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.     

Fabrication of carbon fibre-reinforced aluminium composites with hybridization of a small amount of particulates or whiskers of silicon carbide by pressure casting

An investigation was carried out on the fabrication of carbon fibre-reinforced aluminium matrix composites with hybridization of particulates or whiskers of silicon carbide by pressure casting. A small amount of particulates or whiskers was uniformly distributed among carbon fibres and the preforms prepared from the treated fibres were directly infiltrated by molten aluminium under applied stress. It was found that the longitudinal tensile strengths of hybrid composites were greatly improved, although their fibre volume fractions were very low compared to those of conventional composites. With this hybridization method, it is also practical to tailor the fibre volume fraction of composites from 60 to 25 vol %, which is not possible in direct infiltration of fibre preforms by pressure casting. The results obtained lead to the conclusion that particulate or whisker additions act not directly as reinforcements but as promoters to improve the infiltration performances of fibre preforms, and consequently to increase the strength-transfer efficiency of carbon fibres. The addition of particulates or whiskers can also improve other properties of the composites, such as hardness and wear resistance.     

Interface and fracture of carbon fibre reinforced Al-7 wt% Si alloy

The interface structure in an aluminium-7 wt% silicon alloy reinforced with carbon fibres has been investigated using analytical electron microscopy. Crystals of aluminium carbide (Al₄C₃) have been identified in interface regions and their structure and growth are discussed. Mechanical properties of the composite have been measured and fracture behaviour studied using acoustic emission analysis in parallel with microstructural examination. The results indicated that the aluminium carbide interfacial reaction had produced a strong fibre matrix bond, but reduced the fibre strength and embrittled the matrix. Consequently, whole fibre bundles failed in a brittle manner in the longitudinal direction with limited pull-out of individual fibres. The findings are discussed in relation to the method used to manufacture the composite.     

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.     

Interfacial reaction and strength of SiC fibres coated with aluminium alloys

Silicon carbide fibres (Nicalon) were coated with pure aluminium and aluminium alloys containing silicon. The coated fibres were annealed to produce an interfacial reaction zone between the coated layer and the fibre. The effect of this reaction zone on the tensile strength of the fibre was investigated. During the early stages of growth the reaction zone of the fibre is thin, and the strength of the fibre is controlled by inherent defects so that the fibre retains its original strength. After the early stages, notches are formed in the reaction zone of the fibre on loading at a small strain and the fibre fractures when a notch extends into the fibre. In this stage the fibre strength is dependent on the thickness of the reaction zone. An alloying addition of 1 or 5 at % Si to the aluminium matrix was found to be effective in retarding the growth rate of the reaction zone.On leave from Institute of Metal Research, Academia Sinica, Shenyang, China.On leave from Instituto Superior Tecnico, Departamento de Metalugia e Materais, Lisbon, Portugal.On leave from Hitachi Research Laboratory, Hitachi Ltd, Saiwai-Cho, Hitachi, Ibaraki 317, Japan.     

Nextel™ 610 alumina fibre reinforced aluminium: influence of matrix and process on flow stress

Continuous alumina fibre reinforced aluminium matrix composites are produced using two different liquid metal infiltration methods, namely direct squeeze casting and gas pressure infiltration. Net-shape fibre performs for longitudinal parallel tensile bars are prepared by winding the Nextel™ 610 alumina fibre (3M, St Paul, MN) into graphite moulds. High purity aluminium, two binary (Al–6% Zn and Al–1% Mg) and one ternary (Al–6% Zn–0.5% Mg) aluminium alloys are used as matrix materials. The composite is tested in uniaxial tension–compression, using unload–reload loops to monitor the evolution of Young's modulus. A linear dependence between Young's modulus and strain is observed; this is attributed, by deduction, to intrinsic elastic non-linearity of the alumina fibre. This conclusion is then used to compare on the basis of the in situ matrix flow curve the influence of matrix composition and infiltration process on the composite stress–strain behaviour.     

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.     

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.     

Control of interfaces in Al-C fibre composites

The interface of Al-C fibre composite was modified by coating a silver layer on the surface of carbon fibres prior to making composites, in an attempt to improve the wettability between molten aluminium and carbon fibres during infiltration. An electroless plating technique was adopted and perfected to provide a homogeneous silver coating on the carbon fibre surface. Al-C fibre composites were prepared using a liquid infiltration technique in a vacuum. It was found that silver coating promoted the wetting between aluminium and carbon fibres, particularly with polyacrylonitrile-base carbon fibres. However, due to rapid dissolution of silver in molten aluminium, it was believed that the improved infiltration was not due to the wetting behaviour between molten aluminium and silver. The cleaning of the fibre surface and the preservation of the cleaned carbon surface with silver coating was considered to be the prime reason for the improved wettability. Interfacial reactions between aluminium and carbon fibres were observed. Amorphous carbon was found to react more with aluminium than graphitic carbon. This is believed to be because of the inertness of the graphitic basal planes.     

