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

The chemical reactivity of boron carbide (B4C) with metallic aluminium (Al) was studied at temperatures ranging from 900 to 1273 K (627–1000 °C). Al–B4C powder mixtures were cold pressed, heated for 1–450 h under 105 Pa of purified argon and characterized by X-ray diffraction (XRD) optical metallography (OM), scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). Whatever the temperature in the investigated range, B4C has been observed to react with solid or liquid Al. As long as the temperature is lower than 933 K (660 °C), i.e. as long as Al is in the solid state, interaction proceeds very slowly, giving rise to the formation of ternary carbide (Al3BC) and to diboride (AlB2). At temperatures higher or equal to 933 K, Al is in the liquid state and the reaction rate increases sharply. Up to 1141 ± 4 K (868 ± 4 °C), the reaction products are Al3BC and AlB2: at temperatures higher than 1141 K, Al3 BC is still formed while Al3B48C2 (β-AlB12) replaces AlB2. In the three cases, interaction proceeds via the same mechanism including, successively, an incubation period, saturation of aluminium in B and C, nucleation and growth by dissolution–precipitation of Al3BC and a C-poor boride and, finally, the passivation of B4C by Al3BC. These results are discussed in terms of solid–liquid phase equilibria in the Al–B–C ternary system, with reference to the binary invariant transformation: α-AlB12 + L ⇔ AlB2, which has been found to occur at 1165 ± 5 K (892 ± 5 °C).

Chemical reactivity of aluminium with boron carbide

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

AbstractMetal matrix composites (MMCs) are becoming more popular as structural materials, and joining them is, therefore, of paramount importance. As these new materials become available, it is necessary to define and optimise joining techniques, and a thorough understanding of each process is required. A number of joining processes have been addressed, namely: fusion welding (tungsten and metal inert gas welding, electron and laser beam welding, resistance, capacitor discharge, and plasma welding), solid state bonding (diffusion bonding, friction and magnetically impelled arc butt welding), brazing, and adhesive bonding. The advantages and disadvantages of each technique are discussed with respect to weld-parent material microstructure and the mechanical properties of the bond. Recommendations are subsequently made as to the most appropriate method for bonding aluminium based MMCs.

Joining aluminium based metal matrix composites

Metal matrix composites (MMCs) are becoming more popular as structural materials, and joining them is, therefore, of paramount importance. As these new materials become available, it is necessary to define and optimise joining techniques, and a thorough understanding of each process is required. A number of joining processes have been addressed, namely: fusion welding (tungsten and metal inert gas welding, electron and laser beam welding, resistance, capacitor discharge, and plasma welding), solid state bonding (diffusion bonding, friction and magnetically impelled arc butt welding), brazing, and adhesive bonding. The advantages and disadvantages of each technique are discussed with respect to weld-parent material microstructure and the mechanical properties of the bond. Recommendations are subsequently made as to the most appropriate method for bonding aluminium based MMCs.

Joining of aluminium based metal matrix composites

The interaction of boron carbide and silicon carbide with liquid aluminium at 1000 K was studied when the melt was not stirred. Electron and X-ray diffraction methods were used to identify Al4C3 and AlB2 phases in the Al-B4C system, while the Al4C3 phase only was identified in the Al-SiC system. All the phases were found to grow when the metal crystallized from the supersaturated solution in the shape of rectilinearly faceted crystals. By weighing the specimens prior to and following contact with the liquid metal, kinetic peculiarities of the dissolution of carbide were established. Constants of boron and carbon dissolution from carbides in liquid aluminium were determined. The results are compared with those obtained using quantitative chemical analysis for boron carbide and aluminium carbide. The higher dissolution rate of boron carbide as compared with that of silicon carbide is accounted for by the peculiar structure of the solid.

T.A. Chernyshova

Papers

Interaction kinetics of boron carbide and silicon carbide with liquid aluminium