liquid carrier during wet forming of the preform had a higher tensile strength than the carbon fibre composite made by using the silica binder. After composite heat exposure to 600 °C for 14 h, the carbon fibre composite made by using this phosphate binder with acetone as the liquid carrier during wet forming of the preform showed the best temperature resistance, while the carbon fibre composites made by using this phosphate binder with water as the carrier showed the second best temperature resistance, and that made by using silica binder was the worst. The reason for the better effect of the phosphate binder than the silica binder is probably due to the ability of the phosphate binder and the binder-aluminium reaction product (AIP) to protect the carbon fibres from the undesirable reaction between the carbon fibres and aluminium. The lack of a binder-fibre reaction contributed to making the carbon fibre composites less temperature resistant than the SiC whisker composites. The use of a higher binder concentration is attractive for increasing the temperature resistance of the composites. The binder concentration in the preform can be increased by increasing the binder concentration in the slurry used in the wet forming of the preform.     

Processing and thermal properties of Al/AlN composites in continuous solid–liquid co-existent state by spark plasma sintering

Aluminum nitride-particle-dispersed aluminum–matrix composites were fabricated in a unique fabrication method, where the powder mixture of AlN, pure Al and Al–5 mass%Si alloy was uniquely designed to form continuous solid–liquid co-existent state during spark plasma sintering (SPS) process. Composites fabricated in such a way can be well consolidated by heating during SPS processing in a temperature range between 798 K and 876 K for a heating duration of 1.56 ks. Microstructures of the composites thus fabricated were examined by scanning electron microscopy and no reaction product was detected at the interface between the AlN particle and the Al matrix. The relative packing density of the Al/AlN composite was almost 100% when volume fraction of AlN is between 40% and 60%. Thermal conductivity of the composite was higher than 180 W/mK at an AlN fraction range between 40 and 65 vol.%, approximately 90% of the theoretical thermal conductivity estimated by Maxwell–Eucken’s model. The coefficient of thermal expansion of the composite falls in the upper line of Kerner’s model, indicating strong bonding between the AlN particle and the Al matrix in the composite.     

Dry etching of AlN films using the plasma generated by fluoride

The plasma produced by the mixture of fluoride and argon (SF₆/Ar) was applied for the dry etching of AlN films. Very high etching rate up to 140 nm/min have been observed. The effects of the bias voltage and the plasma component on the etching results were investigated. It shows that AlN can be effectively etched by the plasma with the moderate SF₆ concentration and the etching rate varies linearly with the bias voltage. The FTIR spectra confirm that AlF₃ is formed due to the chemical reaction of Al and F atoms. The mechanism of AlN etching in F-based plasma is probably a combination between physical sputtering and chemical etching and can be briefly outlined: (i) F⁻ ions reacts with Al atoms to form low volatile product AlF₃ and passivate the surface, and (ii) at the same time the Ar⁺ ions sputter the reaction product from the surface and keep it fluoride free to initiate further reaction. AlF₃ formed on the patterned sidewall play a passivation role during the etching process. The etching process is highly anisotropic with quite smooth surface and vertical sidewalls.     

The mechanical alloying mechanism of various Fe2O3–Al–Fe systems

The mixed powders of in situ Al₂O₃ particles and Fe (Al) solid solution were prepared via self-propagating combustion reaction initiated by mechanical alloying (MA), and the MA mechanism of several Fe₂O₃–Al–Fe systems with different Al₂O₃ mass fractions were studied. The adiabatic temperature (T_ad) of each system was calculated to estimate whether the self-propagating combustion reaction could be initiated in theory. The microstructure of the mixed powders was investigated by SEM, EDS and TEM. The phase analysis was evaluated by XRD, and the Fe lattice parameter was calculated from the XRD patterns. The results showed that with the addition of Fe during the MA process, the activation period was prolonged and the sharp increase of temperature occurred, and when the Al₂O₃ mass fraction was decreased to 10.94%, the self-propagating combustion reaction could not occur in theory and practice. When there was no added Fe, the final product was homogeneous Fe (Al) solid solution.     

