# Applications of Magnesium and Its Alloys: A Review
## 3 Magnesium and Its Alloys as an Engineering Material  
### Engineering Properties  
Magnesium is the lightest engineering metal. Pure magnesium has a low density of 1.74 g/cm\({}^{3}\)[31], and it exhibits a higher strength-to-weight ratio and better ductility and castability than aluminum and steel [32]. Compared to other metals and polymeric materials, magnesium presents no toxicity hazard [3] and performs better in thermal and electrical conductivity, vibration and shock absorption, and damping capacity [33]. Magnesium also possesses good machinability and can be shaped with any established method [3]. However, its notable weakness is in its proneness to corrosion. It corrodes quickly under two conditions: (a) when the alloy is constructed with specific metallic impurities or (b) exposure to aggressive electrolyte species. Coating technologies have since been developed and utilized to overcome this weakness, e.g., electrochemical plating, conversion coatings, and anodizing [34]. With a minimum purity of 99.8%, pure magnesium is available commercially [35], and it is suitable for most applications [14]. However, pure magnesium is ideal for metallurgical and chemical uses but not for engineering and structural purposes [31]. Therefore, it must be alloyed for engineering and structural applications to strengthen its weaker properties without sacrificing its key features [8]. Melting and casting magnesium alloys in a vacuum-assisted inert atmosphere is the preferred manufacturing method as magnesium solidifies better than other cast metals. Further, a chemically inactive environment can prevent any contamination from reactive gases [36]. Solid solution strengthening and second phase strengthening are typically used to reinforce magnesium alloys further [35]. The alloying constituents and their chemical compositions can influence the enhanced physical properties of magnesium alloys. Typically, the alloying constituents make up roughly two-thirds of a magnesium alloy. The most common and preferred alloying elements are aluminum and zinc, where aluminum most commonly forms the base of a magnesium alloy [37]. Both aluminum and zinc are economical and highly soluble in magnesium [38]. Aluminum can be used to improve the alloy's strength, hardness, and melting range while tapering its corrosiveness [2]. A range of strength and ductility can be achieved by altering the aluminum content within the alloy [37]. When used together with zinc, it can further enhance the alloy's strength at room temperature. Adding zinc, by itself, can increase alloy fluidity in casting and improves its dimensional stability [2]. When zinc, together with magnesium alloys, is mixed with impurities like nickel and iron, it can strengthen the alloy's resistance to corrosion [31]. Favored magnesium alloys with aluminum include the AZ31 and AZ91 alloys. AZ31 is widely used in the aircraft industry due to its low mass density and good mechanical properties [39]. At the same time, AZ91 alloy remains one of the most popular cast type alloys due to its high strength, excellent corrosion resistance, and good castability [40]. In addition to traditional alloying, there have been, in recent years, significant research progress in magnesium alloy-based nanocomposites, where magnesium and its alloys are further reinforced with nano-sized particles [41]. The characteristics of these magnesium alloy-based nanocomposites are influenced by both the alloying constituents and the reinforced nanoparticles [42]. This domain's heightened research interest stems from the revelation that reinforcements with nanoparticles can improve magnesium's strength and ductility without any adverse effect [41]. While there may be limited commercial applications today, this discovery has posed magnesium alloy-based nanocomposites' to be a candidate for replacing traditionally alloyed magnesium in both engineering and biomedical applications [42].