Metal matrix composites

phase (e.g. polymers or metals) with the stronger but more brittle one (e.g. ceramics), so as to tailor the needed properties of the resulting mixed materials. Among them, metal matrix composites (MMCs) have attracted attention since 1970’s. Metal matrix composites, in general, consist of at least two composites: one obviously is the metal matrix (in most case, an alloy is the metal matrix), and the second component is a reinforcement (in general, an intermetallic compound, an oxide, a carbide or a nitride). MMCs have several advantages that are very important for their use as structural materials. These advantages include a combination of the following properties [62]:

z High strength

z High elastic modulus

z High toughness and impact properties

z Low sensitivity to temperature changes or thermal shock z High surface durability and low sensitivity to surface flaws z High electrical and thermal conductivity

z High vacuum environment resistance

In addition to conductivity of MMCs, the most obvious advantages of MMCs are their resistance to severe environments, toughness, and retention of strength at high temperatures.

The reinforcements of the MMCs can be divided into two major groups, discontinuous and continuous. The continuous reinforcements, such as carbon fibers, glass fibers and silicon fibers, are in the form of continuous long fibers or short fibers. As for discontinuous reinforcements, the most prominent discontinuous reinforcements have been SiC, Al2O3 and TiB2 in both whisker and particulate form.

Continuous fibers could improve the elastic modulus and ultimate tensile strength of composites significantly. Because continuous fibers have a large aspect ratio of axial length

to diametric width, such continuous fibers could effectively carry most of load so as to enhance the elastic modulus and strength of entire composites. From these viewpoints, continuous fibers seem to be perfect reinforcements, but they also suffer some disadvantages.

The strength along the radial direction is greatly lower than that along axial direction, leading to an anisotropy problem [63]. This character will be unfavorable to some engineering applications. In order to improve this issue, most fibers are weaved into different directions to decrease anisotropy.

The composites reinforced by particulates or whiskers usually do not have a strong anisotropic character, sometimes can be almost isotropy. But discontinuous reinforcements would do not effectively share the load. Although particulates or whiskers reinforced MMCs would not have compatible elastic modulus and strength as the continuous fibers reinforced ones, the former can be processed by conventional methods such as extrusion, rolling and forging to meet the desired shapes.

It has long been known that the second phase inclusions or particles can inhibit grain growth in metallic materials. Therefore, one of the critical microstructure parameter is the particle interspacing Ls, which can be roughly estimated from [64]

,

2 / 1

3 2 ⎟⎟

⎠

⎞

⎜⎜

⎝

= ⎛

f

s r V

L π (3)

where <r> is the average particle radius and Vf is the particle volume fraction. Previous reinforcing ceramic particles in the 1980’s usually measure around 10-50 μm in diameter, later improvements have lead to the smaller particles with uniform size distribution in the range of 0.5-5 μm. With the typical reinforcing particles of V^f=20% and <r>=10 μm in typical aluminum base composites, Ls will be around 30 μm. The resulting grain size after

casting would also generally be in this range. Further hot extrusion may refine the grain size down to around 5~10 μm, and toughening effects will only be moderately enhanced. With the abundant resources lately of the nano particles or nano carbon tubes (or wires) fabricated by various physical or chemical means, the reinforcements can be substantially smaller. If the reinforcement size is lowered to be submicron size or nano size, Ls can be reduced to submirco or nano range. It means that the grain size can be refined to submicron or nano grains. For example, Wang and Huang [65] ever added 1 vol% silica with 50 nm to 6061 aluminum alloy and extruded this composite to obtain 0.7 μm grain size.

1.4.1 Processing of metal matrix composites and magnesium matrix composites

There are various processes to produce particulates reinforced MMCs, but they could approximately be divided into solid-state and liquid-state methods.

1.4.1.1 Liquid-state methods

In principle, the liquid state route represents a very simple processing concept whereby particulates or whiskers are mixed into a light alloy melt, subsequently cast and then fabricated in a manner analogous to conventional unreinforced alloys. Sometimes, these particulates or whiskers may be manufactured into performs, and the melted alloy is subsequently introduced into performs. The related processes include the stir casting [66-68], squeeze casting [69-71], molten metal infiltration technique [72], semi-solid slurry stirring technique [73] and plasma spray [74]. Moreover, in order to uniform disperse these particles, Lan et al. [75] reported the use of ultrasonic non-linear effects to disperse nano-sized ceramic particles in molten metal to fabricated the nano-sized SiC particle reinforced AZ91D magnesium composites. This way could effectively disperse SiC particle and reduce severe

clustering occurrence. Detailed properties of composites by various processing routes are listed in Table 1-4.

1.4.1.2 Solid-state methods

The solid state method applies higher diffusion ability at elevated temperatures to sinter the entire composites and the metal matrix is remained to be the solid state during processing.

The most typical solid state methods are the powder metallurgy (PM) [76-79] and diffusion bonding [80]. In addition, the PM process would generally be followed by a secondary processing such as extrusion, rolling, forging and superplastic forming to form final shapes of productions as well as to reduce the porosity. Detailed properties of composites made by the PM method are included in Table1-4.

It is known that many microstructure factors would influence the final performance of MMCs. Table 1-5 [81] presents a list of microstructural factors that influence mechanical properties and fracture in discontinuously reinforced MMCs.

The above mentioned metal based composites, either with the conventional reinforcements ~20 μm or the more advanced ones ~0.5 μm in dimension, have micro-range grain size. According to Eq. (3), when the reinforced particles are smaller, the resultant composite will have finer grain size. The success in fabrication of various nano-sized powders, wires or tubes has offered the new possibility in modifying existing commercial materials in terms of their functional or structural characteristics. At present, nano-sized composites were focused on the polymer matrix modified by ceramic nano particles so as to significantly improve its mechanical or physical properties [82-86]. It is less addressed about nano-sized particulates reinforced metal matrix composites except for few papers [75,79].

The nano-sized particulates reinforced metal matrix composites might be able to stabilize grain size to less than 200 nm and enhance the ductility at elevated temperatures.

Although the nano-sized particulates reinforced metal matrix composites could have a better properties, uniform dispersion of these nano-sized particles would be an extremely difficult task. Because of the high surface area ratio, nano-sized powders tend to cluster together, sometimes forming micro-sized aggregates. After secondary treatments, these aggregates will act as defect to form the initiation of crack to degrade the final performance.

Methods in dispersing the nano powders have been limitedly disclosed, mostly still protected by patents.