Microstructure and interface characteristics of B4C, SiC and Al2O3 reinforced Al matrix composites: a comparative study
Introduction
Aluminium metal matrix composites (Al MMCs) are being considered as a group of new advanced materials for its light weight, high strength, high specific modulus, low co-efficient of thermal expansion and good wear resistance properties. Combination of these properties are not available in a conventional material [1]. The use of Al MMC has been limited in very specific applications such as aerospace and military weapon due to high processing cost. Recently, Al matrix composites have been used for the automobile products such as engine piston, cylinder liner, brake disc/drum etc. [2]. Processing techniques for Al MMCs can be classified into (1) liquid state processing, (2) semisolid processing and (3) powder metallurgy [3], [4]. Particulate reinforced Al composites can be processed more easily by the liquid state i.e. melt-stirring process. Melt stir casting is an attractive processing method since it is relatively inexpensive and offers a wide selection of materials and processing conditions.
The primary function of the reinforcement in MMCs is to carry most of the applied load, where the matrix binds the reinforcements together, and transmits and distributes the external loads to the individual reinforcement [5]. Good wetting is an essential condition for the generation of a satisfactory bond between particulate reinforcements and liquid Al metal matrix during casting composites, to allow transfer and distribution of load from the matrix to the reinforcements without failure. Strong bonds at the interface are required for good wetting. These bonds may be formed by mutual dissolution or reaction of the particulates and matrix metal. The reaction phenomena are very detrimental to the composite as they bring about a decrease of the mechanical properties [6].
Substantial information is available in literatures on wetting and interface of Al-alloy MMCs reinforced with SiC, Al2O3 particulates [7], [8], [9], [10], [11], [12], [13]. Al–SiC system is a reactive system, as it produces Al4SiC4 or Al4C3 compound at the interface of particles and metal [10], [11]. Al4C3 is detrimental for the composites properties. The formation of Al4C3 can be minimized in several ways such as (1) using a suitable coating on particles, (2) using high silicon content Al alloys or (3) using pre-oxidized silicon carbide particulates [5]. The only reaction at the interface of Al–Al2O3 composites is Al2O3 dissolving into aluminium. Small addition of Mg encourages the formation of MgAl2O4 spinal with Al2O3. Some studies on the reactivity B4C in aluminium processed by infiltration and powder metallurgy techniques reported the formation of different compounds at different processing temperature [10], [11], [14], [15]. Good wettability of B4C in aluminium has been found in air due to the formation of boron oxide film around the particles [10], [11]. Literature related on the microstructure and interface of Al–B4C composites processed by a stir cast method in air is very scarce.
The main aims of the present work were to see the feasibility to produce Al–B4C composites by using conventional liquid melt-stirring process in air. For the comparison of the microstructure (particles distribution and pores) and interface of Al–B4C composites, other two composites Al–SiC and Al–Al2O3 were produced by using the melt stirrer method. The microstructure and interfaces were studied using optical microscopy (OM), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX).
Section snippets
Materials and experimental
Three different ceramic particles SiC, Al2O3, B4C were used as reinforcement in pure aluminum (99.99) matrix. Commercial β- and α-type silicon carbide particles with an average particle size of 40 μm, α-Al2O3 particles of 32 μm size and B4C particles of 40 μm size were used in this study. An electrical resistance furnace with a stirring assembly (a graphite impeller) was used for the dispersion of the ceramic particles into liquid aluminum. The reinforcing particles SiC and Al2O3 were preheated at
Results and discussion
Fig. 1 displays optical micrographs taken at low magnification from pure Al and Al–SiC (6, 13 and 20 vol.%) composites sectioned in the longitudinal direction showing distribution of pores. It is seen that the grain size of aluminium with 0% SiC is quite large. With the introduction of SiC particles, the grain size decreases. At the same time, pores seen as dark spot in Fig. 1 increases in number as more SiC particles are introduced in the cast composites. This is due to the fact that the
Conclusions
Particles distribution was found to be better in Al–B4C composites as compared to Al–SiC and Al–Al2O3 composites. A clear interfacial reaction product was found at Al–SiC interface for composites processed for long period, while no reaction product was observed at Al–B4C and Al–Al2O3 interfaces. Two secondary phases in the aluminium matrix away from the interface in Al–B4C composites are thought to be Al2O3 and Al3BC. B4C reinforced Al composite seemed to exhibit a better interfacial bonding as
Acknowledgements
The authors would like to thank ABOS, Belgium for support through a VlIR-ABOS (1998–2002).
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