Products of interaction in the Al-Si alloy-carbon fibre system

Samples of a composite material were obtained by pressure infiltration with AL2 melt of a carbon tape. Products of interaction forming on the fibre-matrix interfacial boundary were investigated. It has been found that Al₄C₃ and SiC phases and silicon crystals are formed. An increase of the contact time between a fibre and the melt, and melt temperature and the infiltration pressure leads an increase in the Al₄C₃ carbide phase quantity and to the bond strength on the fibre-matrix interfacial boundary. Here with tensile strength tests, the character of fractures changes from tensile (with noticeable fibre pull-out from the matrix) — to ductile; shear strength is increased.     

Process,microstructure and properties of squeeze-cast short-carbon-fibre-reinforced aluminium-matrix composites

Two types of high-modulus short-carbon-fibre-reinforced commercially pure aluminium-matrix composites were fabricated in-house using a home-made squeeze caster. The type-I composites were fabricated from short-fibre preforms in which fibres exist as dispersed bundles. The type-II composites were fabricated from preforms in which individual fibres were uniformly dispersed. The detailed processes are described in the text. A three-point-bending strength of higher than 200 M Pa was obtained for the type-1 composite with 17 vol% of fibre. When more fibre was incorporated, both the strength and the ductility decreased due to inadequate infiltration. However, a bending strength of greater than 240 MPa was recorded on a hot-rolled type-I composite with a fibre content as high as 28 vol%. This significant improvement in the mechanical properties is explained by a hot-rolling-inducedvoid-healing effect. The type-II composites, with lower fibre volume fractions than those of the type-I due to their different preforms, exhibited bending strengths up to 166 MPa. Scanning electron microscopy fractography shows that the two types of composites fracture in distinctive manners. Transmission electron microscopy results featured thermal-stress-induced dislocations at carbon-aluminium interfaces as well as submicrometre-sized aluminium carbide, the reaction product, which nucleated from the interface and grew into the matrix interior.     

Process monitoring of partially reinforced metal-matrix composite components by acoustic emission

Partial reinforcement using a fibre bundle embedded in a part of a component has been investigated in the development of a composite piston for use in an internal combustion engine. A trial piston was fabricated by a casting operation in which molten aluminium was poured into a die containing an annular continuous fibre bundle. The most probable defects introduced during the manufacturing operation are (i) microcracks generated in the fibre bundle due to residual thermal stresses and (ii) imperfect impregnation of the molten aluminium into the fibre bundle. Acoustic emission measurements have been used as a technique to detect the presence of defects in the trial pistons. The acoustic emission was measured during cooling of the trial piston after casting. Microcracking in the fibre bundle during cooling could be detected. Imperfect impregnation of the aluminium into the fibre bundle could also be detected. The acoustic emission due to microcracking was found to be strongly dependent on the mechanical properties of the fibres, while the acoustic emission from incomplete impregnation was found to depend on process conditions. It is believed that acoustic emission measurements can not only be used for the detection of microcracks but can also be of value in the selection of fibre materials and in the adjustment of the process conditions.     

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.     

Characteristics of several carbon fibrereinforced aluminium composites prepared by a hybridization method

The properties and microstructures of several high-strength and high-modulus carbon fibrereinforced aluminium or aluminium alloy matrix composites (abbreviated as HSCF/Al and HMCF/Al, respectively, for the two types of fibre) have been characterized. The composites evaluated were fabricated by pressure casting based on a hybridization method. It was found that the strength degradation of high-modulus carbon fibres after infiltration of aluminium matrices was not marked and depended upon the type of aluminium matrix. However, the strength of high-strength carbon fibres was greatly degraded by aluminium infiltration and the degradation seemed to be independent of the type of aluminium matrix. The longitudinal tensile strength (LTS) of CF/Al composites was very different between HMCF/Al and HSCF/Al composites. The HMCF/Al composites had LTS values above 800 MPa, but the HSCF/Al composites had only about 400 MPa. In contrast, the transverse tensile strength of the HSCF/Al composites, above 60 MPa, was much higher than that of the HMCF/Al composites, about 16 MPa. Chemical reactions were evident to the interface of high-strength carbon fibres and aluminium matrices. There was no evidence of chemical products arising between high-modulus carbon fibres and Al-Si alloy and 6061 alloy matrices. However, it was considered that some interfacial reactions took place in pure aluminium matrix composites. Fracture morphology observation indicated that the good LTS of CF/Al composites corresponded to an intermediate fibre pull-out, whereas a planar fracture pattern related to a very poor LTS and fibre strength transfer. The results obtained suggested that interfacial bonding between carbon fibres and aluminium matrices had an important bearing on the mechanical properties of CF/Al composites. An intermediate interfacial bonding is expected to achieve good longitudinal and transverse tensile strengths of CF/Al composites.     

