**3.1 Microstructural characteristics**

**Figure 2** shows the microstructural evolution of as-cast magnesium composites fabricated under four different processing conditions. It is clearly evident from microstructural analysis that the as-cast PP800/PL800 specimen exhibit bi-modal grain size distribution, whereas the as-cast PP700/PL700 specimen represents more or less uni-modal grain size distribution. Such difference in microstructural characteristics arises because of the fact that higher amount of heterogeneous nucleation sites is available for producing Mg crystals in PP800/PL800 specimen as they contain both the SiCNO and Mg2Si particles. However, it is mostly SiCNO particles with negligible amount of Mg2Si particles are observed for the case of PP700/PL700 specimen. This could be also easily inferred from XRD spectra (**Figure 3**). For instance, the intensity of Mg2Si peaks appears to be stronger in composites processed at temperature of 800°C and weak in composites processed at 700°C. Notice the fact that since SiCNO ceramic phases are amorphous in nature, corresponding peaks have not appeared in diffraction spectra. Most importantly, it is also observed that the peaks of SiO2 and Mg(SiO4) phases are apparent only in the as-cast PL700/ PP700 specimen, however, but such peaks are absent in as-cast PL800/PP800 specimen. It is likely expected that because of lower solubility of Si-atoms in the molten magnesium, the chemical reaction takes place between the magnesium and silicon to form Mg2Si crystal according to the following equation [21]:

$$2\,\mathrm{Mg}\,(l) + \mathrm{Si}\,(s) \to \mathrm{Mg}\_2\mathrm{Si}\,(s) \tag{1}$$

Gibbs free energy values ( ∆*Gf* ) are found to be −63.578 kJ and − 57.926 kJ for the

According to the chemical reaction (1), it can be estimated that the change in

#### **Figure 2.**

*Microstructural evolution of in-situ Mg matrix composites in as-cast condition (a) PP700 composite (b) PP800 composite (c) PL700 composite and (d) PL800 composite [1].*

**Figure 3.** *XRD spectra of pure Mg and in-situ Mg matrix composites [1].*

processing temperatures of 800°C and 700°C, respectively. The higher negative value at ∆*Gf* at 800°C represents that the tendency for Mg2Si formation is increased by increasing the pyrolysis temperature from 700 to 800°C. In other words, it is possible to minimize the formation of Mg2Si phase by reducing the processing temperature from 800 to 700°C during in-situ pyrolysis of the polymer precursor.

Inem et al. [22] reported that there is no extensive direct chemical reaction occurs between Mg and SiC particle to form Mg2Si crystal at 900°C in the SiCp particles reinforced AZ91 Mg-alloy. However, they predicted that SiO2 scale that forms on SiC particle can react with molten Mg to form Mg2Si according to the following chemical reaction;

$$4\text{ Mg}(l) + \text{SiO}\_2(s) \rightarrow 2\text{MgO}(s) + \text{Mg}\_2\text{Si}(s) \tag{2}$$

This data indicates that some amount of SiO2 scale must be always present in order to produce any Mg2Si phase in the composite. As it can be seen in **Figure 3**, XRD data shows the presence of SiO2 peaks only in the PL700 specimen but not in the as-cast PL800 specimen. This means that the PL800 specimen consumes SiO2 phase completely to form Mg2Si crystal whereas some free SiO2 is left behind in the PL700 specimen due to lack of formation of Mg2Si particles.

According to constitutional supercooling theory, the ratio of temperature gradient (G) to growth rate (R) determines the grain morphology of the final castings during solidification [23]. This theory predicts that the microstructures can be changed from cellular to columnar dendritic, and then to equiaxed dendritic morphology if the solidification condition possess low G/R ratio [23]. It is worthwhile to mention the fact that constitutional supercooling theory can also be applied to the solidification of metal matrix composites if the tip of the solidification front contains certain level of solute impurities. Kim et al. [24] pointed out that any change in the direction of heat flux resulting in different microstructures during solidification of the metal matrix composites. They explained that if the direction of heat flux is same to that of crystal growth, then equiaxial dendritic