Surface contamination and its effect on the corrosion of rapidly solidified Mg-Al alloy splats

Contamination with copper particles of the surfaces of rapidly solidified Mg-3.5 wt% Al alloy splats during processing is discussed. Two batches of splats produced with copper substrates of different surface finish, were examined by atomic absorption spectrometry (AAS), electron probe micro-analysis (EPMA) and Rutherford back scattering (RBS). Lower copper content was detected on the well-polished splats (splats B) by AAS, while EPMA and RBS analysis with a micro-beam showed fine copper particles on the surfaces of the splats prepared with pistons of inferior surface finish (splats A). Immersion corrosion tests carried out in a 3% NaCl solution saturated with Mg(OH)₂ resulted in higher pit density and earlier pitting times for splats A. Pitting is associated with copper particles (splats A) and with surface cracks and macro-porosity (splats A and B). A mechanism for pitting is suggested in which Mg(OH)Cl is envisaged to be an intermediate reaction product before decomposing to Mg(OH)₂ in the pitting process.     

Phase formation sequence of Cr2AlC ceramics starting from Cr–Al–C powders

The reaction process of Cr₂AlC ceramics was analyzed, in which the samples were prepared for composition Cr:Al:C = 1:1.2:1 by hot-pressing in argon in the range of 850–1450 °C using Cr, Al and graphite powders as the starting materials. X-ray diffraction (XRD), electron probe microanalysis (EPMA) and energy dispersive spectrum (EDS) were employed for identification of phase assembly and analysis of reaction route of the samples. The phase formation sequence of Cr₂AlC was finally analyzed based on phase diagram of the Cr–Al binary system combined with the results of differential thermal analysis (DTA) and XRD. It was found that Cr₅Al₈, Cr₂Al and Cr₇C₃ were the intermediate phases appearing in turn in the heating process. The amount of Cr₂AlC phase was gradually increased with increase in temperature by the reaction between Cr–Al intermetallic compounds, un-reacted Cr and graphite, and it became a pure phase in the sample with disappearance of intermediate phases above 1250 °C.     

Interface microstructure and reaction in Gr/Al metal matrix composites

The interface structure in Gr/Al composites fabricated with liquid metal infiltration has been studied using transmission electron microscopy (TEM). Morphologies of interfacial reaction product, aluminium carbide Al₄C₃, formed at different manufacturing parameters were compared and, the growth mechanism of the carbide was studied by means of high resolution electron microscopy (HREM). It has been shown that the morphology of the carbide is intimately related to the processing parameters with which the composites were produced. There are two kinds of interfaces between the carbide and the aluminium matrix. They have different growth mechanisms and relative growth rates under different growth driving forces. Several crystal orientation relationships between the carbide and the aluminium matrix have been observed.     

Superior high-temperature resistance of aluminium nitride particle-reinforced aluminium compared to silicon carbide or alumina particle-reinforced aluminium

Aluminium-matrix composites containing AlN, SiC or Al₂O₃ particles were fabricated by vacuum infiltration of liquid aluminium into a porous particulate preform under an argon pressure of up to 41 MPa. Al/AlN had similar tensile strengths and higher ductility compared to Al/SiC of similar reinforcement volume fractions at room temperature, but exhibited higher tensile strength arid higher ductility at 300–400 °C and at room temperature after heating at 600 °C for 10–20 days. The ductility of Al/AIN increased with increasing temperature from 22–400 °C, while that of Al/SiC did not change with temperature. At 400 °C, Al/AlN exhibited mainly ductile fracture, whereas Al/SiC exhibited brittle fracture due to particle decohesion. Moreover, Al/AlN exhibited greater resistance to compressive deformation at 525 °C than Al/SiC. The superior high-temperature resistance of Al/AlN is attributed to the lack of a reaction between aluminium and AlN, in contrast to the reaction between aluminium and SiC in Al/SiC. By using Al-20Si-5Mg rather than aluminium as the matrix, the reaction between aluminium and SiC was arrested, resulting in no change in the tensile properties after heating at 500 °C for 20 days. However, the use of Al-20Si-5Mg instead of aluminium as the matrix caused the strength and ductility to decrease by 30% and 70%, respectively, due to the brittleness of Al-20Si-5Mg. Therefore, the use of AIN instead of SiC as the reinforcement is a better way to avoid the filler-matrix reaction. Al/Al₂O₃ had lower room-temperature tensile strength and ductility compared to both Al/AlN and Al/SiC of similar reinforcement volume fractions, both before and after heating at 600 °C for 10–20 days. Al/Al₂O₃ exhibited brittle fracture even at room temperature, due to incomplete infiltration resulting from Al₂O₃ particle clustering.     

