Interlayer structure of carbon fibre reinforced aluminium wires

As the extent of the interfacial reactions controls the properties of metal matrix composites, the microstructural features and the chemical composition of the interlayers in aluminium wires reinforced with unidirectional carbon fibres (volume fraction app. 55%) have been investigated. High voltage and high resolution transmission electron microscopy of fibres, matrix, and interlayers, combined with analytical methods (electron energy loss spectroscopy and energy filtered microscopy) revealed a nanometre-sized C/Al interdiffusion layer and aluminium carbide needles or platelets of 10–50 nm thickness and 50–500 nm length in the matrix material, starting from the interlayer, the extension of which strongly correlates with the duration of melt contact. The observed interlayer phenomena impose restrictions to the process parameters, as by massive interface reactions the fibre strength is degraded, and the formation of brittle reaction products such as Al₄C₃ provides sites for initiation of fibre cracking and can cause composite failure. With a newly developed continuous process, which is capable of infiltrating endless products, the fibre/melt contact duration could be reduced to less than one second resulting in carbide formation lower than 0.2 wt% as confirmed by chemical analyses. So it was possible to achieve strength values of the composite wires that are as high as the theoretical prediction.     

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.     

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.     

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.     

Studies on carbon fibre reinforced aluminium composite processed using pre-treated carbon fibres

Carbon fibre reinforced Al-12% Si alloy composite has been fabricated by pre-treating the fibres with K₂ZrF₆ followed by molten alloy infiltration and subsequent hot pressing of the preforms. The infiltration conditions were arrived at based on the measurement of tensile strength of the fibres extracted from the preforms. The fibre volume per cent of 20 was found to result in composite tensile strength of about 240 MPa as compared to tensile strength of 100 MPa for the unreinforced matrix. Characterization of the interface revealed the formation of ZrSi₂ and diffusion of potassium and aluminium into the fibre. The interfacial bonding was strong as is evinced by the absence of fibre pull-out on to the fracture surface.     

Effects of fibre volume fraction on the stress transfer in fibre pull-out tests

The elastic stress transfer taking place across the fibre/matrix interface is analysed for the fibre pull-out test by means of both micromechanics and finite element (FE) analyses. A special focus has been placed on how fibre volume fraction, V_f, affects the interface shear stress fields in the model composites containing both single and multiple fibres. In a so-called ‘three-cylinder model’, where a fibre, a matrix and a composite medium are coaxially located, the constraint imposed on the central fibre due to the surrounding fibres is properly evaluated. It is shown in the FE analysis that the differences in stress distributions between the composite models containing single and multiple fibres become increasingly prominent with increasing V_f. The principal effect of the presence of surrounding fibres in the multiple-fibre composite model is to suppress effectively the development of stress concentration near the embedded fibre end and thus eliminate the possibility of debond initiation from this region for all V_f considered. This is in sharp contrast to the single-fibre composite model, in which the interfacial debond can propagate from the embedded end if V_f is larger than a critical value. These findings are essentially consistent with the results from micromechanics analysis on the same specimen geometry. The implications of the results for the practical fibre pull-out test as a means of measuring the interface properties are discussed.     

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.     

Electroless nickel coated short carbon fibres in aluminium matrix composites

Electroless nickel coated fibres have been used as reinforcement for the fabrication of aluminium matrix composites by liquid processing. Uniform, continuous and well-adhered nickel coatings are obtained with different phosphorus contents. Coated carbon fibres were mixed with AA6061 aluminium powders, compacted and heated at temperatures from 650 to 950 °C to study the reactivity, the nickel diffusion, and the resulting interfaces. Coatings improve the wetting behaviour of carbon fibres by molten aluminium because of the formation of a transient Al–Ni intermetallic at the matrix–fibre interface, limiting fibre segregation to obtain a homogeneous reinforcement distribution. Finally, the mechanical properties of the composite have been measured through nanoindentation tests.     

