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

containing entropy stabilized alloy systems. Increase in Mg content enhances the complexity of the microstructures as per the **Figure 5(b)-(e)** [32].

Another light weight Mg-HEA, Al60Cu10Fe10Cr5Mn5Ni5Mg5, fabricated using vacuum induction melting followed by die casting, exhibits three different phases (1) Al3Fe4, (2) Al7Cu4Ni and (3) Mg2Cu6Al5 in the SEM image (**Figure 6**) [34].

Tun et. al. fabricated Mg80Al5Cu5Mn5Zn5 HEA through disintegrated melt deposition followed by extrusion: The alloy showed two IM and one SS phase in the microstructure [53]. The microstructure contained Al6Mg and Al2CuMg intermetallics and an HCP phase containing Mg, Mn and Al. The microstructure of Mg80Al5Cu5Mn5Zn5 HEA is shown in **Figure 7(a)**. Phases mentioned as 1, 2 and, 3 are the HCP phase, Al6Mg and Al2CuMg phases respectively.

The density of any high entropy system is highly dependent on the elements present in the alloy. Mg and Li system produces extremely light weight alloys while addition of Al increases density but it strengthens the system as well. This increases the specific strength, which makes magnesium based high and medium entropy systems a matter of interest these days. The highest density in any magnesium containing multi-element alloy system is 5.06 g/cc in equiatomic MgMnAlZnCu produced using induction melting followed by cooling in brine, whereas Mg80Al5Cu5Mn5Zn5 exhibits the lowest density of 2.15 g/cc. The other available density data is given in **Table 3**.

The hardness of Mg-HEAs generally decreased with increase in composition of Mg, the maximum hardness with equiatomic composition was found to be 428 HV [32]. MgMnAlZnCu HEA exhibits the highest hardness when processed at higher cooling rate due to formation of Al-Mn icosahedral quasicrystals [31].

Mg-HEAs with a low magnesium content exhibit higher yield strength. The Al60Cu10Fe10Cr5Mn5Ni5Mg5 exhibits a yield strength of 743 MPa, while magnesium rich systems can result in yield stress as low as 211 MPa. The yield strength data

**Figure 6.** *SEM micrographs of* Al60Cu10Fe10Cr5Mn5Ni5Mg5 *(Reprinted with permission from Ref. [34]).*

#### **Figure 7.**

*SEM micrograph of* ð Þa Mg80Al5Cu5Mn5Zn5 *, (b)* Mg35Al33Li15Zn7Ca5Y5 *and (c)* Mg35Al33Li15Zn7Ca5Cu5 *(image (a) adapted from Ref. [53], image (b) and (c) Reprinted with permission from Ref. [33]).*


**Table 3**

*Enthalpy of mixing of elements in a pair.*

testifies for the brittleness caused due to presence of high amount of Mg. The behavior is predominantly because of HCP structure induced by higher percentage of Mg which inhibits dislocation by insufficient number of slip systems. Although by adding Li with 10.3 wt% in Mg the crystal structure changes from HCP to BCC. The properties of Mg-HEAs are better than conventional Magnesium alloys. The reason behind this lies in the formation of a complex mixture of intermetallics and solid-solution phases in the system according to a study on Mg80Al5Cu5Mn5Zn5 by Tun et. al. [53].

It can be inferred from existing literature, Mg-HEAs synthesized by MA can find applications in structural as well as hydrogen storage systems [39, 41, 54–57] although more extensive research is required to find a better alloy and processing techniques; whereas induction melting in different atmospheres and casting process were used for alloys to be used in load bearing components. Mechanical properties are available for a few alloys which makes it difficult for authors to give a comprehensive analysis. MgMoNbFeTi2 and Y doped MgMoNbFeTi2 were synthesised using mechanical alloying followed by laser cladding. Increase in Yttria content resulted in enhanced mechanical properties of the alloy [36]. It is well established that for enhancement of mechanical properties, Mg alloys are doped with reactive elements like Re, Y and Hf.

**Table 4** represents the manufacturing route for various Mg containing HEAs, the phases present in corresponding alloys, the presence of intermetallic phases and the physical and mechanical properties of the alloys. It is evident from the data that the presence of intermetallics enhance the mechanical properties of the alloys. The increased amount of Mg reduces the alloy density and makes it a suitable candidate for aerospace applications. Further researches on Mg-HEAs need to aim on creating light weight and strong alloys at the same time, this could be done by *thermomechanical processing*, which is not yet explored for Mg-HEAs.

