It depends. The explanation will vary for different constituent elements and phases involved. For example, in Al-containing high entropy alloys, Al will tend to decrease its free energy by forming more thermodynamically stable Al2O3 (due to its high and negative enthalpy of formation), in which Oxygen diffusion is very low that improves oxidation resistance. A similar explanation can be applied for Cr an Y containing HEAs. As regarding the oxidation resistance of solid solution versus intermetallics, one among possible explanations can be the initial stability of the phase and affinity of constituent elements to oxygen.
I have a rejection to call any mixer whether it is terminal solids solution or intermediate phase as a high entropy mixer using the terminology mostly prefers by chemist but not any one deals with the thermodynamics of multicomponent system.
The maximum entropy criterion can be used only for the stability considerations for those closed systems where change is taking place either under the constant volume and energy or the constant pressure and enthalpy. (SEE: E. A. Guggenheim)
Otherwise, the stability considerations under the isothermal changes for closed systems should be defined in terms of Helmholtz or Gibbs free energies depending upon whether the system is isochoric (no external surface and body forces are applied) or isobaric (under the constant surface traction and body forces).
In the first case: DelF= DelU - T DelS and
the second case: Del G= DelH -T DelS,
where Del operator refers the specific formation quantities with respect to the standard states of constituents species in their elementary forms.
The direction of the phase transition under the isothermal condition is always towards to those states which have lower Gibbs or Helmholtz free energies depending upon whether one has isobaric (Del G Less Then Zero) or isochoric( DelF Less Then Zero) systems. For isobaric systems: Del G = 0 and Del^2 G GRT Zero is implies absolute stability, where Del is the first order variance operator.
Thermodynamically if the free energy for oxides is lesser then other state, oxide formation is preffered. but without understanding the kinetics it is difficult to come at any conclusion.
the diffusion reaction for oxidation need to occur to form oxides. also one has to check whether the metal/alloy can show passivity or not. because if it is passive it will definitely show lower free energy for oxidation but the kinetic aspects does not let it to oxidize.
Dr. Saha and others what they mean by saying "high entropy intermetallic alloys" is that their Gibbs free energy of formations are less than the corresponding terminal solid solutions by assuming that the configurational enthalpy of mixing stays invariant. ( DEl G= DELH - T DELS)
On the other hand high entropy indicates that those intermetallic alloys are highly disordered like random solid solutions or metallic glasses. Actually most of the intermetallic alloys have long range ordered structures at room temperatures such as beta brass and they do have high oxidation resistance compared to alpha brass (copper rich terminal solid solution). One shouldn't forget oxidation is an electron transfer reaction where the metallic substance plays the role of donor and oxigence as an acceptor.
There may a tentative explanation of poor oxidation resistance of terminal solid solution of metallic alloys if one considers the fact that oxidation phenomena is closely related to the electron transfer from metallic species to the atomic oxygen, which has roughly two electron holes in its outer orbitals (acceptor). Since intermetallic compounds are usually stochiometric (OR they show very limited deviations from stoichiometry) dissimilar metallic constituents are bond together by ionic bonds which are fully saturated therefore electron transfer from those donor species to the atomic oxygen would be very improbable (i.e., high oxidation resıstance). On the other hand in terminal solutions one has metallic bonding having unbound electrons in the conduction band mostly localized at the surface layer, which
are responsible for oxidation process!!!
I suppose, that, first of all, the existence of the mentioned phenomenon should be established by the appropriate experimental results. If indeed the oxidation resistance of solid solution of certain element in another is lower than that of the intermetallic compound with similar chemical composition? I am not sure. But if so, classical calculations of Gibbs free energy of the participating phases (pure metals vs oxides) should be done to conclude something. In principle, a formation of oxide is a result of oxygen diffusion into a metallic solid solution. Basically, if this diffusion will be reactive on not, depends on strength of "impurity" atom bonding in metallic crystal vs its covalent bonding with oxygen (as was explained in the previous answers). It seems that a covalent bonding is a stronger one. But, on the other hand, specific "impurity" metallic atoms' concentration in certain volume should achieved for an oxide formation. Therefore, if self-diffusivity in the discussed HEA is quite low, oxide form will not be able to form. On the other hand, is we are speaking about an equimolar multi-component alloy, there are no "impurity" metallic atoms may be suggested. In this case, only the element with the highest affinity to oxygen may form an oxide. Even in this case, a concentration of this element in certain volume should be high enough, so a self-diffusivity remains important. Since it low, oxidation may be completely suppressed. Anyway, I think, any attempts to generalize are problematic. Each case should be discussed separately.
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