**1. Introduction**

298 Recent Trends in Processing and Degradation of Aluminium Alloys

Vinogradov, A. (1998). Acoustic emission in ultra-fine grained copper. *Scripta Mateialia,*

Valiev, R.Z.; Ismagaliev, R.K & Alexandrov, I.V. (2000). Bulk nanostructured materials from

severe plastic deformation. *Progress in Materials Science*, Vol.45, No.2, pp.103-189,

Vol.39, No.6, (August 1998), pp.797-805, 1359-6462.

ISSN 0079-6425.

Aluminum has played and continuous to play a key role in the development of metal matrix composites (MMCs) reinforced with a variety of ceramic materials including Al2O3, TiC, B4C, and SiC. From the wide range of MMCs systems studied thus far and on account of the attractive properties of SiC, Al/SiC composites have drawn the attention of a plethora of research scientists and technologists. Like with any other composite material, the materials behavior lies much in the matrix characteristics as in the reinforcement properties. Several aspects are to be considered with regard to the metallic matrix, namely, composition, response to heat treatments, mechanical and corrosion behavior. And since aluminum offers flexibility in terms of these aspects, accordingly, a number of aluminum alloys have been used in studies intended for research and technological applications. The choice, however, for one or another alloy depends also on other factors as the composite processing route, which in turn can be dictated by the volume fraction of the reinforcement in the composite. For instance, the stir casting route is more suitable for low volume fractions (< 20%), whilst the infiltration routes are more appropriate for high volume fraction of the reinforcement (> 40%). Another important factor for selection of the aluminum alloy is the composites application and specific requirements in service. For instance, one composite may behave better under certain loads or in corrosive environments.

The aim of this chapter is to provide the readers with an insight into the factors that affect the properties of Al/SiC composites and the most important response parameters, associated to mechanical, heat-treatment, and corrosion behavior. The chapter is organized based on a hierarchical concept, starting with the role of alloy composition, followed by the resulting mechanical properties and its dependence in heat treatments, closing with the corrosion behavior. At the same time, this is derived from the central paradigm of materials science and engineering, based on the correlation: *processing* → *microstructure* → *properties* → *performance*. A review of the main findings in studies related to mechanical properties, heat treatments and corrosion behavior is presented. In view of that, the chapter is provided with references from earlier-to-the most recent studies on the behavior of Al/SiC composites, on the basis of the importance of aluminum alloy characteristics.

## **2. Factors and response variables related to Al alloys for Al/SiC composites**

Aluminum alloy composition is one of the factors that most notoriously influence the properties of Al/SiC composites. Other factors are processing time and temperature,

Aluminum Alloys for Al/SiC Composites 301

into the alloy matrix, and on other factors such as the SiC volume fraction and shape (fibers, whiskers or particles). In some cases, processing is made with the metal in the liquid state and in others, in solid state, as in the powder metallurgy route. Due to the inherent advantages of using the aluminum alloy in molten state, a copious number of researchers have used liquid aluminum in their work. Hence, this section revolves around the implications of using liquid aluminum for the processing and characterization of Al/SiC composites. With the metal in liquid state, the SiC reinforcements can be incorporated by way of stirring or mixing, followed by casting into metallic molds to produce ingots, which are then remelted to produce parts by other routes, like squeeze casting. Alternatively, the alloy may be incorporated into a porous preform formed by the SiC reinforcements, via the infiltration route, which in turn can be conducted either under the application of forces or vacuum or by capillary action. The latter is the so-called non-assisted, spontaneous or pressureless infiltration. In both cases – mixing and infiltration – wettability of the SiC reinforcements by the aluminum alloy is a crucial prerequisite, in tandem with an optimum fluidity of the alloy. One of the main problems faced when processing Al/SiC composites with the metal in molten state is that liquid aluminum tends to attack SiC according to the following reaction

 4Al(*l)* + 3SiC(*<sup>s</sup>*) ↔Al4C3(*s*) + 3Si(in *l* Al) (1) The Al4C3 compound has deleterious effects within the composite because, firstly, as a brittle phase degrades the mechanical properties, and secondly, it reacts with liquid water or with moisture in the ambient, debilitating even more the composite, according to the following

 Al4C3(*s*)+ 18 H2O(*l)* → 4Al(OH)3(*s*) + 3CO2(*g*) + 12H2(*g*) (2) ΔG25 oC= -1746 kJ/mol

 Al4C3(*s*)+ 12 H2O(*<sup>g</sup>*) → 4Al(OH)3(*s*) + 3CH4(*g*) (3) ΔG25 oC= -1847 kJ/mol Several approaches have been proposed in the literature to prevent or retard aluminum carbide formation and overcome its harmful effect; these include: i) modifications to the aluminum alloys compositions, ii) coatings on the SiC reinforcements, iii) varying processing temperature and contact time, iv) artificial or intentional oxidation of the SiC reinforcements, and v) incorporation of silica (SiO2) powders in the SiC preforms

One of the first and most successful approaches to avoid the attack of SiC by liquid aluminum and its consequences, and at the same time improve wetting, was the modification of alloy composition with silicon and magnesium. It is well established that silicon in the aluminum alloy plays a key role, as it lowers the melting point of the alloy and decreases the contact angle between the solid and the liquid, thus enhancing wettability

On the other hand, from equation (1) and in the light of Le Chatelier's principle it is simple to see the beneficial effect of silicon, since it will tend to reverse the direction of the reaction. However, an uncontrolled excess of silicon in the alloy may have an adverse effect because above a critical level –the eutectic point – it tends to increase the viscosity of the alloy. In the

case of the Al-Si system it corresponds to 12.6 wt. % Si at the temperature of 577 °C.

(Iseki et. al., 1984):

reactions (Kosolapova, 1971; Park & Lucas, 1997):

(Rodriguez-Reyes et.al., 2006).

(Pech-Canul et. al., 2000 (a)).

atmosphere type, and reinforcement characteristics. The alloy chemistry impinges on the mechanical properties and corrosion behavior, and when applicable, on the composite response to solution treatment and age hardening. It is interesting to note, however, that the wide variety of Al/SiC composites reported thus far have been prepared both with commercial and experimental alloys.
