**4.1 Microstructural characteristics**

**Figure 8** shows the microstructural evolution of as-cast PP900-AZ specimen. It can be evident that the intermetallics of β-Mg12Al17 phase are segregated at the grain boundaries as seen in **Figure 9(a)**. Most interestingly, majority of the SiCNO particles (black color) are entrapped within discontinuous network of β-Mg17Al12 phase at the vicinity of grain boundaries of PP900-AZ specimen as shown in **Figure 9(b)**. **Figure 9(d)** represents EDS spectra showing the presence of Si, C, and O atoms along with Mg and Al peaks. Microstructural analysis reveals no signature of Mg2Si particles in the PP900-AZ specimen fabricated at 900°C. This is because of the fact that the existence of large cluster of Al-atoms (of about 9 wt%) in the molten slurry leads to maximize the probability of interrupting the diffusion path for Si-atoms to form any Mg2Si crystals on heterogeneous substrates of SiCNO particles. This explanation is justifiable due to slower inter-diffusion rate of Al-atoms in the Mg solution as reported by Brennan et al. [1, 26]. However, Sachin et al. [27] observed the formation of in-situ Mg2Si ceramic phase in the ultrasonic agitated molten AZ91 Mg-alloy after the addition of Si particles. It should be kept in mind that the polymer precursor approach does not involve any ultrasonic assisted vibration treatment of the molten Mg-alloy [1]. Yang et al. [28] mentioned that an ultrasonic vibration

### **Figure 9.**

*Microstructural evolution of AZ91 matrix composites (a)* β*-Mg12Al17 intermetallics at the grain boundaries (b) and (c) encapsulation of SiCNO particles within the* β*-Mg12Al17 phase, and (d) EDS spectrum of polymer derived ceramic (SiCNO) particles [1].*

can produce transient micro "hot spots" that can take temperature of about 5000°C and pressure above 1000 atmospheres in the melt [1]. Such a drastic variation in temperature–pressure accelerates the reaction kinetics of Mg2Si formation as explained by Sachin et al. [27]. In addition, Sachin et al. [27] introduced the native powder of Si particles into the Mg-alloy melt which results in intimate physical contact between Si particles and the Mg melt. However, Si-atoms are introduced in the form of cross-linked polymer in the polymer precursor approach [1].

During solidification, molten Mg-alloy can be first transformed into primary α-Mg and β-Mg17Al12 phases in accordance with phase diagram. The primary α-Mg phase has limited amount of solubility with Al-atoms depending up on the temperature (maximum solubility of 11.8 at% Al-atoms at 437°C to 1 at% at room temperature). Therefore, Al-atoms have a greater tendency to push away any SiCNO particles to the grain boundaries which eventually leading to particle entrapment by β-Mg12Al17 phase. Hashim et al. [29] pointed out that grain boundary segregation of SiC particles occurs in Al-based MMCs owing to poor wettability between Al melt and SiC particles. Despite the fact that SiC and SiCNO particles are chemically distinct, it is justifiable to consider both of them as equivalent in terms of wettability properties with Al-atoms. The formation of Mg2Si crystals was suppressed as most of the SiCNO particles are entrapped by β-Mg17Al12 phase. Under this situation, diffusion of Si-atoms from SiCNO ceramic phase could not take place across the domains of supersaturated α-Mg and β-Mg17Al12 phases during solidification. Therefore, the probability of forming Mg2Si crystal within PP900-AZ composite can be ruled out completely [1].

**Figure 10** represents the microstructural characteristics of as-cast PP900-AE specimen. As shown in **Figure 10**, it can be observed that Mg2Si crystals exhibit dendritic morphology (average particle size of 50–100 μm) along with dispersion of AlxREy intermetallics (acicular shaped gray color particles) in the matrix. The particle size of the fewer AlxREy precipitates are appeared to much finer in size

#### **Figure 10.**

*Microstructural evolution of AE44 matrix composites (a) dispersion of coarsened Mg2Si crystals and AlxREy intermetallics (b) dendritic morphology of Mg2Si crystal (c) dispersion of fine-sized AlxREy intermetallics and (d) uniform dispersion of fine-sized SiCNO particles [1].*

*Solidification Processing of Magnesium Based In-Situ Metal Matrix Composites by Precursor… DOI: http://dx.doi.org/10.5772/intechopen.94305*

as indicated in **Figure 10(c)**. It can be seen that SiCNO particles (a width of 100 to 200 nm and a length of 0.5 to 1 μm) are distributed homogenously throughout the matrix (**Figure 10(d)**). During in-situ pyrolysis, the chance of forming Mg2Si crystals seems to be limited again for the same reasons mentioned earlier for PP900-AZ composite [1]. However, the surrounding medium for nucleating Mg2Si crystals is completely different during solidification [1]. In AE Series Mg-alloy, the liquid phase was converted in to primary α-Mg phase and AlxREy phase. Most of the Al-atoms are expected to chemically bond with RE elements in the molten Mg-alloys to form an acicular AlxREy precipitates. Moreover, Chen et al. [30] found that addition of RE elements have positive effects on the nucleation of Mg2Si crystals for the case of Al-Mg-Si alloys. Therefore, the most preferential sites for nucleating Mg2Si crystals could be the AlxREy precipitates in AE-44 Mg-alloy [1]. Once Mg2Si crystal is nucleated, and it persists its growth along the preferential growth direction **<100>** to form an equilibrium crystal shape of octahedron morphology. However, the morphology of Mg2Si crystal changes from octahedral to dendritic shaped crystal because of SiCNO particles in the surrounding medium (**Figure 10(b)**) which may impose space constraint for this equilibrium growth direction (**Figure 10(c)**).
