**1. Introduction**

In-situ metal matrix composites offer superior microstructural/mechanical characteristics compared to their conventional counterparts. In-situ MMCs are characterized by uniform dispersion of fine-sized thermodynamically stable ceramic particles, clean and unoxidized ceramic-metal interfaces having high interfacial strength, improved hardness/yield strength and elastic modulus [1]; In-situ composite fabrication techniques are gaining attraction among the researchers since this techniques overcome several processing related issues such as non-uniformity of particle

distribution, poor wettability and weak interfaces associated with composites produced by conventional processing techniques [2–3]. In-situ processing methods use a chemical reaction between liquid–gas, liquid–liquid, liquid–solid, and mixed salts to generate fine ceramic particles in the metal matrix. For instance, silicon, dissolved in copper, may be converted into a dispersion of silica particles by reacting with oxygen diffusing in from the environment [4]. Enormous data are available in the composite literature regarding the processing of Al-based in-situ composites whereas data on in-situ Mg-based composites seems to be very limited [5–6].

In last two decades, investigators have been collaborating to enhance the high temperature creep resistance of metal matrix composites by utilizing polymer precursor approach [7–19]. A noteworthy feature of this polymer precursor approach is that no chemical reaction between polymer precursor and the host Mg is required to produce in-situ ceramic particles because the polymer contains the constituents of ceramic phases within organic molecules itself [14–15]. The pyrolysis is a highly reactive process, accompanied by the evolution of hydrogen, which disperses the ceramic phase into nanoscale or sub-micron constituents [1]. However, the two critical main issues were identified in earlier work [14–15] as follows: (i) the chemical reaction between polymer precursor and magnesium melt results in the formation of brittle Mg2Si particles at pyrolysis temperature ranging from 800 to 1000°C. These brittle Mg2Si ceramic phase significantly reduce the ductility of fabricated composite. In addition, formation of Mg2Si phase leads to reduction in the amount of polymer precursor available for in-situ generation of sub-micron or nano-sized SiCNO particles, and (ii) most of the PDC particles are pushed away by the solidification front and get segregated at the grain boundaries. Such grain boundary segregation limits any enhancement in the mechanical properties of the final in-situ MMCs. These two critical issues can be mitigated by adopting the following research methodologies [13] namely (i) The intensity of Mg2Si formation can be minimized by lowering the pyrolysis temperature from 800 to 700°C. (ii) The issue of grain boundary segregation can be reduced by adopting friction stir processing (FSP) treatment. The severe plastic deformation within the nugget zone is expected to create fragmentation in the larger SiCNO particles leading to nanosized particles, and stirring action arising during FSP tool aids in achieving the uniform dispersion of particles throughout the Mg matrix [1].

The microstructural design of light weight MMCs for creep resistant applications involves the uniform dispersion of thermally stable nano-scale ceramic particles throughout the grain matrix [1]. The potential for incorporating nano-sized PDC particles into a light weight metal is made possible by polymer precursor approach [1]. The direct injection of liquid polysilazane precursor into molten metal and the subsequent casting process seems to be most advantageous and viable route of producing P-MMCs. If nano-scale particles of PDCs are dispersed uniformly in the magnesium matrix, it can be expected to impede dislocation motion and resist coarsening at high temperatures, thereby holding the promise of a new genre of light weight polymer-derived P-MMCs [1]. Therefore, the present chapter mainly focuses on understanding the correlation between structure-properties of in-situ magnesium matrix composites containing SiCNO particles via polymer precursor approach.

## **2. In-situ polymer precursor approach**

Rishi Raj and M. K. Surappa jointly [14] patented a novel pyrolysis infiltration process (PIP) to fabricate Mg-based P-MMCs using liquid metallurgical route. The PIP process has great generality because different organic precursors for producing oxide, carbides, nitrides, and borides are commercially available as indicated in **Table 1** [1].

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


**Table 1.**

*Types of organic precursors that can serve as a source for ceramics in MMCs [1, 15].*

Also, the process would permit the addition of the desired volume fractions of the ceramic enabling the nanostructural design, and production of P-MMCs with a wide range of mechanical properties, meant especially for high temperature creep resistant applications [1]. When the melting point of a metal lies below 800°C, the Si-based polymer can be added into the molten metal by a stir-casting method. Magnesium and aluminum matrix composites are typical examples for this PIP approach. The conversion of the polymer into the ceramic phase releases hydrogen and small amounts of hydrocarbons. During in-situ processing these gaseous species must migrate to the free surface in a way to avoid any porosity in the final castings. This can be an issue in aluminum composite castings; although it does not appear to constrain the production of magnesium composite castings. Originally, the standard procedure recommended by inventor of this PIP process [14] is to add the cross-linked polymer powder just above the melting point and then superheat the metal to 800°C to convert the organic into the ceramic phase. Later, they realized that direct injection of the liquid polymer is even most simpler [15], and holds greater potential for futuristic development of polymer derived metal matrix composites (P-MMCs).

**Figure 1** depicts the schematic diagram of casting setup used to fabricate in-situ Mg/Mg-alloys based P-MMCs [1]. The systematic procedures for composite fabrication can be described in the following steps; [1].


The designation of these fabricated Mg matrix composites has been indexed as PP700, PP800, PL700, and PL800. Mg-alloys based P-MMCs were designated as PP900-AZ, and PP900-AE which were fabricated using AZ91 and AE44 Mg-alloys as matrix materials, respectively. Here, PP refers to polymer derived composites made using cross-linked powder, and PL indicates polymer derived composites made using as-received liquid ceraset. The last 3 digits refer to process or pyrolysis temperature. In addition, the castings from pure magnesium, and unreinforced Mg-alloys were also fabricated using similar processing parameters.

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