Interaction of boron fibres with aluminium melt during metallization

The aim of the paper is to investigate interphase interactions on the fibre-melt interface boundary during metallization of boron fibres by aluminium.Using methods of selective etching and weighing it has been stated that chemical interaction of boron fibres with the aluminium melt develops in 2 stages: firstly - boron solution in liquid aluminium occurs, secondly - formation of chemical compounds AlB₂, AlB₁₂ on the interface boundary.Mechanical tests and fractographic analysis have stated that if the fibre material is soluted in the melt then fibre strength raises due to the lower influence of surface defects; the appearance of aluminium borides on the interface boundary is followed by a decrease in the initial fibre strength.     

Weibull statistical analysis of tensile strength data of short mullite fibre reinforced aluminium alloy composite

Abstract

A statistical evaluation by means of Weibull statistics was carried out on the tensile strength data of a short mullite fibre reinforced aluminium alloy composite, which was prepared by squeeze casting. The results show that the material has a high and reliable tensile strength. The area fractions of the fibres on the cut surface and on the fracture surface of specimens have been statistically analysed. The fibre distribution shows heterogeneity in the microsturcture. On the cut surface the average area fraction of fibres which make large angles with the normal of the cut surface (denoted as A _fl ) is slightly less than that of those fibres which make a small angle with the normal of the cut surface (denoted as A _fs ). However, on the fracture surface of the composite, A _fl is much bigger than A _fs , and the lower the tensile strength of the specimen, the bigger is A _fl on the fracture surface. Debonding of the interface between the large angle fibres and the matrix is an important cause of failure of the composite, and the non-uniform distribution of the large angle fibres is one of the main causes of the large scatter in the data.     

Electron paramagnetic resonance studies of interfacial solvent absorption in quartz fibre-reinforced epoxy composites

A new method for the in situ study of composite fibre/matrix interface regions has been developed and applied to the study of solvent absorption mechanisms. The technique involves the selective placement of stable free radical spin probes at the interface in composite systems via covalent bonding to the fibre surface. In this case, quartz fibres were labelled with 4-aminotetramethylpiperidinyloxy via a diisocyanate coupling agent and used as uniaxial reinforcements in an epoxy matrix. Electron paramagnetic resonance (EPR) spectra of the spin-labelled composites were obtained as a function of temperature before and after exposure to toluene. The rotational correlation times (τ_r) and corresponding activation energies were evaluated. The results were compared to those obtained for various control experiments. It was found that solvent preferentially absorbs at the fibre/resin interface as opposed to the bulk matrix. Possible interfacial solvent absorption mechanisms are discussed with respect to the EPR data.     

Interfacial reactions in a Ti-6Al-4V based composite: role of the TiB2 coating

The characterization of TiB₂/C-coated SiC fibres and their interface region in a Ti-6Al-4V based composite has been performed by using scanning electron microscopy (SEM), energy-dispersion X-rays (EDX) and Auger electron spectroscopy (AES). The features of the as-received fibre and the reactivity between fibre and matrix occurring during preparation of the composite have been studied in this paper. The interaction of the TiB₂ external coating of the fibre with both the adjacent carbon layer and the titanium-based matrix is already appreciable in the as-received composite: TiB needles grow from TiB₂ towards the matrix and a new layer containing C, Ti and B appears between TiB₂ and C. The thicknesses of the original carbon and TiB₂ fibre coatings decrease in the composite from 1000 nm to 400 and 800 nm, respectively. The TiB₂ inhibits the reaction between SiC and Ti: there is no evidence of Si _x Ti _y brittle phases.     

