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Page 2 of 21 Zhou et al. J Mater Inf 2022;2:18 https://dx.doi.org/10.20517/jmi.2022.27

for improving the tensile properties of as-printed single-phase HEAs. In contrast, multiple strengthening and
toughening mechanisms occur in as-printed multiphase HEAs, which can synergistically enhance their mechanical
properties. Furthermore, machine learning provides an effective method to design new alloys with the desired
properties and predict the optimal AM parameters for the designed alloys without tedious experiments. The
synergistic combination of machine learning and AM will significantly speed up scientific advances and promote
industrial applications.
Keywords: High-entropy alloys, additive manufacturing, precipitation hardening, strengthening mechanism,
machine learning

INTRODUCTION
High-entropy alloys
High-entropy alloys (HEAs) and multiple-principal element alloys were first proposed by Yeh et al. and
[1,2]
Cantor et al., respectively, in 2004 . Unlike most conventional alloys based on one principal element, this
[3]
concept provides a new criterion for developing new alloys with five or more principal elements . The
concentration of each principal element generally ranges from 5 to 35 at. %. The properties of HEAs can be
modified by changing the type and content of the base elements or by adding some other trace elements.
Therefore, various HEAs have been developed with a broad range of microstructure and properties, such as
single-phase HEAs (face-centered-cubic (FCC), body-centered-cubic (BCC), hexagonal-close-packed
[4-6]
(HCP) and eutectic HEAs (EHEAs) and precipitation-strengthened HEAs . Single-phase FCC HEAs are
[2]
mainly based on the transition elements of Fe, Co, Cr, Ni and Mn . Eutectic HEAs (FCC + BCC) are
[7-9]
developed by alloying with other elements, such as Al, Nb and Cu . Single-phase BCC systems are mainly
based on some refractory elements, such as V, Nb, Mo, Ta, W, Ti and Hf. It has also been reported that
rare-earth elements are strong stabilizers for the formation of HCP-phase HEAs, such as YGdTbDyLu,
[10,11]
GdTbDyTmLu and HoDyYGdTb .

[12]
Compared with traditional alloys, HEAs mainly have the following four unique effects : (a) high-entropy
effect in thermodynamics, which benefits the formation of the single phase; (b) sluggish diffusion effect
kinetics caused by multiple principal elements; (c) severe lattice distortion effect induced by the variation in
atomic diameter and mixing enthalpy; (d) cocktail effect in properties that can be easily optimized by
changing the elements and microstructures. These beneficial effects make HEAs very promising as
engineering and functional materials. As an essential criterion for evaluating their engineering applications,
the tensile properties of HEAs have been widely studied. Figure 1 summarizes the room-temperature tensile
properties of HEAs classified based on their crystal structures. HEAs with a single-phase FCC structure
[13-24]
possess superior tensile ductility but suffer from poor yield strength . To improve the tensile strength of
single-phase FCC HEAs, the hard BCC phase is introduced to the FCC system by adding Al, Cu, Ti, and so
on [9,22,23,25-28] . He et al. illustrated that the BCC phase content increased with increasing Al content, which
resulted in high hardness and tensile strength. However, the ductility was sacrificed when the BCC phase
[22]
exceeded a certain content . Furthermore, some dual-phase HEAs, such as AlCoCrFeNi and
2.1
CoCrFeNiNb , show excellent wear properties even at elevated temperatures [29,30] . Single-phase BCC HEAs
x
usually contain refractory elements, such as Ti, Nb, Mo, Ta and Hf. Like β-Ti alloys, these single-phase BCC
HEAs show almost negligible work hardening capability, resulting in a poor uniform elongation [31-37] .

It is noteworthy that many single-phase HEAs suffer from insufficient strength, especially at elevated
temperatures. Therefore, precipitation-strengthened (PS) HEAs have received extensive attention and
research in recent years. As shown in Figure 1, PS HEAs show superior tensile strength and uniform
[6,38-47]
elongation to single-phase and eutectic HEAs . Precipitation in HEAs can be generally divided into two