For elemental phase (= terminal solid solution based on one metal element) consisting of one major element, Yeh et al. [1], Murty et al. [2] and Gao et al. [3] proposed a small-negative enthalpy of mixing ΔHmix. Why is that?
[1] Yeh, J.-W. (2013)_ Alloy Design Strategies and Future Trends in High-Entropy Alloys
[2] Murty, B. S., Yeh J.-W., Ranganathan S., Bhattacharjee P. (2014): High-entropy alloys. 1 ed: Elsevier; 2014.
[3] Gao, M. C., Yeh J.-W., Liaw P. K., Zhang Y. J. C. S. I. P. (2016): High-entropy alloys
ΔHmix for elemental phase is small-negative because the enthalpy of mixing of two elements is typically lower than their individual enthalpies. This is because of the formation of new bonds between atoms during the mixing process, which results in a decrease in Gibbs free energy and an overall decrease in total enthalpy.
The ΔHmix of a solution can be either positive or negative (interaction of pairs of components). More soluble solutions are close to ideal. A low negative term can be caused by an imperfect definition of the interaction of components...
Hi
From Ref:
Elemental phases based on one major element have small ∆H mix and ∆S mix, and compound phases have large ∆H mix but small ∆S mix; on the other hand, solid-solution phases containing multiple elements have medium ∆H mix and high ∆S mix.
Weak Interaction(affinity) in Elemental phases is the result for small negative of ∆H mix.
Moderate Affinity between elements in Solid Solutions and more ordering to
have medium ∆H mix and high ∆S mix.
Strong Affinity between elements and little ordering in Compounds to have large ∆H mix but small ∆S mix.
thanks & regards,
g.sudhakar
phd(materials engineering)
hcu.
The answer by Sagayaraj and Sudhakar do give a correct explanation. I also recomend to the followers of this question to read Miedema (papers from 1973). And I also would like to add the following, which is merely saying the same thing as Sagayaraj and Sudhakar, with different words.
The larger the electronegativity difference between the constituents elements the stronger the bond between their atoms, leading to higher
∆H mix (more of the free energy is burried into the bond formation). This also affects the configurational and vibrational entropy terms as well, giving smaller values. A strong enough electronegativity difference would create short range order, especially in dilute solid solutions, not allowing B-B type bonds in the A-B solid solution, which effectively means lower configurational entropy. At the same time, again due to the strong bonds between A-B atoms vibrational entropy becomes smaller as well.
Regards
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