The physical and mechanical properties that can be obtained with metal matrix composites (MMCs) have made them attractive candidate materials for aerospace, automotive and numerous other applications. More recently, particulate reinforced MMCs have attracted considerable attention as a result of their relatively low costs and characteristic isotropic properties. Reinforcement materials include carbides, nitrides and oxides. In an effort to optimize the structure and properties of particulate reinforced MMCs various processing techniques have evolved over the last 20 years. The processing methods utilized to manufacture particulate reinforced MMCs can be grouped depending on the temperature of the metallic matrix during processing. Accordingly, the processes can be classified into three categories: (a) liquid phase processes, (b) solid state processes, and (c) two phase (solid-liquid) processes. Regarding physical properties, strengthening in metal matrix composites has been related to dislocations of a very high density in the matrix originating from differential thermal contraction, geometrical constraints and plastic deformation during processing.

Particulate reinforced metal matrix composites — a review

A review is presented of those factors responsible for promoting the integrity and longterm durability of metal-polymer bonds used in the fabrication of aircraft and aerospace structures. Using a multidisciplinary approach and a variety of surface analytical techniques such as extended resolution scanning electron microscopy (XSEM), X-ray photo-electron spectroscopy (XPS), ellipsometry, and a new technique called surface behaviour diagrams (SBD), investigators at the author's laboratories have evolved several important concepts. First, it has been determined that the initial integrity of metal-polymer bonds depends critically upon the morphology of the surface oxide on the metal. For aluminium and titanium, the metals studied, it is demonstrated that certain etching or anodization pretreatment processes produce oxide films on the metal surfaces which, because of their porosity and microscopic roughness, mechanically interlock with the polymer forming much stronger bonds than if the surface were smooth. Second, the long-term durability of metal-polymer bonds is shown to depend strongly on the environmental stability (or lack of stability) of the same oxide which is responsible for good initial bond strength. For aluminium moisture intrusion at the bond line causes the oxide to convert to an hydroxide with an accompanying change in morphology and bond strength. For titanium the oxides appear to be much more stable than those on aluminium but under severe environmental conditions the oxide undergoes a polymorphic transformation which may lead to bond degradation. Third, it is observed that significant improvements in durability of adhesive bonds to aluminium can be achieved using an extremely simple treatment in which monolayer films of certain organic acids are applied to the adherend oxide to protect it against the effects of moisture.

Adhesion and Durability of Metal/Polymer Bonds

Review of surface pretreatments for aluminium alloys

The solidification microstructure of particulate reinforced aluminium/SiC composites

When metal matrix composites are produced by molten metal methods there are some unique factors which have to be considered. In this paper, the microstructure of SiC-reinforced aluminium alloys produced by this method are considered. It is shown that the stability of SiC in the melt is dependent on the matrix alloy involved and that only alloys with high silicon contents have a low reactivity with this reinforcement. With other alloy matrices, SiC reacts to form Al 4 C 3 , and the nature of this reaction and its kinetics are considered in this paper. Initially, the reaction rates are very rapid but almost saturate after about 1 h. It is also shown that the distribution of the reinforcing particles is dependent on the solidification rate because particles are rejected and pushed ahead of the meniscus. At low solidification rates, and hence for large cell sizes, the reinforcing particles are clustered and form a network which delineates the cell walls. Because the SiC particles are in the interdendritic regions they will be associated with any coarse intermetallic particles present and this can influence the fracture behaviour.

