Interfacial reactivity of aluminium/fibre systems during heat treatments

The interfacial reactivity of specimens composed of aluminium coated on SiC-based fibres, carbon fibres and protected carbon fibres, was investigated. The woven fibres were coated with aluminium by physical vapour deposition and the obtained materials were heat treated in a furnace which was connected to a mass spectrometer. It was shown that reactions occur between CO and CO₂ gases, which are released by the fibres, and aluminium, when the temperature is above 650°C. These gases react during their passage through the aluminium layer and form aluminium carbide. Aluminium carbide is also produced by reactions between the solid-species constituents of the fibres and the metal. The amount of aluminium carbide formed at the fibre/metal interface during heat treatment was determined by hydrolysis. It was thus possible to ascertain that the aluminium carbide is mainly formed by the latter reactions. The efficiency of various protective coatings against the formation of aluminium carbide was also investigated.     

Effect of high temperature annealing on texture and microstructure on an AISI-444 ferritic stainless steel

AISI 444 is a Mo-alloyed ferritic stainless steel which presents good naphthenic corrosion resistance, making it attractive for applications in petroleum refining plants; however, good formability is also important. To achieve good formability with this alloy the annealing process is crucial. The annealing temperature in ferritic stainless steel is usually around 850 °C, which falls in the range of sigma phase precipitation. A means to avoid this precipitation is to anneal at temperatures around 1000 °C followed by rapid cooling. Annealing at high temperatures can cause grain growth and carbide or nitride precipitation which can result in a reduction of room temperature toughness. In this paper, the rolling and recrystallization textures of AISI 444 steel were studied in samples cold rolled with different thickness reductions (30%, 60%, 80% and 90%) followed by annealing at 955, 980 and 1010 °C. Aspects of grain size and carbide precipitation after annealing were characterized using EBSD and AFM. The material drawability was analyzed through strain rate or Lankford (r) coefficients calculated from texture results.     

Fracture of aluminium-coated carbon fibres

A degradation in the ultimate tensile strength (UTS) of aluminium-coated carbon fibres was associated with the formation of a reaction layer of aluminium carbide during annealing treatments 475° C for high tensile fibres (HT) and 550° C for high modulus fibres (HM). It was established that for a given annealing treatment, the UTS depended on the square root of the original coating thickness and proposed that fracture was controlled by cracks in the aluminium carbide, with a specific surface energy () and intrinsic crack length (c ₀) of 2.33 J m^–2 and 30 nm for HT fibres, and of 0.64 to 0.77 J m^–2 and 20 nm for HM fibres.     

Interface analysis in Al and Al alloys/Ni/carbon composites

Nature of fibre/matrix interfaces existing in Al/C composites were investigated depending on the presence of a nickel interlayer deposited on carbon fibres and on the composition of the aluminium matrix. Auger and electron microprobe analyses were used. The role of the nickel layer on the chemical evolution of the system after a 96 h heat treatment at 600°C is discussed. The presence of this nickel layer limits the diffusion of carbon into aluminium, and thereby, eliminates the formation of a carbide interphase, Al₃C₄, which is known to lower the mechanical properties of Al/C composites. The mechanisms differ according to the composition of the matrix. In the case of pure aluminium, an Al-Ni intermetallic is formed after thermal annealing. It does not react with the carbon fibre and so inhibits the growth of Al₃C₄. In the case of the alloyed matrix (AS7G0.6), the dissolution of the Ni sacrificial layer, after annealing, does not lead to the same Al-Ni intermetallic but a thin nickel layer remain in contact with the carbon fibre avoiding formation and growth of Al₃C₄ carbide. This difference of behaviour is tentatively ascribed to the presence of silicon that segregates at the fibre/matrix interface.     

Preparation and microstructural evolution of carbon/carbon composites

Carbon/carbon (C/C) composites with characteristic matrix-crack pattern are key intermediate materials for preparation of carbon/silicon carbide (C/C–SiC) composites. The C/C composites were prepared by pyrolyzing carbon fiber/phenolic resin preform. The change of density, open porosity, mass loss and specially the microstructural evolution of the composites during pyrolysis at 200–900 °C was analyzed, which provided important information for preparation of C/SiC composites by infiltration of silicon. An increasing number of regular spacing cracks were formed above 400 °C. After pyrolysis at 900 °C, the pore volume was 0.17 cm³/g, and the pores in the radius range of 2.44–122.19 μm occupied 81% of the pore volume.     

