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Page 4 of 45 Mooraj et al. J Mater Inf 2023;3:4 https://dx.doi.org/10.20517/jmi.2022.41

Figure 1. Schematic illustration of a typical protocol for high-throughput development of HEAs. (A) Selection of elements for a
prospective alloy system; (B) high-throughput computational methods are used to select a range of promising compositions that can be
explored experimentally; (C) high-throughput manufacturing and characterization of the promising compositions selected via
[47]
computation to determine the target composition. The lower illustration in (C) is quoted with permission from Borkar et al. , copyright
2016, Elsevier. HEA: high-entropy alloy.

Figure 2. (A) Schematic illustration of the composition space of conventional alloys, equiatomic HEAs, and non-equiatomic HEAs.
This figure is quoted with permission from Li et al. [49] ; (B) ashby chart of the yield strength vs. fracture toughness of many material
[5]
groups showing high/medium entropy alloys have excellent damage tolerance, adapted from George et al. , copyright 2019, Springer
Nature. HEA: High-entropy alloy.

The second widely accepted definition is based on the mixing entropy of an alloy system, assuming an ideal
random solution state. The mixing entropy is calculated as where R is the ideal gas
th [48]
constant, n is the number of principal elements, and x is the atomic fraction of the i element . Yeh et al.
i
separated the alloy design space into three regimes where a low entropy alloy has ΔS < 0.69R, a medium
[51] mix
entropy alloy has 0.69R < ΔS < 1.61R and a HEA has 1.61R < ΔS . It should be noted that the
mix mix
definition of mixing entropy above includes the assumption that the random solution state is defined as the
liquid state or a high-temperature solid solution state such that the atoms have enough energy to maintain
[51]
completely random configurations . However, it has been pointed out by Miracle et al. that the above
threshold for HEAs would exclude certain non-equiatomic alloys with five principal elements, as
[48]
calculations show that such alloys exhibit ΔS < 1.61R . Miracle et al. also pointed out that others have
mix
suggested using a threshold of 1.36R < ΔS which would include the alloys that were excluded by the
mix
threshold proposed by Yeh et al., making the entropy definition more consistent with the principal element
[48]
definition .