Microstructure and interface characteristics of B₄C, SiC and Al₂O₃ reinforced Al matrix composites: a comparative study

Abstract

Three aluminium metal matrix composites containing reinforcing particles of B₄C, SiC and Al₂O₃ (0–20 vol.%) were processed. The stir-casting manufacturing route followed by hot extrusion was utilized, being one of the cost-effective industrial methods. A clear interfacial reaction product/layer was found at Al–SiC interface for composites held for a relatively long processing time (>30 min). No reaction product was observed at Al–B₄C and Al–Al₂O₃ interfaces at the resolution limit of the SEM used. On the other hand, two secondary phases (alumina and another phase containing aluminium, boron and carbon) were found in the aluminum matrix away from the interface in Al–B₄C composites. From the fracture surface analysis, B₄C reinforced Al composite seemed to exhibit a better interfacial bonding compared to the other two composites.

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).