Recrystallization in a directionally solidified cobalt-base superalloy

The recrystallization behavior in a range of annealing temperature from 1020 to 1280 °C in a directionally solidified cobalt-base superalloy was studied. Local recrystallization first appeared at 1040 °C. The recrystallized volume increased rapidly as increasing the annealing temperature. Pinning effect of all carbides (M₂₃C₆, M₇C₃ and MC) was observed and large amount of twin formed at low annealing temperature. The size of the recrystallized grains increased significantly at high annealing temperatures accompanied with the sharp decrease of twin. The effect of annealing temperature and the role of carbide and twin on the development of the recrystallization were discussed.     

Fracture behaviour of hybrid glass matrix composites: thermal ageing effects

The thermal aging of a glass matrix composite reinforced by short carbon fibres as well as by ZrO₂ particles (hybrid composite) was investigated at temperatures in the range 500–700 °C for exposure durations of 24 h in air. The mechanical properties of as-received and aged samples were evaluated at room temperature by using the three-point flexure chevron notch technique. The fracture toughness values of as-received specimens were in the range 2.6–6.4 MPa m^1/2. Fracture toughness was affected by the thermal aging conditions. For thermal aging at temperatures <700 °C, degradation of fibre–matrix interfaces occurred and therefore the apparent fracture toughness and flaw tolerant resistance decreased. For the most severe ageing conditions tested (700 °C/24 h), fracture toughness values dropped to 0.4 MPa m^1/2. Significant degradation of the material was detected for this aging condition, mainly characterised by porosity formation in the matrix as a result of softening of the glass and oxidation of the carbon fibres.     

Icosahedral Al62Cu25.5Fe12.5 phase in the presence of carbon

Previous studies have suggested that contamination by carbon of the i-Al₆₂Cu_25.5Fe_12.5 quasicrystalline phase can cause destabilization of the aperiodic structure. Hence, the possibility of carbon diffusion into AlCuFe quasicrystalline thin films and the possible subsequent degradation of the quasicrystalline structure were investigated at room temperature through to 600 °C. The study shows that a carbon layer deposited on the AlCuFe quasicrystalline thin film did not diffuse into the AlCuFe over the temperature range tested whatever the oxide thickness between carbon and alloy. Moreover, the carbon did not react with any of the alloy elements as has been shown with aluminium in the presence of oxygen. Post-deposition annealing at 600 °C of the amorphous alloy, fabricated by simultaneous electron beam evaporation on an amorphous carbon substrate used for transmission electron microscopy (TEM), also leads to a pure quasicrystalline phase thin film without any carbon diffusion from the substrate.     

Influence of microstructure on the strength of Nicalon-reinforced aluminium metal-matrix composites

The microstructure and mechanical properties of two aluminium-based composites reinforced with Nicalon fibre are investigated. During composite processing, aluminium carbide forms at the interface as a result of a reaction between aluminium and free carbon in the fibre. Magnesium, when present in the aluminium matrix, diffuses into the outer (~ 200 nm) layer of the fibre where it reacts with the silicon oxycarbide constituent to form magnesium-containing oxide and also to free carbon for the production of more interfacial aluminium carbide. These chemical reactions affect to differing degrees the strength of a fibre, as measured after extraction from the two composites, and influence the respective fibre/matrix interfacial friction stress and composite strength. A simple rule-of-mixtures approach based upon the measured strength of extracted fibres gave some agreement with longitudinal properties of the composite, but treatment of the fibres as bundles, using a Weibull probability distribution of properties, provided more accurate predictions.     

Influence of high-temperature exposure on mechanical properties of zircon-silicon carbide composites

Thermal stability of zircon matrix composites uniaxially reinforced with either uncoated or BN-coated silicon carbide monofilaments was determined by measuring mechanical properties and fibre-matrix interfacial characteristics in the as-fabricated state and after annealing treatments between 25 and 1430 °C for times up to 100 h. Composites reinforced with uncoated silicon carbide filaments retained their mechanical properties and fibre-matrix interfacial characteristics up to 1350 °C for 100 h. In contrast, composites reinforced with BN-coated silicon carbide filaments displayed changes in mechanical properties and fibre-matrix interfacial characteristics when annealed beyond 1300 °C for 100 h. Both types of composite displayed a significant reduction in strength and toughness after annealing at 1430 °C for 20 h. These results are consistent with changes in fibre-matrix interfacial properties, and with changes in mechanical characteristics of zircon matrix and silicon carbide filament as a result of the high-temperature annealing treatments.     

