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Page 2 of 45 Mooraj et al. J Mater Inf 2023;3:4 https://dx.doi.org/10.20517/jmi.2022.41

INTRODUCTION AND MOTIVATION
Throughout history, metallurgists have altered the properties and compositions of alloys to achieve higher-
performance materials. Traditional alloy design strategies involved microalloying trace elements into a
primary base element, resulting in the discovery of many valuable alloys such as Cu-based bronze, Fe-based
steels, and Ni-based superalloys. Over time the increasing demand for high-performance materials has led
[1]
to increasingly complex alloys . This trend has peaked in the past 20 years with the introduction of multi-
[2]
principal element alloys or high-entropy alloys (HEAs) . Unlike traditional alloys, HEAs do not contain a
single primary element; instead, multiple elements in the alloy are mixed in relatively similar (almost
equiatomic) concentrations. Cantor and Yeh first popularized this new alloying strategy concept in 2004
when they independently published works describing the manufacture and design philosophy of this new
[3,4]
class of alloys . Since the publication of these two works, the field of HEAs has exploded as such a new
[5]
alloy design paradigm opens up a vast compositional space that was previously unexplored . Although
some fundamental questions such as phase selection and diffusion kinetics in HEAs remain elusive, many
[6-8] [9] [10-12]
HEAs have shown high strength , large ductility , exceptional hardness and wear resistance , and
[13]
superior corrosion resistance .
Despite the great potential that HEAs present for researchers, some crucial challenges must be overcome to
increase their viability for future applications. While HEAs open up an uncharted multicomponent
compositional space for material design, the vast compositional space makes it impractical to explore via
[14]
traditional metallurgical techniques . Additionally, the cost of HEAs can vary wildly due to the variety of
[15]
possible elemental combinations. Some alloy systems only contain cheap transition metals (Fe, Ni, Cr)
[16]
that may be easy to scale, while other systems contain refractory elements (W, Nb, Ta) , which can
significantly raise the cost of material. Finally, processing history significantly affects the microstructure and
material properties even for a given nominal alloy composition. Many processing conditions including
temperature, cooling rate, mechanical deformation, and irradiation can play a significant role in the
formation of constituent phases and microstructures in HEAs [17-20] . Hence, processing imposes an additional
and orthogonal dimension that multiplies with the huge compositional dimension and makes it more
difficult to efficiently identify high-performance alloys using conventional alloy development strategies [21-23] .
Thus, it is paramount for researchers to utilize efficient workflow to minimize the cost and experimental
trials to study HEAs.

Over the past decade, many high-throughput material development techniques have emerged to tackle the
combinatorial nature of HEAs. These techniques include magnetron sputtering, diffusion multiples, and
additive manufacturing. Magnetron sputtering uses a magnetically confined plasma to accelerate positively
charged ions toward a target material, leading to the sputtering of the target atoms onto a substrate to form
[24]
a thin film with a thickness ranging from a few nanometers to a few microns . A combinatorial materials
library can be built by sputtering multiple elemental targets onto a single substrate [24-29] . The diffusion
multiples method involves arranging different metals such that they are physically touching. Then this
configuration is heated to an elevated temperature that enables atomic diffusion across the interfaces
between the different metals. This process leads to a compositional gradient near the interface that serves as
[30-34]
a compositional library . Despite the large compositional space that diffusion multiples and magnetron
sputtering can achieve, these approaches encounter some difficult issues. Both techniques involve samples
of microscopic length scales, and thus, the microstructures and material properties observed from these
libraries may not be representative of these materials at bulk scales. In addition, magnetron sputtering
10
involves extremely high cooling rates on the order of 10 K/s, which are substantially higher than those
involved in routine metal manufacturing [35,36] . As such, the phases and microstructures in sputtered thin
films are almost exclusively polymorphic or even amorphous and thus do not represent the microstructures
of bulk materials for most practical applications.