**5. Synthesis of Mg HEA**

**Figure 4** shows Al is the highest alloyed element with Mg followed by Cu and Li in HEA system. Mg is a reactive metal with a low melting point. It burns with a shiny white light in air. Considering its high vapour pressure, Mechanical Alloying (MA) would be a suitable process compared to melting and casting for synthesis of alloys containing Mg. MA is a common route for synthesis of Mg containing HEAs [33, 36–42]. Youssef et. al [37] and Ornov et. al [43] were the first to study Mg containing HEAs synthesized by MA. Initial results were promising for the future of Mg-HEAs. AlFeCuCrMgx (x = 0, 0.5, 1, 1.7 mol) was synthesized by MA process to understand the effect of composition of Mg on phases and solid solution formability. It has been found that Mg addition stabilizes BCC structure via dissolution of FCC structure [43]. Cold welding supersedes fracturing in alloys with higher Mg. Lattice distortion was found to increase with Mg % in the alloys [43]. A low density Mg containing HEA with FCC phase transformed to more stable HCP structure on annealing and reported high hardness of 5.8 GPa [37]. Due to continuous cold welding and mechanical impact by the balls the atomic density of the powder increases and hence number of slip systems and ductility increases. So, the milling time must be increased with the increase in Mg composition [43]. Maulik and Kumar confirmed lattice distortion, which increased with further addition of Mg

*Magnesium Containing High Entropy Alloys DOI: http://dx.doi.org/10.5772/intechopen.98557*

[43]. Although, for sintered alloys, lattice distortion first increased and then decreased with Mg content [38]. Equiatomic MgAlSiCrFe HEA was synthesized by MA and was found to be stable at moderate temperatures (300-500°C), also Si did not dissolve completely even after 60 hours of milling [44]. In the pioneer works on Mg containing HEAs, a similar phenomenon could be identified that conventional threshold limits of Δ*Hmix* and *δ* do not support the formation of solid solution but there is still formation of SS [6, 7, 35, 43, 45].

**Figure 4.** *Elements vs number of times alloyed with Mg.*


#### **Table 2** *Physical properties of elements.*

The Δ*Hmix* values in **Table 2** are taken from [46, 47] in which the values have been calculated using Miedema Model [48–52]. A significant portion of the high entropy alloys are produced using the induction melting route [32, 34, 35]. It can be easily inferred from the **Table 2** that there is unavoidable formation of intermetallic and Al-Mn quasicrystals which is majorly because of high negative values of Δ*Hmix* for various binary pairs given in **Table 2**.