Electrical and mechanical properties of electrically conductive polyethersulfone composites

Electrically conductive polyethersulphone (pes) composites containing carbon fibres, nickel fibres, stainless steel fibres or aluminium flakes at various volume fractions up to 40% were fabricated and tested. For electromagnetic interference (emi) shielding effectiveness > 50 dB, the minimum filler volume fraction was 40% for carbon fibres of length 200 or 400 μm, 20% for nickel or stainless steel fibres, and 30% for aluminium flakes. The tensile strength first increased and then decreased with increasing filler content, such that the highest tensile strength occurred at 30 volume% (vol%) for carbon fibres (of length 200 or 400 μm) as the filler and at 10 vol% for nickel or stainless steel fibres. However, for carbon fibres (of length 100 μm) and aluminium flakes, the tensile strength increases up to at least 40 vol%. The best overall performance was provided by aluminium flakes at 40 vol%; the resistivity was 7 × 10⁻⁵ Ω cm, the emi shielding effectiveness was > 50 dB and tensile strength was 67 MPa. The resistivity of the aluminium flake composites was not affected by heating in air at 140°C for up to at least 144 h.     

In situ fabrication of titanium carbide reinforced copper MMC

Abstract

The in situ fabrication of titanium carbide reinforced copper and aluminium bronze (AB2) composites by carbothermal reduction of titanium in an induction furnace has been investigated. An inert atmosphere was maintained with carbon monoxide created as a byproduct from the heat of reaction between the induction field, graphite crucible and graphite lid. Titanium carbide particles of the order 1–3 μm were formed in aluminium bronze at approximately 1250°C and, in copper, particles of order 1–6 μm were produced at approximately 1330°C. Dispersion concentrations of titanium carbide of 20% and 6.5% were obtained for copper and aluminium bronze respectively. In addition, evidence is presented indicating that iron could be used as a dispersion medium for titanium carbide particulates in aluminium bronze alloys.     

Atomic-layer doping in Si by alternately supplied PH3 and SiH4

Atomic-layer doping of P in Si epitaxial growth by alternately supplied PH₃ and SiH₄ was investigated using ultraclean low-pressure chemical vapor deposition. Three atomic layers of P adsorbed on Si(100) are formed by PH₃ exposure at a partial pressure of 0.26 Pa at 450°C. By subsequent SiH₄ exposure at 220 Pa at 450°C, Si is epitaxially grown on the P-adsorbed surface. Furthermore, by 12-cycles of exposure to PH₃ at 300–450°C and SiH₄ at 450°C followed by 20-nm thick capping Si deposition, the multi-layer P-doped epitaxial Si films of average P concentrations of 10²¹ cm⁻³ are formed. The resistivity of the film is as low as 2.4×10⁻⁴ Ω cm. By annealing the sample at 550°C and above, it is found that the resistivity increases and the surface may become rough, which may be due to formation of SiP precipitates at 550°C and above. These results suggest that the epitaxial growth of very low-resistive Si is achieved only at a very low-temperature such as 450°C.     

Effect of precursor solution on the formation of perovskite phase of Pb(Mg1/3Nb2/3)O3 thin films

Precursor solutions for Pb(Mg_1/3Nb_2/3)O₃ (PMN) synthesis were obtained by Pechini's method. The influence of the concentration of organic materials on the phase formation has been studied. For this purpose, PMN solutions were prepared with different precursors and were characterized by thermogravimetric and differential thermal analysis. The obtained solutions were deposited onto a Si (100) substrate by dip coating and pre-treated in a hot plate at 300 °C for 1 h. The films were annealed at 600, 700, 800 and 900 °C for 1 h and characterized by X-ray diffraction. The perovskite phase was formed after annealing at 600 and 700 °C when the solution of PMN was prepared with a lower amount of organic material and starting with niobium oxide. By increasing the temperature to 800 or 900 °C, only the formation of pyrochlore phase was observed. With the solution prepared from niobium ethoxide, only the presence of pyrochlore phase was observed independently of the annealing temperature.     

Combined impurity gettering effects of helium-induced cavities and oxygen precipitates created by plasma immersion ion implantation

Helium bubbles are formed in silicon by high dose implantation using the plasma immersion ion implantation technique. Thermal annealing of the implanted samples at a temperature above 700 °C causes the helium to diffuse out of silicon and form cavities which can be observed by cross-sectional transmission electron microscopy. The well-defined band of helium-induced cavities both with and without oxygen precipitates (formed by subsequent oxygen implantation and annealing) have been studied as gettering layers for copper and gold. Our secondary ion mass spectrometry and Rutherford backscattering spectrometry results demonstrate that the presence of oxygen does not increase significantly the gettering efficiency of these cavities. The combined cavity/oxygen gettering sites also remain stable up to 1200 °C     

