**Table 7.**

*Types of phase based on the values of thermodynamic parameters, Gibbs free energy ΔG\_mix, mixing enthalpy ΔH\_mix and mixing Entropy ΔS\_mix [60].*

there is high possibility of ordering within a high entropy solid solution, as also said above [35]. Simple solid solution of HEA could be obtained between 0.041≤*δ*≤ 0*:*24. Solid solution formation is also depending on the values of Valence Electron Concentration (VEC), which are being discussed in section 10.

**Figure 10** shows the effect of VEC on solid solution formation in LHEAs. VEC is the total number of free electrons including the d-orbital electrons that can participate in the formation of chemical bond. Generally, VEC also helps to understand the type of phase formation in the alloy. FCC phases are found to be stable at VEC 8, and BCC phases are stable at VEC<6.87. Combination of BCC + FCC are stable for the value of VEC between 6.87 *<* VEC < 8.

**Figures 9b** and **10a-c**, shows the effect of VEC on solid solution formation with respect to **Δ***Hmix*, *χ* and **<sup>Δ</sup>***Smix <sup>R</sup>* . All of the three graphs shows exact same values for solid solution, Intermetallic compound and combination of solid solution and intermetallic compound formation. Solid solution is observed between 2.7< VEC<6.5; Intermetallic compounds are observed between 1.92<VEC<4.35 and combination of solid solution and intermetallic compounds are observed between 2.8<VEC<8.1.

All the parameters can be understood as the pure ss of LHEA will form for the following parameters.


If an alloy follows these certain condition, we can obtain the single phase LHEAs.

It is observed that alloys with Ca have a higher tendency to form IM, similar result have been observed by Nagase et. al. The melting point (MP) of constituting elements also plays a significant role, as a higher difference in MP leads to segregation upon cooling. It should also be accepted that formation of IM cannot avoided in Complex concentrated systems as there is always a possibility of two random elements having higher negative enthalpy of mixing. It is important to note that till date no Mg containing HEA has shown LPSO. LPSO containing Mg alloys if possible could have exhibited better properties and stability. VEC rule is

well-established in HEAs, but its drawback is that it does not discuss about lattice other than BCC and FCC, in the case of Mg HEAs, most of the lower density alloys crystallizes in HCP.
