V. D. Scott

Corrosion behaviour of aluminium-based metal matrix composites

The corrosion characteristics, in 3.5 wt% NaCl solution, of aluminium alloy composites containing a range of reinforcements have been investigated using potentiostatic measurements and simple immersion tests. Complementary microstructural studies carried out on corroded surfaces and sections through corroded material have identified a number of preferential corrosion sites; these include the fiber/matrix interface, especially where it contains chemical reaction products resulting from composite fabrication, as well as second phases and pores in the metal matrix. The effect on corrosion behaviour of the different reinforcements, with particular reference to their chemistry and geometry, is discussed, as is the influence of composite manufacturing route.

Effect of heat treatment in air on the structure and properties of barium osumilite reinforced with Nicalon fibres

Microstructural studies have been carried out on glass-ceramic matrix composites, consisting of barium osumilite reinforced with Nicalon fibres, which have been subjected to heat treatment in air in the range 600–1100 °C. Parallel studies have involved the measurement of the friction stress between fibre and matrix and the flexural strength of the composite. The matrix was shown to consist of barium osumilite, hexacelsian, mullite and a silica-rich glass, the thermal mismatch of these different phases leading to the development of appreciable strains. Whilst high-temperature treatments caused the formation of voids due to flow of the glassy phase, the major factor controlling the mechanical properties of the composite was the fibre/matrix interface. A change in microstructure, from a weak carbon-rich interface to one where the fibre and matrix were strongly bonded together by a silica layer, was thus reflected in an increase in the interfacial friction stress and a change in fracture behaviour from one showing fibre pull-out and delamination to one with brittle characteristics.

Interface characterization and fracture of calcium aluminosilicate glass-ceramic reinforced with Nicalon fibres

An investigation of the structure and properties of a calcium aluminosilicate glass-ceramic reinforced with Nicalon fibres is described. Microstructural analysis of the interface showed that during manufacture of the composite a reaction zone rich in carbon formed between the Nicalon fibre and the anorthite matrix. Tensile strengths were approximately 330 MPa for unidirectional material and around 210 MPa for a (0°/90°)3s. composite, little more than half that predicted by the mixtures rule. Flexural strengths were, however, higher than tensile strengths, by a factor 1.5–2.5 depending on lay-up. Studies carried out on specimens heat treated in air for 24 h at temperatures in the range 600–1200 °C showed a progressive change of interface microstructure in the outermost regions of the specimens due to oxidation of the carbon-rich layer; at 1000 °C and above the carbon had disappeared to leave voids and silica-rich bridges between fibre and matrix. These changes affected the strength of the interfacial bond, as measured by an micro-indentation technique, and also the degree of fibre pull-out produced in mechanical tests. Thus as-received material exhibited appreciable pull-out whilst heattreated samples were characterized by brittle behaviour in the outer (oxidized) regions. Nevertheless, the composites whilst in the unstressed condition appeared to survive these short-term exposures to oxidizing environments. An interfacial shear stress of around 5 MPa was calculated by applying the Aveston, Cooper and Kelly theory to crack spacings measured in our room-temperature deformation experiments, a value which agreed well with the 3–5 MPa obtained by the micro-indentation method.

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.

Microstructural studies of aluminium-silicon alloy reinforced with alumina fibres

The microstructure of an alumina fibre reinforced Al-7wt% Si alloy has been investigated. It was shown that the Al-Si eutectic structure which characterized this alloy was markedly changed by the presence of the fibres, with coarsening of silicon particles and a reduction in primary aluminium grain size. The coarse silicon particles exhibited twinning but no orientation relationship with the aluminium. Fine silicon precipitates were also present and these had a cube-cube orientation relationship with the aluminium lattice. Lath-like intermetallics, FeSiAl5 and FeSi2Al4 with monoclinic and tetragonal structures, were identified which existed in equilibrium and had the epitaxial relationship (001)mono//(001)tet and [100]mono//[100]tet. The iron was a contaminant introduced in the course of composite fabrication.Dislocations were a common feature of the aluminium matrix, with a typical density of ∼4×107mm−2. Nevertheless, dislocation hardening of the metal matrix was not detected. No evidence of Mg2Si precipitates in the metal matrix was found, but the small addition (0.2wt%) of magnesium to the alloy was discovered to segregate at the fibre-aluminium interface. This segregation was believed to result in improved wettability of the two constituents, encouraging the formation of a strong fibre-matrix bond, and producing desirable properties of the composite in the transverse direction.

