Microstructural characterization of a silicon carbide whisker reinforced 2124 aluminium metal matrix composite

The microstructure of a silicon carbide whisker (SiC_w) reinforced 2124 aluminium metal matrix composite was characterized using scanning transmission electron microscopy (STEM). The SiC whiskers ranged in length from approximately 2 to 10 µm, and demonstrated good bonding to the aluminium matrix. In a few cases, the interface between SiC whiskers and the aluminium matrix exhibited wavy characteristics. The size of subgrains in the aluminium matrix was found to be dependent upon that of SiC whiskers. In addition, two types of intermetallic compounds were observed in the composite.     

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.     

Joining of tungsten carbide to nickel by direct diffusion bonding and using a Cu–Zn alloy

The objective of this work was to study various aspects of liquid and solid state diffusion bonding of cylindrical samples of WC (with 6% Co) and commercially pure nickel (99.5%) produced by direct bonding and brazing using a 25 μm thick 70Cu 30Zn (wt%) alloy as joining element. Joining experiments were carried out on WC/Ni and WC/Cu Zn/Ni combinations at temperature of 980 °C using 1, 15, 25 and 35 min holding times in argon (Ar). The results show that it is possible to create a successful joint at temperature and times used. Joining occurred by the formation of a diffusion zone. The joining interface is feasible because it presents a homogeneous interface with no several interfacial cracking and porosity. In both combinations, it can be observed a diffusion of cobalt decreasing in the direction of the metal, as well as, the diffusion of nickel decreasing in the direction of the ceramic.     

Ageing behaviour and toughness of silicon carbide particulate reinforced Al-Li-Cu-Mg-Zr metal matrix composites

The effects of lithium content on the ageing characteristic and notched tensile properties of particulate reinforced Al-Li-Cu-Mg-Zr based metal matrix composites (MMCs) have been investigated. MMC sheet containing 20 wt% silicon carbide particulate produced by a conventional powder metallurgy route aged at a similar rate as unreinforced sheet, and the highest strengths were achieved in samples containing 2–2.5 wt% Li. A proprietary processed 8090 Al-Li alloy MMC sheet aged more rapidly, however, and gave considerably higher strengths. The toughness of Al-Li-Cu-Mg-Zr MMC sheet, as indicated by the notched tensile behaviour, can be improved by reducing the lithium content albeit at the expense of strength.     

Ageing and toughness of silicon carbide particulate reinforced Al-Cu and Al-Cu-Mg based metal-matrix composites

Metal-matrix composites (MMC) comprising powder aluminium alloys reinforced by particulate ceramic are being developed for widespread aerospace structural applications ranging from fuselage and missile components to undercarriage parts. Most interest is centred on MMCs with an Al-Cu-Mg (2124) matrix alloy. These MMCs possess high levels of specific stiffness with high specific strengths but can exhibit lower ductility and toughness than conventional unreinforced aluminium alloys.To overcome these problems the effects of alloy composition on the ageing behaviour and notched tensile properties of Al-Cu-Mg and Al-Cu based alloys reinforced with 20 wt% silicon carbide particulate have been investigated.Al-Cu-Mg MMCs gave higher strengths and moduli than unreinforced sheet. Lowering the copper and magnesium content resulted in reduced strength but did not affect the rate of age hardening. The Al-Cu-MMCs gave the lowest strengths but the absence of natural ageing may prove advantageous, enabling sheet to be formed and subsequently heat-treted to the peak strength condition.     

Liquid phase bonding of siliconized silicon carbide

Aluminium was used as a braze to join siliconized silicon carbide to itself. Brazes were carried out in the 700–1100 °C temperature range, in vacuum. A thick reaction layer forms in the ceramic adjacent to the braze film, due to reaction between the metal braze and the free silicon present in the ceramic matrix. The silicon concentration of the braze film reaches values well above the maximum liquid solubility at the brazing temperature. A pseudotransient aluminium-silicon liquid phase promotes the formation of a 100% silicon braze film when either high temperatures, long holding times or very slow cooling rates are used. The dominant mechanism responsible for the formation of the braze microstructure is the preferential unrestrained solidification growth of Si plates on the braze plane, supported by fast liquid Si diffusion. Strong joints were produced and, when pure silicon brazes formed, four-point bend strengths over 200 MPa were obtained at testing temperatures as high as 700 °C. Fracture occurs either in the reaction layer-ceramic boundary or in the braze, the crack propagation plane changing from one side of the braze-ceramic interface to the other and through the braze itself.     