Microstructural aspects of aluminium-silicon carbide particulate composites produced by a casting method

The hydration of aluminium surfaces prepared for adhesive bonding by anodization in phosphoric acid has been studied using surface behaviour diagrams These surface behaviour diagrams, which are similar to phase diagrams for equilibrium bulk phases, trace the evolution of the aluminium adherend surface composition, obtained by X-ray photoelectron spectroscopy, during the hydration process When supplemented with high-resolution scanning electron micrographs and Auger depth profiles, the surface behaviour diagrams show that hydration proceeds in three steps The first step is reversible and consists of the adsorption of water by the monolayer of AlPO4 initially present on the surface It involves no change in the oxide morphology The second, which appears to be rate-controlling, involves the slow dissolution of the phosphate followed by rapid hydration of the exposed alumina to the oxyhydroxide, boehmite During this stage, extensive morphological changes occur as the boehmite fills the pore cells and bridges the whiskers of the original surface The third step consists of the nucleation and growth of the trihydroxide, bayerite, on top of the boehmite Using these results as examples, we propose the surface behaviour diagram approach as a new tool for the study of surface reactions in general

Application of surface behaviour diagrams to the study of hydration of phosphoric acid-anodized aluminium

The corrosion and passivity of aluminum exposed to dilute sodium sulfate solutions

The electrochemical response of Al alloys containing various concentrations of Mo in solid solution was studied using potentiodynamic polarization. The evolution of the chemical composition of the passive film during exposure to deaerated O.1N KCl was analyzed using x-ray photoelectron spectroscopy. Exposure of Al-Mo alloys to the chloride solution results in an increase in the corrosion potential and a decrease in the cathodic Tafel slope. The breakdown potential is also found to increase with higher concentrations of Mo in the alloy. These results correlate with an increase in the amount of Mo in the oxide film during polarization, even though the concentration of Mo in the passive film is well below that in the alloy. This work suggests that the oxidized Mo in the passive film inhibits the nucleation of pits in the alloy by improving the integrity of the passive film.

Corrosion Behavior of Aluminum‐Molybdenum Alloys in Chloride Solutions

The adsorption of selected organic hydration inhibitors onto Forest Products Laboratory (FPL)-etched aluminium surfaces and the subsequent hydration of the treated surfaces have been studied using X-ray photoelectron spectroscopy (XPS) and surface behaviour diagrams (SBDs) supplemented by Fourier Transform Infra-red Spectroscopy (FTIR). Wedge tests were used to evaluate performance of these inhibitors in improving bond durability and the locus of failure was identified by XPS and high resolution scanning electron microscopy (X-SEM). The results indicate that nitrilotris methylene phosphonic acid (NTMP) and related compounds adsorb to the alumina surface via the POH bonds of the phosphonic acid groups, resulting in a displacement of water normally adsorbed onto the surface. A model of adsorption was developed which suggests that after treatment with very low concentrations of inhibitor (∼1 ppm), only one leg of the NTMP molecule adsorbs onto the surface although at higher concentrations (∼100 ppm) all three legs adsorb. Hydration is a three-step process: (i) reversible physisorption of water; (ii) slow dissolution of the inhibitor followed by rapid hydration of the freshly exposed Al2O3 to boehmite (AlOOH); and (iii) further hydration of the AlOOH to bayerite [Al(OH)3]. Analysis of the adsorption, hydration, and wedge test results using different inhibitors suggests the following five inhibitor characteristics that promote good bond performance: (i) displacement of water and occupation of all active sites on the Al2O3 surface; (ii) formation of strong inhibitor surface bonds; (iii) insolubility of the resulting inhibitor-aluminium complex in aqueous solutions; (iv) compatibility with the adhesive or primer; (v) coupling of the inhibitor to the adhesive.

Use of hydration inhibitors to improve bond durability of aluminium adhesive joints

En alliant Mo et Al en solution solide, on ameliore considerablement la resistance a l'attaque localisee en deplacant la courbe de polarisation anodique dans la direction noble. L'alliage Al-6Mo resiste a la corrosion par piqure dans des solutions contenant du chlorure saturees en air

J. S. Ahearn

Papers

Application of surface behaviour diagrams to the study of hydration of phosphoric acid-anodized aluminium

The corrosion and passivity of aluminum exposed to dilute sodium sulfate solutions

Corrosion Behavior of Aluminum‐Molybdenum Alloys in Chloride Solutions

Use of hydration inhibitors to improve bond durability of aluminium adhesive joints

Influence of Molybdenum on the Pitting Corrosion of Aluminum Films