**3.2 Mechanical properties**

**Figure 6** shows the Vickers hardness data of as-cast in-situ Mg-based P-MMCs along with commercial pure Mg. It was found that the value of hardness of as-cast P-MMCs is enhanced by about 40–75% as compared to that of the base metal. Such enhancement is caused by the reinforcement of PDC particles, and grain size reduction of the as-cast composites. Notice that all hardness measurements are performed in the grain matrix or matrix rich regime of the composites, away from

## **Figure 5.**

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

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

**Figure 6.** *Micro-hardness properties of pure Mg and in-situ Mg matrix composites [1].*

the particles at the grain boundaries but taking care to avoid indentation near the micro-pore area in the as-cast composites in a way to minimize the influence of porosity on the hardness of composites [1]. The hardness data reveals considerable scatter in the values of micro-hardness varying from 45 ± 2 to 56 ± 2 HV. Such a variation arises because of the differences in grain size, volume fraction of in-situ formed PDC particles, and the intensity of grain boundary segregation and formation of Mg2Si phases in the composites fabricated under different processing conditions [1]. Furthermore, the presence of Mg2Si crystals in composites fabricated at 800°C exhibits high hardness as compared to that of the composites fabricated at 700°C. This can be attributed to Taylor strengthening mechanism due to thermal mismatch between the matrix and in-situ formed Mg2Si particles leading to produce significant amount of dislocations at the particle/matrix interface [1].

**Figure 7** shows the typical compression true stress-true strain curves of the as-cast composites fabricated under different processing conditions. As depicted in **Figure 7**, the values of compressive yield stress (CYS) and ultimate compression stress (UCS) of all the as-cast composites are significantly higher when compared to pure magnesium [1]. For instance, it can be noticed that the yield stress for the PP800/PL800 specimen is in the range of 77–90 MPa, whereas 85–88 MPa for the PP800/PP700 specimen, but it is only 60 MPa for pure Mg. Moreover, the values of UCS of PL700/PP800 specimen is in the range of 235–237 MPa, enhanced by 10% and 82% as compared to that of PL800 specimen and pure Mg, respectively [1]. Further, the value of strain to failure of PP800/PL800 specimen experiences the lowest strain to failure (9–10%) when compared to that of PL700 specimen (16–18%) and pure Mg (20–22%). The lowest ductility can be closely associated with the presence of brittle Mg2Si ceramic phase which often impairs the room temperature plasticity of the composites fabricated at 800°C [1].

**Figure 8** illustrates the morphology of the fractured surface for the as-cast PL700 and PL800 composites after compression. As shown in **Figure 8**, while the fracture surface of PL700 composite shows the mixed mode of failure including both ductile and cleavage patterns, PL800 composite exhibits cleavage mode

**Figure 7.** *Compression behavior of pure Mg and in-situ Mg matrix composites at room temperature [1].*

#### **Figure 8.**

*SEM micrographs of the fracture surface of as-cast Mg matrix composites tested at room temperature (a) and (b): PL700 composite and (c) and (d): PL800 composite [1].*

representing brittle fracture. It can be understood clearly that the mechanical properties (CYS, UCS and ductility) of as-cast composites can be improved significantly if one could avoid the formation of brittle Mg2Si ceramic phase in the molten magnesium. This can be achieved by lowing the process temperature from 800 to 700°C during in-situ pyrolysis. Such improvement in mechanical properties of the final composites arises because of two primary reasons namely (i) minimal amount *Solidification Processing of Magnesium Based In-Situ Metal Matrix Composites by Precursor… DOI: http://dx.doi.org/10.5772/intechopen.94305*

of grain boundary segregation of PDC particles tending to provide reasonable strengthening of the magnesium matrix, and (ii) reduction in the amount of Mg2Si crystal within the magnesium matrix leads to cause significant plastic deformation of the composites [1].
