On the formation of disordered solid solutions in multi-component alloys

• The work aims at understanding the basic question: “what constitutes an high entropy alloy?”.
• Role of parameters used for the formation of HEA (ΔRmax, ’δ’, Δχ, etc.) is critically analyzed.
• Focus is on the ranges prescribed for the parameters.
• Attention is paid to standard pitfalls observed (inclusion of B2 compounds under HEA).

In high entropy alloys (HEA) a disordered solid solution (DSS) is entropically stabilized in competition with possible intermetallic compounds or phase separation. Ranges for many parameters (ΔRmax, ‘δ’, ΔHmix, ΔSmix, Ω) have been prescribed in literature for the formation of HEA. In the current investigation, we show that not withstanding the centrality of entropy in the stabilization of a DSS, other factors (parameters) like valence electron concentration, atomic size differences between the elements and enthalpy of mixing, play a significant role in the formation of phases. To illustrate the issues involved in the formation of DSS and to understand the phase formation, selected ternary (CoFeNi, CrFeNi), quaternary (CoCrFeNi, CuCoFeNi, AlCrFeNi) and penternary alloys (AlCuCoFeNi) are chosen for the experimental investigations and for the calculation of different parameters. It is shown that the ranges for the relevant parameters prescribed in literature are not satisfactory. It is possible that a (single) DSS can form, even if the entropy of mixing is small, provided all the relevant parameters are within favorable ranges. Contrastingly enough, violation of any of the parameters results in the formation of multiple phases or compounds. Strict and reduced definitions of HEA are invoked to understand the results.