**9. Design, phase prediction and future of Mg HEA**

This section gives an elementary guideline for design, phase prediction and future trends of magnesium containing HEAs. Before doing that it is more important to focus on possible applications of such alloys e.g. die casting, paneling of aircraft, weight reduction in automobiles and biomedical implants. Magnesium components are used in automobiles as instrument-panel beam, transfer case, steering components, air bag housing, seat tanks, fuel tank cover and radiator support. Typically Mg constitutes 4 kgs of normal cars weight which is fairly less. Mg can absorb 16 time more vibrations compared to Al, hence alloys of Mg can be used for shock absorbing applications. The major reason for limited use of Magnesium alloys is low strength compared to Aluminum alloys and Steels. The concept of maximization of entropy via mixing multiple elements in near equiatomic ratios to creating "base" or solvent less alloy. Till date, various HEAs have proven their strength. The implementation of this system to Mg could be breakthrough for automotive, space, missiles and aircraft industry. To do so authors provide a preliminary scientific approach to develop Mg HEAs.

Mg has high solubility with Li ( 17 at. %), Al (12 at. %), In (19 at. %) and it is completely soluble with Cadmium. Although Li and In are soft metals like Mg. It has very high tendency to form intermetallic compounds with metals such as Al, Zn, Cu, Y, Zr and Ca etc. It forms the famous quasicrystals when alloyed with Mn. Mg finds place in a wide spectrum of alloys, compounds and systems. Perhaps it is most interesting element, yet to be studied thoroughly in the complex concentrated systems or HEAs. The short range order in HEAs is recently reported to be a core effect for strengthening in such alloys. Mg provides a great avenue for tailoring heterogeneities in HEAs. In the light of limited data of Mg containing HEAs with only 35 compositions, it is hard for authors to suggest the role of thermodynamic and kinetic criteria to develop Mg-HEAs. There is an insufficient data to develop an analogy on the phase development and phase evolution of Mg containing HEAs.

It has been understood that critical values of theoretical parameters used for conventional heavy HEAs are not sufficient and effective in case of LHEAs containing especially Mg owing to its larger size, high |ΔHmix| with most of elements leading to immiscibility (ΔHmix is +ve) and intermetallic formation (ΔHmix is –ve). Yang et. al. proposed new limits to the critical value for light weight HEAs. The modified threshold values suggest that SS will form at ΔHmix ∈ [-1, 5] kJ/mol; δ < 4.5% and Ω > 10. Mg amongst all the elements in the reported alloys has higher radius due to which poly-disparity constant "δ" value is higher and hence does not support complete SS formation. It is evident that Ω > 10 is an ideal condition for SS formation but in various cases a pure SS is obtained when Ω < 10, which is an unknown at present and shall be governed by the effect of individual elements for instance; in few of such alloys Mg was alloyed with refractory elements such as Mo and Nb and in this case, δ had a high value. In few alloys the effect of high configurational entropy of mixing is overshadowed by high negative enthalpy of mixing and high atomic mismatch. HEAs containing Mg, Li and Al open a new

avenue for scientific research and new outlook for understanding of such complex systems. It is simply understood that if an alloy contain more elements with HCP crystal structure (Mg, Ti, Sc, Co, Zn, Cd, Zr and Y), it should probably result in HCP structure of alloy. This is evident from the authors analysis. It can be concluded that for designing HEAs with Mg, Li and Al, ΔHmix should be given priority over entropy of mixing. It is crucial to study the possible binary and ternary combinations out of the sought HEA composition.

For the design of LHEAs, Mg alone should not be essentially alloyed with Al and/ or Li. At the same time elements soluble with Al or Li can be used to further increase the disordered structure. As it has been also observed that the presence of d orbital element is necessary to obtain high entropy effect in the alloy [35]. To understand the phases, pseudo binary phase diagrams can also be produced using Thermo-Calc, molecular dynamics simulations or extrapolating the experimental data. Due to lack of experimental data, the extrapolation may not be accurate. Phase formation and phase stabilization can be understood by studying the thermodynamic parameters. Few graphs have been plotted shown in **Figures 8**–**10** using the basic thermodynamic parameters such as **Δ***Hmix*, **Δ***Smix*, *δ*,*χ*, **Ω** and VEC from the data shown in **Table 4**, which are considered as the phase formation parameters. Values of these parameters can be calculated from the equations given in the **Table 6** below.

**Table 7** shows the type of compound formation based on the values of **Δ***Hmix and* **Δ***Smix*. Larger negative values of mixing enthalpy may lead to the compound formation. For solid solution formation, higher negative values of entropy is required. Positive values of mixing enthalpy and mixing entropy may lead to the elemental or compound segregation within alloys.

**Figure 8** shows the graph between the **Δ***Hmix* and **<sup>Δ</sup>***Smix <sup>R</sup>* , where R is the Gas constant of the data shown in **Table 5**. Graph shows that pure solid solution is stable at entropy higher than 1.13 kJ/mol. IM are observed in both low entropies and higher entropy region as well. Pure solid solution is observed at medium negative values of **Δ***Hmix:*One exceptional is when solid solution has been obtained at higher

**Figure 8.** *Graph between enthalpy and entropy from the data shown in Table 4.*

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

#### **Figure 9.**

*(a) Graph between atomic mismatch factor and VEC; (b) graph between the atomic mismatch factor and omega; (c) graph between atomic mismatch factor and VEC. Data has been taken from Table 5.*

negative value of **Δ***Hmix*. That is due to the presence of Ca in the alloy. Mg tends to form stable compounds with the Ca: Presence of Ca enhances the solid solution formation in the presence of Mg.

Effect of *δ* is need to be understood properly. As per the Hume Rothery solid solution guidelines, the atomic mismatch factor must be minimum (*δ* ≤15%). The atomic size of light weight elements, Al, Mg, In, Li, Ca is higher as compared to the d block and other elements, due to which there is higher tendency of phase separation. **Figure 9a-c** shows effect of *δ* in solid solution formation. The data is used from **Table 5** to plot the graphs against **<sup>Δ</sup>***Smix <sup>R</sup>* and VEC. **Figure 9a** shows that the pure solid solution are observed only between 0.043≤ *δ*≤ 0*:*24. Pure intermetallic compounds are observed between 0.067 ≤*δ*≤0*:*133, and combination of intermetallic and solid solution is observed for rest of the regions as solid solutions are stable only at higher values of entropy, and medium atomic mismatch factor.

**Figure 9b** shows the effect of *δ* with the **Ω**; Ideally, for solid solution formation the value of **Ω** should be near 1.1 [60], but solid solutions are also observed at higher values. **Figure 9b** shows that solid solution forms within 0.41 ≤*δ*≤0*:*24 and intermetallic compounds are observed at medium values of 0.066≤ *δ*≤ 0*:*133*:* Combination of solid solution and intermetallic compounds are observed between 0.037≤ *δ*≤ 0*:*25*:* Similarly **Figure 9c** shows that solid solutions are stable between 0.043 ≤*δ*≤0*:*24, longer range of atomic mismatch factor, and intermetallic compounds are observed between 0.067 ≤*δ*≤0*:*13. Combination of solid solution and intermetallic compound is observed at all other regions. These graphs show that

#### **Figure 10.**

*(a) Graph between VEC and enthalpy; (b) graph between VEC and electro-negativity; (c) graph between VEC and entropy. Data has been taken from Table 5.*


**Table 6.**

*List of equations for calculation of thermodynamic parameters.*