Influence of spray atomization and deposition processing on microstructure and mechanical behaviour of an aluminium alloy metal-matrix composite

An aluminium alloy metal matrix discontinuously reinforced with silicon carbide particulates, was synthesized using the spray atomization and co-deposition technique. Microstructural characterization studies were performed to provide an understanding of the intrinsic effects of carbide particulate co-injection into the aluminium alloy metal matrix. Results reveal the ageing kinetics to be altered by the reinforcing ceramic particulates. Ambient temperature tensile tests revealed that the presence of particulate reinforcement in the aluminium alloy metal matrix degrades both strength and ductility. The results obtained are discussed in relation to thermal conditions during spray co-deposition and contributions from reinforcement to intrinsic microstructural effects and mechanical response.     

Fabrication of alumina short fibre reinforced aluminium alloy via centrifugal force infiltration

Abstract

A new casting process for fabricating short fibre reinforced metal matrix composites via the centrifugal force infiltration of fibre preforms with molten aluminium alloys is described. The effect of processing variables, such as pouring temperature, preheated mould temperature, and time of application of centrifugal force, on the infiltration kinetics and resultant microstructure is discussed. Composites having fibre volume fractions of 4·5, 8·0, 12, and 16% were obtained via this method. Comparisons with existing infiltration technology and the mechanical properties of the composite are presented.

MST/2000     

Fracture behaviour and pullout analysis in ceramic matrix composites using m0dified Rosen's model

Abstract

A modified Rosen's model is proposed to predict the ultimate tensile load and pullout in ceramic matrix composite materials. The model assumes a Weibull distribution for the fibre strength and that the matrix is already cracked. The solid is discretised in fibre segments between matrix cracks. After fracture of one fibre segment, the load is redistributed among the intact neighbouring segments. This in turn produces further fibre failures and subsequent load transfer. Then, the load is increased on the testpiece until new segments are fractured, and the procedure is iterated until one fracture path isfound. The model is applied to a silicon carbide fibre/calcium aluminium silicate (SiC_f/CAS) material and compared with experimental results. It is concluded that upper and lower bounds can be obtained using different load transfer assumptions.     

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.     

Crack‐Tip Stress Field and Fatigue Crack Growth Monitoring Using Infrared Lock‐In Thermography in A359/SiCp Composites

Abstract: This paper deals with the study of fracture behaviour of silicon carbide particle‐ reinforced aluminium alloy matrix composites (A359/SiCp) using an innovative non‐destructive method based on lock‐in thermography. The heat wave, generated by the thermo‐mechanical coupling and the intrinsic energy dissipated during mechanical cyclic loading of the sample, was detected by an infrared camera. The coefficient of thermo‐elasticity allows for the transformation of the temperature profiles into stresses. A new procedure was developed to determine the crack growth rate using thermographic mapping of the material undergoing fatigue. The thermographic results on the crack growth rate of A359/SiCp composite samples with three different heat treatments were correlated with measurements obtained by the conventional compliance method. The results obtained by the two methods were found to be in agreement, demonstrating that lock‐in thermography is a powerful tool for fracture mechanics studies. The paper also investigates the effect of heat treatment processing of metal matrix composites on their fracture properties.     

A micromechanics-based strength prediction methodology for notched metal-matrix composites

An analytical micromechanics-based strength prediction methodology was developed to predict failure of notched metal-matrix composites. The stress/strain behaviour and notched strength of two metal-matrix composites, boron/aluminium (B/Al) and silicon carbide/titanium (SCS-6/Ti-15-3), were predicted. The prediction methodology combines analytical techniques ranging from a three-dimensional finite element analysis of a notched specimen to a micromechanical model of a single fibre. In the B/Al laminates, a fibre failure criterion based on the axial and shear stress in the fibre accurately predicted laminate failure for a variety of lay-ups and notch-length-to-specimen-width ratios with both circular holes and sharp notches when matrix plasticity was included in the analysis. For the SCS-6/Ti-15-3 laminates, a fibre failure criterion based on the axial stress in the fibre correlated well with experimental results for static and post-fatigue residual strengths when fibre/matrix debonding and matrix cracking were included in the analysis. The micromechanics-based strength prediction methodology presented here offers a direct approach to strength prediction by modelling behaviour and damage on a constituent level, thus explicitly including matrix non-linearity, fibre/matrix interface debonding and matrix cracking.