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The four core effects of HEAs

Due to their multi-component composition, HEAs exhibit different basic effect than the other traditional alloys that are based only on one or two elements. Those different effect are called "the four core effects of HEAs" and are behind a lot of the particular microstructure and properties of HEAs. The four core effects are high-entropy , severe lattice distortion, sluggish diffusion, and cocktail effects.

High entropy effect

The high entropy effect is the most important effect because it can enhance the formation of solid solutions and makes the microstructure much simpler than expected. The prior knowledge expected to multi component alloys to have many different interactions among elements and thus form many different kinds of binary, ternary, and quaternary compounds and/or segregated phases. Thus, such alloys would possess complicated structure brittle by nature. This expectation in fact neglects the effect of high entropy effect. Indeed, according to the Second law of thermodynamics, the state having the lowest mixing free energy $\Delta G_{mix}=\Delta H_{mix}-T\Delta S_{mix}$ among all possible states would be the equilibrium state. Elemental phases based on one major element have small $\Delta H_{mix}$ and $\Delta S_{mix}$, and compound phases have large $\Delta H_{mix}$ but small $\Delta S_{mix}$; on the other hand, solid-solution phases containing multiple elements have medium $\Delta H_{mix}$ and high $\Delta S_{mix}$. As a result, solid-solution phases become highly competitive for equilibrium state and more stable especially at high temperatures.

Severe lattice distortion effect

Because solid solution phases with multi-principal elements are usually found in HEAs, the conventional crystal structure concept is thus extended from a one or two element basis to a multi-element basis. every atom is surrounded by different kinds of atoms and thus suffers lattice strain and stress mainly due to atomic size difference. Besides the atomic size difference, both different bonding energy and crystal structure tendency among constituent elements are also believed to cause even higher lattice distortion because non-symmetrical bindings and electronic structure exist between an atom and its first neighbours. This distortion is believed to be the source of some of the mechanical, thermal, electrical, optical, and chemical behaviour of HEAs. Thus, overall lattice distortion would be more severe than that in traditional alloys in which most matrix atoms (or solvent atoms) have the same kind of atoms as their surroundings.

Sluggish diffusion effect

As explained in the last section, an HEA mainly contains random solid solution and/or ordered solid solution. Their matrices could be regarded as whole-solute matrices. In HEAs, those whole-solute matrices diffusion vacancy are surrounded by different element atoms, and thus have a specific lattice potential energy (LPE). This large fluctuation of LPE between lattice sites leads to low-LPE sites can serve as traps and hinder atomic diffusion. This leads to the sluggish diffusion effect.

Cocktail effect

Cocktail effect is used to emphasise the enhancement of the properties by at least five major elements. Because HEAs might have one or more phases, the whole properties are from the overall contribution of the constituent phases. Besides, each phase is solid solution and can be viewed as a composite with properties coming not only from the basic properties of constituant by the mixture rule but also from the interactions among all the constituant and from the severe lattice distortion. The Cocktail effect take into account the effect from the atomic-scale multicomponent phases and and from the multi phases composite at the micro scale.