**4. Why Mg HEAs**

Since the inception, there have been several doubts, questions and strong arguments against HEAs. Most of them are related to scalability and process engineering in industries, repeatability, reliability, applications and high density of HEAs. Among all these drawbacks, process engineering of HEAs is least studied, while the major problem are repeatability and their high density. To obtain a high level of repeatability, an extensive standard of alloy preparation and their further processing must be established throughout the world. The problem of high density is being solved with the development of light weight HEAs (LHEA) containing low density elements such as Al, Ti, Li and Mg. LHEAs consisting of Mg and Li are

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

found to be lightest [25]. Mg based alloys have several applications in aircraft and automobile panels, bio-implants and energy storage. Mg alloys show exceptional and beautiful microstructures with Long Period Stacking Order (LPSO) phases. LPSO phases are most important feature of few Mg based alloys systems Mg-TM-RE (TM: transition metal, RE: rare earth metal) alloy as it enhances the mechanical properties at room and elevated temperatures [26]. *LPSO phases are not yet reported in Mg-HEAs and could be one of the most interesting breakthrough in Light-weight High Entropy Alloys (LHEAs)*.

There are several other factors which makes Mg-HEAs a topic of interest for example, Magnesium alloy anodes tends to increase the efficiency of Mg-ion batteries which may replace Li-ion batteries in future; Mg is a fast biodegradable and bio-compatible material. Magnesium alloys do not possess enough strength compared to the bone tissue. Mg containing HEAs could be the answer to this problem. Lynette W. Cheah found that for every 10 % mass reduction in vehicle, fuel consumption may reduce by 7 % [27]. If the parts of vehicle are made of strong alloys containing Mg and/or Al instead of Iron, the weight reduction will be 45 and 29 % respectively. This will significantly lower the carbon emission and save fossil fuel. The disadvantage with high reactivity of Mg and low strength of its alloys can be avoided by the virtue of introducing severe lattice distortion, high entropy effect and cocktail effect. This means that Mg must be alloyed with three or more elements to increase the configurational entropy of alloy to attain higher stability and strength.

Clearly, Mg containing HEAs are materials for future.

Alloying has a positive impact on the properties of Mg, and it has been proven that Al addition increases hardness, strength and castability without significantly affecting the density [28]. Ca enhances the thermo-mechanical properties, increases creep resistance and refines the grains. Nd and Ni both increase the strength of Mg when added separately. Cu enhances mechanical properties and aid thermal stability. Ce addition improves corrosion resistance; Mn increases saltwater corrosion resistance in Mg-Al alloys; Zn increases corrosion resistance in Mg-Ni-Fe alloys; Sn prevents cracks during Mg-Al alloy processing and Sr increases creep resistance [28, 29]. Every element has a unique effect on alloy, as shown in **Figure 2** and hence, HEAs must be exploited to establish a synergy between different alloying elements in Mg to produce an alloy with high strength to weigh ratio. In **Figure 2**,

**Figure 2.** *Development of magnesium alloys.*

Mg containing HEAs may have property in a combination of all (cocktail effect). Most Mg containing HEAs show light weight and moderate strength which is described in the mechanical properties section 6. A combination of the light weight achieved due to Mg being one of the base metals and the superior properties of the other principal elements makes HEAs special.

**Figure 3** shows graph of tensile yield strength versus elongation of a range of Mg-Al, Mg-Zn, Mg-Zn-RE, Mg-Gd-RE alloys and Mg containing HEAs [30–35]. The available data on Mg containing HEAs are used in this plot and the other Mg alloy range is obtained from a study by Sankaran and coworkers [30]. This diagram shows the wide range of tensile strength and elongation depending on the compositions of alloys, which not only contributes to the materials property but also adds new possible alloys for a wide range of applications.
