Processing of metal matrix composites and magnesium matrix composites

1.3.1.1 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, such as the stir casting [34-36] or semi-solid slurry stirring technique [37]. Sometimes, these particulates or whiskers

may be manufactured into preforms, and the melted alloy is subsequently introduced into preforms, for example, the squeeze casting [38-40] or molten metal infiltration technique [41].

Moreover, in order to uniformly disperse these particles, Lan et al. [42] reported the use of ultrasonic non-linear effects to disperse nano-sized ceramic particles in molten metal to fabricate the nano-sized SiC particle reinforced AZ91D magnesium composites. This way could effectively disperse SiC particle and reduce severe clustering occurrence. Detailed properties of Mg-matrix composites by various processes are listed in Table 1-4.

The interfacial reaction between the matrix and reinforcement in metal matrix composites systems is a very important issue. A good interfacial bond can effectively transfer load to reinforcements so as to enhance the stiffness and strength. For instance, Zheng et al.

[40] compared the difference of magnesium matrix composites mixed with SiC preforms in

the Al(PO₃)₃ binder between the one without the binder. If the composites are mixed into the Al(PO3)3 binder, the interfacial MgO phase would be formed mainly by the reaction between liquid Mg and Al(PO₃)₃ binder:

Al(PO3)3 + 9Mg→9MgO + Al +3P. (4)

Naturally, the composite with MgO to connect the matrix and reinforcement could increase the effective load transferred from matrix to reinforcement to result in a higher elastic modulus and strength. Zheng et al. [39] also observed similar interfacial reaction of MgO phase formation between the Al18B4O33 whiskers and AZ91, according to the following chemical equations:

Al18B4O33 + 33Mg→ 33MgO + 18Al + 4B, (5) ΔG1073K = -1783 kJ/mol.

Al18B4O33 + 33Mg→33MgAl2O4 +6Al +16B, (6) ΔG1073K = -1128 kJ/mol.

Both of the chemical reactions are thermodynamically possible at 800^oC. However, Eq. (5)

has lower ΔG value, which implies that the formation of MgO is thermodynamically more favorable than that of MgAl2O4 (the spinel phase).

In the particulate reinforcement system, Bochenek and Braszczynska [35] observed the formation of MgO and Mg₂Si compounds in the MgAl₅ alloy matrix composites reinforced with SiC particles. They reported that the SiO2 film covering silicon carbide particles might have been reduced by molten magnesium according to the following two-stage reactions:

2Mg + SiO2 → 2MgO + Si, (7) ΔG1000K = -255 kJ/mol.

2Mg + Si → Mg2Si, (8)

ΔG1000K = -55 kJ/mol.

Moreover, Lan et al. [42] also found the existence of SiO₂, MgO and Mg₂Si after solidification of the AZ91D matrix composites reinforced with SiC particles. They used X-ray photoelectron spectroscopy (XPS) to identify the Si-2p spectra, and that of Si2p of AZ91D/5SiC consisted of three peaks at 99 eV, 100.3 eV and 102.8 eV. These were identified as those due to Si, SiC and SiO₂, respectively. They also illustrated the formation of Mg₂Si, SiO2 and MgO, as shown in Fig. 1-3. Moreover, Ye and Liu [43] reported that the composition of the matrix and the reinforcement materials, as well as the microstructure and porosity of the reinforcement materials would predominantly influence the interfacial reactions.

From the above examples, it needs to pay attention to the interfacial reaction between the molten metal and reinforcement. Promising chemical reactions will effectively enhance the stiffness and strength, but detrimental chemical reactions may form some extra undesired precipitates.

1.3.1.2 Solid-state methods

The solid state method utilizes higher the 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) [44-47] and diffusion bonding [48]. In addition, the PM process would generally be followed by a secondary processing such as extrusion, rolling, forging and superplastic forming to form the final shapes of products as well as to reduce the porosity. Detailed properties of Mg-matrix composites made by the PM method are included in Table 1-5.

The interfacial reactions between the matrix and reinforcement prepared by the PM method were less frequently reported. Because the PM working temperature is below the melting point of the matrix, the interfacial reaction could not rapidly occur. However, once the sintering temperature reaches close to the liquid line, the interfacial reactions may occur.

For example, Li et al. [49] reported that MgO, MgAl2O4 and Mg2Si were the main reaction compounds in the Al-Mg alloy matrix composite reinforced with SiO₂-based glass particles fabricated by the PM process by press sintering at 610^oC. The main interfacial reactions of glass/Al-Mg alloy composite are:

SiO2(s) + 2Mg(l) → 2MgO(s) + Si(s), (9) 2SiO2(s) +2Al(s) + Mg(l) →MgAl2O4(s) + 2Si(s), (10) 2Mg(l) + Si(s) → Mg2Si. (11)

1.3.1.3 Other processing methods

In addition to these two categories of processing techniques described above, a number of other techniques have been explored for the fabrication of magnesium matrix composites, including in-situ synthesis [51] and spray forming [52,53]. The in-situ synthesis is a process wherein the reinforcements are formed in the matrix by a controlled metallurgical reaction.

During fabrication, one of the reacting elements is usually a constituent of the molten matrix alloy. The other reacting elements may be either externally-added fine powders or gaseous phases. Mabuchi et al. [51] utilized this concept and rapid solidification to fabricate the Mg-Mg2Si composite. Spray forming is also a potential processing to fabricate Mg based metal matrix composites with particulate reinforcement [52,53]. The method is that reinforcing particles are synchronously injected into the stream of the atomized matrix materials to form a bulk MMC before the solidification of an atomized matrix material dropping onto a substrate.

From the above introduction about the metal matrix composites, it is known that many microstructure factors would affect the final performance of MMCs. Table 1-6 [50] presents a list of microstructural factors that influence mechanical properties and fractures 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. Nevertheless, according to Eq. (3), L_s in the modified alloy with V_f = 3% (0.03) and <r> = 20 nm will be 167 nm. Fortunately, 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. However, nano-sized composites were focused on the polymer matrix modified by ceramic nano particles so as to significantly improve its mechanical or physical properties [55-59]. It is less frequently

addressed about the nano-sized particulates reinforced metal matrix composites except for a few papers [33,42,47,54]. 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 better properties, uniform dispersion of these nano-sized particles would be an extremely difficult task. Due to the high surface area ratio, nano-sized powders tend to cluster together, and they sometimes form micro-sized aggregates. After secondary treatments, these aggregates will act as defect to form the crack initiation to degrade the final performance.

Methods in dispersing the nano powders have been limitedly disclosed, mostly still protected by patents. How to effectively and simply disperse nano-sized particle into metal matrix will be one of the major efforts for the current proposed research work.