**3.7 Selection of matrix material: AZ91 magnesium alloy (Mg-9Al-0.8Zn-0.2Mn)**

Constituents of composite materials are matrix and reinforcement. Matrix plays a major role in binding and protecting the reinforcement, distributing the load, enhancing the final property and giving better finish to final product. Matrix is usually aluminium, magnesium and titanium for structural applications.

AZ91 magnesium alloy possesses good mechanical properties. It has less susceptibility to hydrogen porosity and less reactive iron that can be poured in steel mould. Among the various AZ magnesium alloy, AZ91 is conferred as one of the best magnesium alloy suitable for thin walled applications. Usage of AZ91 magnesium alloy instead of aluminium alloy reduced the weight by about 25%, but the geometry and production tools remained the identical [17].

## **3.8 Reinforcement**

Reinforcement can enhance desired properties of the conventional materials. The desired properties depend upon the working environment in which the material is exposed like strength, stiffness, corrosion resistance, wear resistance, reduced weight, dimensional stability, fatigue and extends up to attractiveness and cost. Reinforcements can be classified into two groups: continuous and discontinues. Metal matrix Composites processed by them are termed as continuously reinforced metal matrix composite and discontinuously reinforced composite. They are further sub divided into categories like short fibres, whisker, particulates, continuous fibre and preform/ wire (fibre solid). Some important parameters considered are as follows for the selection of reinforcement [15].


In order to attain the desired properties in (AZ91 magnesium alloy) like improved wear resistance and corrosion resistance, right choice depends upon the types of reinforcement, production method and compatibility with matrix. Most widely used reinforcement in Metal matrix composites is ceramics [18]. Usually ceramics such borides, oxides and carbides are widely used reinforcements.

One of the best advantage of Magnesium/Magnesium alloys based composites over aluminium composites is that the wettability of liquid magnesium with ceramics reinforcement phase is better. Use of titanium reinforcement in Magnesium MMCs gave better results which had the potential to create a new material with excellent mechanical properties, wear and corrosion resistance [19]. Carbon nanotube and graphene are also gaining high interest in the field of materials. Graphene possesses better mechanical, thermal and electrical properties and also light weight with density of 1.06 g/cc. Addition of graphene as reinforcement in magnesium composites has enhanced the strength of metals. Titanium oxide are usually used for coating purpose to enhance corrosion resistance property. Titanium oxide used as reinforcement with magnesium matrix showed improved hardness and strength [20].