Oxidation bonding of porous silicon carbide ceramics

A oxidation-bonding technique was successfully developed to fabricate porous SiC ceramics using the powder mixtures of SiC, Al₂O₃ and C. The oxidation-bonding behavior, mechanical strength, open porosity and pore-size distribution were investigated as a function of Al₂O₃ content as well as graphite particle size and volume fraction. The pore size and porosity were observed to be strongly dependent on graphite particle size and volume fraction. In contrast, the degree of SiC oxidation was not significantly affected by graphite particle size and volume fraction. In addition, it was found that the fracture strength of oxidation-bonded SiC ceramics at a given porosity decreases with the pore size but increases with the neck size. Due to the enhancement of neck growth by the additions of Al₂O₃, a high strength of 39.6 MPa was achieved at a porosity of 36.4%. Moreover, such a porous ceramic exhibited an excellent oxidation resistance and a high Weibull modulus.     

Polypropylene reinforced with silicon carbide whiskers

Thermal, crystallization and mechanical behaviour of isotactic polypropylene (iPP) reinforced with advanced silicon carbide whiskers (SiC_w) has been investigated. It is well established that the existence of chemical and physical interactions between the matrix and the reinforcement enhances the cohesive strength at the interphase thus improving the mechanical performance of the composite. In order to improve chemico-physical interactions between the components of the inorganic–organic composite system, their affinity has been enhanced in two ways: by coating the whiskers with a thin layer of acrylate-grafted polydivinylbenzene and by using as matrix a chemically modified polypropylene. The mechanical properties of the resulting composite materials have been compared and related to the dispersion grade of the whiskers within the matrices and the morphology of the samples. This revised version was published online in November 2006 with corrections to the Cover Date.     

Joining of reaction-bonded silicon carbide using a preceramic polymer

Ceramic joints between reaction-bonded silicon carbide (RBSiC) were produced using a preceramic polymer (GE SR350 silicone resin) as joining material; samples were heat treated in an argon flux at temperatures ranging from 800–1200°C without applying any pressure. The strength of the joints was determined by four-point bending, shear and indentation tests. Microstructural and microchemical analyses were performed by optical microscopy, SEM, TEM and AEM. The room-temperature strength of the joints increased with the joining temperature. Maximum values as high as 220 MPa in bending and 39 MPa in shear tests were reached for samples joined at 1200°C. No detectable residual stresses were observed both in the joining material and the joined parts, and the fracture mechanism was nearly always cohesive. The joint thickness was shown to depend on the processing temperature, and ranged from about 2–7 m. The joining material was a silicon oxycarbide amorphous ceramic, with no oxygen diffusion occurring between this and the RBSiC joined parts. The lack of compositional gradients, precipitates or reaction layers indicate that the SiOC ceramic acted as an inorganic adhesive, and that the joining mechanism involved the direct formation of chemical bonds between the RBSiC parts and the joining material. © 1998 Chapman & Hall     

High-temperature diffusion doping of porous silicon carbide

The results of experiments on high-temperature (2000–2200°C) diffusion doping of porous silicon carbide (PSC) by vanadium and erbium are reported. It is established that the specific features of diffusion processes in PSC at these temperatures are determined by modification of the porous structure due to the transport of vacancies. Based on a comparison of these results to available data on the low-temperature (900–1000°C) diffusion, it is concluded that the mechanisms of diffusion in PSC at low and high temperatures are different and that SiC with a porous structure is an effective medium particularly for low-temperature diffusion.     

Thermal diffusivity of chemically vapour deposited silicon carbide reinforced with silicon carbide or carbon fibres

The thermal diffusivity of chemically vapour deposited silicon carbide reinforced with either Nicalon SiC yarn or PAN-precursor carbon fibres was measured by the laser-flash method during various time-temperature treatments. The diffusivity was found to depend on the degree of densification, the direction of heat flow with respect to the fibre orientation, and the thermal history. Structural modifications, confirmed by X-ray diffraction, produced large permanent changes in the thermal properties of the SiC-SiC composites when heated above 1200° C, while only minor changes were seen in C-SiC composites heated above 1500° C. On sabbatical leave of absence from the Société Européenne de Propulsion, Bordeaux, France.