Eigenspannung im mit gasdruckunterstütztem Feingussverfahren hergestellten langfaserverstärkten Aluminium‐Matrix‐Verbundwerkstoff

Residual Stress in Continuous Fibre Reinforced Aluminium Matrix Composites Prepared by Modified Investment Casting The residual stresses between matrix und fibres in the continuous γ‐Al₂O₃ fibre reinforced aluminium alloy (AlZn6Mg1Ag1) matrix composites prepared by modified investment casting were measured with x‐ray diffraction as well as simulated with FEM. It was indicated as expected that tensile residual stress exists in the Matrix und compressive residual in the fibre. The average value of the residual stress in both matrix and fibre in the composites is not very significant. However it is distributed very unevenly. Next to the interface between matrix and fibre there is a small zone in the matrix with relative great tensile residual stress. The effect of fibre volume percentage on the residual stress in the composite was also analysed. With increase of the fibre volume percentage the tensile residual stress in the matrix increases while the compressive residual stress in the fibre decreases. If the fibre volume percentage in the composite exceeds 65 %, the maximal tensile residual stress will reach the yield stress of the matrix alloy and local plastic deformation will occur.     

Characterisation of interfaces in Al–Zn–Mg alloy reinforced with Sicabo fibres

Abstract

The microstructure of a metal matrix composite consisting of an Al–Zn–Mg alloy reinforced with SiC coated boron fibres has been examined by electron microscopy, electron probe microanalysis, and by optical microscopy. Considerable amounts of Mg₂Si phase were found to be segregated at the fibre/matrix interface. This intermetallic was not formed by a reaction between the fibre and matrix during the fabrication process, a liquid infiltration technique, but as a result of silicon impurities present as contaminants in the melt. It was concluded that the interface phase was precipitated from the metal matrix in the later stages of solidification without any nucleation role being played by the fibre. The Mg₂Si phase appears to be brittle and was present in amounts likely to have a deleterious effect on the strength of the composite.

MST/871     

Influence of fibre taper on the work of fibre pull-out in short fibre composite fracture

A model has been formulated to determine the work of pull-out, U, of an elastic fibre as it shear-slides out of a plastic matrix in a fractured composite. The fibres considered in the analysis have the following shapes: uniform cylinder and ellipsoidal, paraboloidal or conical tapers. Energy transfer at the fibre–matrix interface is described by an energy density parameter which is defined as the ratio of U to the fibre surface area. The model predicts that the energy required to pull out a tapered fibre is small because the energy transfer at the fibre–matrix interface to overcome friction is small. In contrast, the pull-out energy of a uniform cylindrical fibre is large because the energy transfer is large. The pull-out energies of the paraboloidal and ellipsoidal fibres lay between those for the uniform cylindrical and the conical fibres. With the exception of the uniform cylindrical fibre which yields a constant energy density, tapered fibres yield expressions for the energy density which depend on the fibre axial ratio, q. In particular, the energy density increases as q increases but converges at large q. By defining the critical axial ratio, q ₀, as the limit beyond which u is independent of the fibre slenderness, our model predicts the value of q ₀ to be about 10. These results are applied to explain the mechanisms regulating fibre composite fracture.     

Evaluation of continuous alumina fibre reinforced composites based upon pure aluminium

Abstract

Composites of super purity aluminium unidirectionally reinforced with Altex or Nextel 610 continuous alumina basedfibre have been made by liquid metal infiltration. The composites were well consolidated, with fibre volume fractions V_f of 0.4 and 0.6 for the Altex composites and 0.7 for the Nextel composite, the higher values being obtained where preforming involved the use of sized fibre tows. Matrix porosity was very low and there was no evidence of any deleterious reaction product having formed at the fibre/matrix interface. Monotonic longitudinal tensile tests of the composites gave Youngs modulus values between 125 and 250 GPa, in line with rule of mixtures (ROM) predictions and evidence of effective load transfer between fibres. The onset of yielding in longitudinal composites was commensurate with the yield stress of unreinforced super purity aluminium for V_f = 0.4 (～20 MPa), but increased to 225 MPafor V_f =0.7. The tensile strengths of the Altex composites were 760 and 930 MPa, values in accord with ROM predictions based upon equal load sharing of fibres up to the mean filament failure stress. Although the Nextel composite had a higher tensile strength of 1250 MPa, this was significantly lower than the ROM value of 1650 MPa and was better described by fibre ‘bundle’ theory. Predictions of the accumulation of fibre damage, by statistical analysis, indicated that filament breakage commenced at an applied stress of ～50 MPa for the Altex composite and ～ 500 MPa for the Nextel composite. Despite damage at the lower stress, however, the Altex composites were able to tolerate many more ‘double’ fibre breaks than the Nextel composite, the failure of which coincided with the onset of the first double break. Transverse tensile tests of the composites gave Young's modulus values between 80 and 170 GPa, in line with ROM predictions. The yield stress increased with increasing V_f, from 10 to 60 MPa, this behaviour being attributed to plane strain deformation caused by the virtually non-deformable fibres constraining matrix flow. The tensile strengths showed a similar trend, with 84 MPa for V_f =0.4 increasing to 168 MPa for V_f = 0.7.     

