**4. Structural applications**

The magnesium is the third most commonly used structural material after aluminum and steel and is significantly used in automotive, power tools, aerospace, and 3C (computer, communication, and consumer products). Currently, magnesium application in automotive includes transfer case, radiator support, instrument penal beam, and steering components. Magnesium with minimal density is used in mass saving applications to replace aluminum and steel, but magnesium alloys' strength is low. The hard and tough ceramics are added in magnesium alloys using proper manufacturing techniques to improve the strength for structural applications. The extrusion of the metal matrix composites provide strength comparable to aluminum alloys. AZ, ZK and AM series of the magnesium alloys are very common to fabricate the metal matrix composites. The magnesium based metal matrix composites are developed because the wrought magnesium alloys are very less formable at room temperature than aluminum alloys due to hexagonal closed packed crystal structure, although ductility appears reasonable [13].

The materials selection for structural applications is very complex in which component geometries, loading conditions, manufacturing process, materials properties, and the cost is very important. The bending mode is the primary loading condition in automotive structures. Thus, the calculation should be based on bending stiffness and strength. The thickness and mass ratio of magnesium alloy (AZ91) should be 1.67 and 0.39 times the steel to get similar bending strength. AZ91 AZ31 and AZ61 are the most commonly used magnesium alloy in the automobile. The addition of proper reinforcement in AZ series of the magnesium alloys have improved the strength and ductility. AZ61 and AZ80 have higher strength but less extrudability. The higher strength alloy ZK60 is designed for racing cars, bicycle parts, but extrusion speed is very low. The implementation of severe plastic deformation on magnesium alloys have improved the mechanical strength drastically. The new magnesium composites are being developed with a higher extrusion rate and maintaining good mechanical properties such as AM30 (higher strength applications) and ZE20 (higher ductility applications) reinforced with Al2O3, WS2, TiC, and SiC. The presence of aluminum contents in magnesium-based composites improves the strength, hardness, and corrosion resistance but reduces the ductility. The aluminum contents within range of 5–6% yield optimal strength and ductility. Zinc is next to the aluminum alloying element to improve the corrosion resistance but reduces the ductility. Manganese does not affect the tensile strength but increases the yield strength. It is 0.4% recommended by ASTM specification B93-94a to improve the corrosion resistance [14, 15].

The small addition of ceramics (Al2O3, TiC, B4C, SiC, WS2, MoS2 etc.) in magnesium improves the ductility and likely reduces the grain size and weakens the texture. The combined addition of zinc and cerium enhances the strength and ductility closer to aluminum. The addition of lithium, zirconium, and cerium has improved the formability and strength of the composites.