升温时效处理7075铝合金在含SRB模拟海水中的应力腐蚀行为

7系铝合金是一种可热处理强化的铝合金,通过改变热处理工艺的方法可以提高铝合金的抗应力腐蚀性能.本文研究了不同升温时效处理下,铝合金在含硫酸盐还原菌(SRB)模拟海洋溶液中的应力腐蚀行为,并通过应力-应变曲线和断口形貌,对比分析了铝合金在无菌溶液和SRB溶液中的应力腐蚀行为差异.研究表明,升温时效处理可以提高合金的自腐蚀...     

Microstructure and mechanical properties of cast (Al–Si)/SiCp composites produced by liquid and semisolid double stirring process

Abstract

A stirring process containing two steps, i.e. liquid and then semisolid stirring, was used to produce SiC particle reinforced aluminium matrix composites. The major advantages of this process are that full wetting of SiC particles by molten aluminium can be readily achieved at relatively low stirring rates, and undesirable Al₄ C₃ is not formed at the Al/SiC interface due to lower processing temperatures. Cast Al–Si matrix composites reinforced with 15 and 20 vol.-%SiC particles were produced in the present work. The mechanical properties of the composites were evaluated under the conditions of investment mould casting and heat treatment. For the composites obtained without employing semisolid stirring, the aggregation of SiC particles observed in the microstructure of composites resulted in quite poor mechanical properties. Observations and analyses indicated that some Al/SiC interfaces were very clean, and a reaction product of spinel MgAl₂O₄ was also found at some Al/SiC interfaces. Silicon dioxide (SiO₂ ) was found to exist on the surface of as purchased and 250°C dried SiC powders. This SiO₂ is involved in the spinel reaction at the interface between the SiC particles and the matrix in the present Al/SiC composites.     

Reaction mechanism,kinetics and high temperature transformations of geopolymers

The reaction kinetics and mechanism of geopolymers are studied. The dissolved silicate concentration decreases from the beginning of the reaction. A characteristic time ‘t _0,vit’ for the setting of the reaction mixture is derived from isothermal Dynamic Mechanical Analysis experiments. ‘t _0,vit’ increases with SiO₂/R₂O but goes through a minimum for increasing water content. The reaction is slower for K compared to Na-silicate based systems. ²⁹Si and ²⁷Al solution NMR are used to probe the molecular changes. ²⁷Al NMR and FTIR reveal that an ‘intermediate aluminosilicate species’ (IAS) is formed from the start of the reaction. The concentration decrease of OH⁻ during low-temperature reaction is related to the formation of IAS. The rate law of this process seems to be obeyed by a total reaction order of 5/3, with a partial order of 1 for OH⁻ and 0 for Na⁺ in the silicate solution. During first heating after polymerization water is lost leading to a distortion of the Al environment. According to XRD, no crystallization occurs below 900 °C. However, between 950 and 1100 °C a crystallization exotherm of nepheline is observed with DSC for a geopolymer with SiO₂/Na₂O = 1.4. Neither T _g of the amorphous geopolymer, nor the shrinkage and expansion around T _g during first heating, cause a measurable heat effect.