SiC fibre-reinforced lithium aluminosilicate matrix composites fabricated by the Sol-Gel process

SiC fibre-reinforced lithium aluminosilicate (LAS) matrix composites were fabricated by (i) the hydrolysis-condensation reaction of relevant metal alkoxides, and (ii) the non-aqueous condensation route using diphenylsilanediol. Optimum reaction conditions of LAS sol formation for thin coating applications in the hydrolysis-condensation route were derived from rheological measurements. The SiC fibre-LAS composite fabricated by the hydrolysis-condensation route shows a uniform microstructure. The debonding/fibre pullout was more pronounced in the niobium-doped composites. The fracture toughness,K _1c, also increased from 11 MPa m^1/2 for the undoped composite to 16 MPa m^1/2 for the 5 wt% Nb-doped composite, suggesting that the fibre pullout contributes significantly to the toughening of SiC fibre-LAS composites. The crystallization of matrix at high temperatures (after hot pressing at 800C) eroded SiC fibres internally. This was attributed to the diffusion of aluminium atoms into the fibre, forming aluminium carbide above 1200C.     

Experimental push out testing and analysis of fibre reinforced composites: applicability and test considerations

Abstract

The applicability of the push out technique to fibre composite systems with both excellent and poor interfacial bonding has been considered through experimental push out studies on model polymer matrix and metal matrix composite systems, respectively. Some factors which affect the reliability and reproducibility of data from the push out test have also been established. The findings are applicable to ceramic matrix composites. The interfacial properties of the model steel fibre/epoxy matrix, copper fibre/epoxy matrix, steel fibre/aluminium matrix, and SiC fibre/aluminium matrix composite systems were studied. Factors which influence the reproducibility of interfacial properties such as the interfacial bond strength and the matrix shrinkage pressure as determined from the push out test have been identified. These include the thickness/fibre diameter ratio, the relative diameter of the fibre and the specimen, and the aspect ratio of the fibre in the push out specimen. It was also important to establish the nature offailure at the interface.     

Rate of formation of intermetallic compounds in aluminium matrix-carbon fibre composites

This paper describes the formation of aluminium carbide at the interface between aluminium and carbon fibres as a result of annealing for 4 h at 550°C or above, or for 1 h at 620°C or above. For a quantitative analysis of the reaction, measurements were made of the weight reductions of carbon fibre that occurred as a result of annealing, these reductions being proportional to the quantity of aluminium carbide formed. The relationship between the amount of aluminium carbide formed, the tensile strength and the surface characteristics of carbon fibres is discussed.     

High-strength graphite fibre/lithium aluminosilicate composites

Graphite fibre/lithium aluminosilicate composites of matrix composition Li₂O Al₂O₃·nSiO₂ wheren=3, 4, and 8, have been developed with a high volume fraction of undirectionally aligned graphite fibres. Graphite fibre/ceramic matrix tapes were produced on a drumwinding apparatus and hot-pressed to final dimensions. This composite system exhibits a combination of useful properties, including high strength, low density, excellent thermal shock resistance and impact strength. Shear strength, cyclic modulus of rupture and modulus of elasticity and optical and electron microscopic evaluation of microstructure are discussed. Modulus of rupture as a function of vol % fibres was studied. The high modulus of rupture and impact strength are discussed in terms of the relative properties of fibre and matrix.