Microstructure property relationship in Pyrex glass composites reinforced with Nicalon fibres

Detailed microstructural studies have been carried out on a series of composites consisting of Pyrex glass reinforced with Nicalon fibres. A variety of techniques has been employed, including X-ray and electron diffraction, electron-probe microanalysis and thin foil analytical electron microscopy. In parallel, mechanical tests have been performed on the composites and measurements have been made of the fibre-matrix bond.Substantial amounts of cristobalite have been identified in the matrix, up to ∼48% by volume in some cases. At such levels, microcracking is a common occurrence due to the high differential contraction between the matrix constituents upon cooling, which leads to matrix disintegration upon mechanical testing. A second microstructural feature which affects the mechanical behaviour of the composite concerns the fibre-matrix interface and, in particular, the chemistry of the outermost (∼200 nm) surface regions of the fibre. The amount of graphite here is shown to affect directly the strength of the fibre-matrix bond and, in turn, the degree of fibre pull out and the mechanical properties of 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.

Microstructure and micromechanics of the interface in carbon fibre reinforced Pyrex glass

Detailed microstructural studies have been carried out on composites consisting of Pyrex glass reinforced with carbon fibres. Analysis of the fibre-matrix interface showed that some reaction had taken place during fabrication of the composite and that a carbide or oxycarbide layer had formed between the glass and the carbon fibre. The measured interlaminar shear strength of the composite indicated that the layer was not a source of weakness and appeared to be well bonded to the matrix. Substantial fibre pull-out had occurred, however, to expose clean fibre surfaces and smooth sockets. These observations led to the conclusion that the interfacial shear process was confined substantially to the outer layers of the carbon fibre. Confirmatory evidence for the low interfacial friction stress was available from micro-indentation tests which showed fibre displacement relative to the matrix at loads of less than ∼10 kPa. Heat treatment of the composite at 500°C in air caused preferential oxidation of the carbon fibre. Where fibres met the specimen surface, oxidation had proceeded down the fibre to produce a smoothly tapering shape. The rate of oxidation was estimated to be 3 μm h−1 parallel to the fibre axis, but much less than this in a direction perpendicular to the fibre, 0.5 μm h−1, due to the relatively slow diffusion rate of oxygen through glass.

Comparison of tunnel and jet methods for cavitation erosion testing

Abstract The cavitation erosion behaviour of a number of metals has been studied using a sea water cavitation tunnel test and a fresh water jet cavitation method. Erosion parameters, including erosion rate and depth of damage, have been obtained for both systems and the data shown to correlate with mechanical properties of the metal, such as plasticity and hardness. Analysis of eroded specimens subjected to interrupted jet cavitation tests, combined with parallel microstructural studies on all eroded specimens, have shown that the erosion damage produced by the two test methods, whilst differing in degree, is generally similar in character. Hence it may be concluded that the jet cavitation test provides, with qualification, a satisfactory alternative for the more traditional cavitation tunnel technique.

Joining molybdenum to aluminium by diffusion bonding

The joining of molybdenum to aluminium and aluminium-copper alloy using diffusion bonding has been investigated. Bond strengths have been measured by means of a simple shear jig and the joint microstructures characterized by electron microscopy and electron-probe microanalysis. Successful joints were produced by using a copper foil interlayer to form a eutectic liquid during the bonding process which helped disrupt the oxide film on aluminium and promote metal diffusion across the joint interface. When bonding commercial-purity aluminium to molybdenum, the iron present as an impurity caused a ternary eutectic liquid to form and, after solidification of the liquid phase, a thin film of Al7Cu2Fe was left behind on the aluminium. Failure of this joint occurred at a shear stress of 75 MPa, with the fracture path contained within the aluminium. With super-purity aluminium, a binary eutectic liquid was produced and the ensuing interface reaction resulted in a multi-layered structure of molybdenum-containing phases. The bond failed at the molybdenum interface at a stress of 40 MPa. When bonding aluminium-copper alloy to molybdenum without a copper interlayer, general melting at the interface via eutectic phase formation did not occur and the interface showed only localized reaction. The joint failed by separation from the molybdenum, at a stress of 25 MPa. When, however, a copper interlayer was used, fairly thick regions of multi-layered molybdenum intermetallics formed and the remaining surface was covered by a layer of Al7Cu2Mo phase. Failure of this joint occurred at a stress of 70 MPa, mainly by separation at the molybdenum interface.