**3.4 Metal matrix composites**

Metals play significant and versatile role extending itself from house hold to engineering application. Metal Matrix Composite (MMC) as the name implies has reinforcement (fibres or particles) surrounded by matrix of metal. The matrix is the monolithic material. The main matrix materials used are aluminium, titanium, magnesium and copper. Reinforcement phase not only carries the structural task but also used to enhance the property like wear resistance, resistance to degradation by fluid and so on. Metal matrix composite finds its wide spread opportunities as it tailors the need to achieve a better property beyond those unattainable by monolithic materials. These composites potentially have high stiffness, strength and high temperature resistance over polymer matrix composites. However metal matrix composites can operate in varied temperatures and has better electrical and thermal conductivity.

In recent years, metal matrix composite materials are gaining more attention due to their greater wear resistance, higher thermal conductivity when compared to unreinforced metals and alloy. Apart from these advantages, Metal matrix composites are environmentally suitable as they do not absorb water like PMC. They use low density materials like aluminium and magnesium as matrix.

## **3.5 Hybrid composites**

Hybrid composite materials are consisting of a matrix and possess two or more reinforcements having different properties. The performance of the hybrid composite material is the joint effect of all the individual constituents. They are usually employed for low density and high strength applications particularly in the field of automotive sector and aerospace sector.

## **3.6 Magnesium based composites**

Magnesium alloys and its composites find a significant place as next generation materials as a substitute for aluminium and steel. Magnesium offer better mechanical and physical properties like high strength, good vibration dampening capacity, better castability, ease of machinability, good recyclability, environmental friendly and above all low density 1.74 g/cc (pure magnesium) than the conventionally used materials in the automotive sectors like iron and aluminium [2]. Pure Magnesium though light in weight but has limited use due to its insufficient strength and corrosion resistance [12]. Alloying magnesium with one or more element like aluminium, zinc, manganese, zirconium, rare earths and so on gave rise to new phase of magnesium alloys which enhanced the properties of pure magnesium. Magnesium alloys not only stand for their better properties but also as environmental friendly material [13].

Among various grades of magnesium alloys, A-Z magnesium alloy are used for structural applications. It has good castability and high specific strength. AZ91 magnesium alloy finds numerous applications in building structural components in aerospace and automotive sector like chassis, wheel, and indoor frame so on [14]. But its negative effect is poor corrosion resistance, low wear resistance in some service environments. It exhibits poor corrosion resistance property in sea salt environment but generally has good corrosion resistance during atmospheric exposure condition [15]. However, its application cannot extend beyond the limit due to some of its negative effects. Reinforcement of AZ91 magnesium alloy with suitable particles can overcome the shortfalls of alloy. Due to growing demand and to meet out the requirements, hybrid composite material is more advantageous than the conventional and composites materials. There are various researches on magnesium metal matrix composite reinforced with SiC, Al2O3, Ti, TiO2, Carbon nanotubes, Graphene, TiC to establish the correlation between mechanical properties and microstructure characteristics of composites, corrosion resistance, wear resistance property [16] etc. It was observed that there were not much research reports on the influence of titanium oxide and graphene and titanium & graphene reinforced with AZ91 magnesium alloy as hybrid composites.

*Experimental Investigation of Mechanical and Wear Behaviour of AZ91 Magnesium Hybrid… DOI: http://dx.doi.org/10.5772/intechopen.104703*