Behaviour of coatings on reinforcements in some metal matrix composites

Coating on reinforcements affects the interface bonding of a composite, and is therefore usually used for improving the composite's properties. The behaviour of SiC coating on carbon fibre in reinforced aluminium metal castings, Fe on carbon fibre-reinforced copper and alumina coating on K₂O · 6TiO₂ whisker-reinforced aluminium composites were investigated, respectively, by modern techniques such as TEM, SEM etc. with the goal of controlling the interfacial interaction and wettability of reinforcement with the matrices. SiC coating produced by a polycarbosilane solution process effectively improved the strength because it successfully controlled oxidation of the carbon fibres themselves and the harmful reaction between the carbon fibres and molten aluminium during the fabrication process and heating process of the composites. The metal coating, Fe, made by electrical plating, strengthened the bonding of carbon fibres with copper by changing the bonding state of the interface from a mechanical one to a partly chemical one. Therefore the strengths of the resulting composites were improved. The alumina coating on K₂O · 6TiO₂ also controlled the diffusion of the K element from the whiskers into the aluminium matrix and altered the reaction with aluminium, and led to the optimization of interfacial bonding between the whiskers and a superior composite.     

Structural and optical characterization of β-FeSi2 layers on Si formed by ion beam synthesis

Structural and optical properties have been investigated for surface β-FeSi₂ layers on Si(100) and Si(111) formed by ion beam synthesis using ⁵⁶Fe ion implantations with three different energies (140–50 keV) and subsequent two-step annealing at 600 °C and up to 915 °C. Rutherford backscattering spectrometry analyses have revealed Fe redistribution in the samples after the annealing procedure, which resulting in a Fe-deficient composition in the formed layers. X-ray diffraction experiments confirmed the existence of /gb-FeSi₂ by annealing up to 915 °C, whereas the phase transformation from the β to phase has been induced at 930 °C. In photoluminescence measurements at 2 K, both β-FeSi₂/Si(100) and β-FeSi₂/Si(111) samples, after annealing at 900–915 °C for 2 h, have shown two dominant emissions peaked around 0.836 eV and 0.80 eV, which nearly coincided with previously reported PL emissions from the sample prepared by electron beam deposition. Another β-FeSi₂/Si(100) sample has shown sharp emissions peaked at 0.873 eV and 0.807 eV. Optical absorption measurements at room temperature have revealed the allowed direct bandgap of 0.868–0.885 eV as well as an absorption coefficient of the order of 10⁴ cm⁻¹ near the absorption edge for all samples.     

Evaluation of thermal stability of ultrafine grained aluminium matrix composites reinforced with carbon nanotubes

The effect of carbon nanotubes on the thermal stability of ultrafine grained aluminium alloy processed by the consolidation of nano-powders obtained by mechanical alloying was evaluated via measurements of grain size and mechanical property changes upon annealing at various temperatures. It was found that the grain size of the samples containing carbon nanotubes is stable up to high temperatures and even after annealing at 450 °C (0.7T_m) no evident grain growth was observed. The limited grain boundary migration was attributed to the presence of entangled networks of carbon nanotubes located at grain boundaries and to the formation of nanoscale particles of aluminium carbide Al₄C₃. It was also revealed that carbon nanotubes decompose at a relatively low temperature of 450 °C and form fine Al₄C₃ precipitates. This transformation does not significantly affect the mechanical properties due to the nanoscale size of the carbides.     

Corrosion Behavior of Vanadium Carbide Coating Produced by a Thermal Diffusion Process

Vanadium carbide coating was deposited on carbon steel (EN9) and mild steel using a thermal diffusion process. Samples were immersed in a molten salt bath containing appropriate carbide forming compounds at temperatures ranging from 850°C to 1100°C. Samples were treated for various lengths of time in order to obtain coatings of various thicknesses. The corrosion behavior of the vanadium carbide coatings was evaluated by accelerated electrochemical tests. The corrosion resistance of the carbide coating was found to be superior to that of the untreated base alloys.     

Improvement of Ta-based thin film barriers on copper by ion implantation of nitrogen and oxygen

Magnetron sputtered polycrystalline Ta and Ta(Si) barriers for copper metallization schemes were modified by nitrogen as well as oxygen high dose ion implantation to improve their thermo-mechanical stability. Ion bombardment changed the initial polycrystalline microstructure to amorphous-like. In contrast to pure Ta, Ta(Si) layers were already amorphous or nanocrystalline after deposition. In this case, the annealing temperature at which formation of a well crystallized structure occurs increased by approximately 100 K as a result of the implantation. In order to demonstrate the improvement in the barrier properties of the implanted Ta films, the intermixing of Ta and Cu at the interface of corresponding layer structures was measured as a function of the annealing temperature by depth profiling using Auger electron spectroscopy (AES). The thermal stability of Ta and Ta(Si) barriers increased from 600 °C/1 h for the non-implanted layers up to 750 °C/1 h after implantation of nitrogen or oxygen.