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Page 24 of 45 Mooraj et al. J Mater Inf 2023;3:4 https://dx.doi.org/10.20517/jmi.2022.41

Functionally graded materials
LAM can produce compositionally graded materials, making it a powerful tool for rapid combinatorial
[160]
material exploration . DED is the more common method used to produce graded materials, as the
multiple nozzles can be coaxially aligned with the laser, ensuring that each nozzle’s flow rate can be
individually adjusted to spatially control the deposited alloy composition [162,163] . By dynamically changing the
flow rate during AM, a compositional gradient can be formed, allowing for the exploration of a large
compositional space within the same sample [164,165] . This method is schematically illustrated in Figure 11A,
where a compositionally graded wall is produced starting from pure Cantor alloy at the base and increasing
[166]
the content of refractory metals with increasing the build height . Pegues et al. used this method to add
[166]
Nb, Ta, and Ti6Al4V to CoCrFeMnNi to produce 3 different materials libraries . They explored the effect
of these additions on the microstructure and mechanical properties of the resulting alloys. Utilizing this
method allowed them to efficiently explore a large compositional space without producing large samples.
Micro-hardness tests on all three libraries revealed that the addition of the refractory elements resulted in
[166]
increased hardness, likely due to the formation of secondary intermetallic phases such as a Laves phase .

While DED is the most common method to produce gradient materials compositionally, Wen et al. used L-
[167]
PBF to produce a gradient material, as shown in Figure 11B . Normally, L-PBF is considered undesirable
for compositionally graded materials as the powder composition cannot be systematically controlled once
loaded in a chamber. Wen et al. addressed this issue by adding a partition within the powder hopper so that
two different powders could be loaded together. Then a mixer is placed below the hopper that mixes
powders along the width of the mixer. This mixture forms a compositional gradient in the laser scan plane
when the powder bed is deposited. Thus, a horizontal compositional gradient forms rather than the typical
vertical gradients achieved in DED combinatorial studies [168-170] . Wen et al. used CoCrFe medium entropy
alloy and Inconel 718 as the feedstock powders to prove this new technique’s concept. At the pure CoCrFe
end of the alloy gradient, a pure FCC phase structure was formed and as the Ni content increased due to the
addition of Inconel 718, a secondary HCP phase was formed. The HCP phase content increased with
increasing Ni content. The decrease in hardness occurred with increasing Ni-content, which is likely due to
the larger sub-grain size observed near the Inconel 718 end.

Li et al. used DED to explore the effects of compositional and cooling rate changes on the microstructure
[171]
and mechanical properties of the Al-Co-Cr-Fe-Ni alloy system . First, they produced a pure CoCrFeNi
substrate via casting. Then, they deposited varying amounts of Al on the substrate using a LENS system
which formed different compositions of Al CoCrFeNi along the substrate surface, ranging from x = 0.51 to
x
x = 1.25, as shown in Figure 11C. The laser was also used for remelting straight lines parallel to the
compositional gradient with different laser powers and scan speeds which induced different cooling rates in
the compositional library. Three compositions from the library were also chosen to produce casting
counterparts to achieve cooling rates far below what is achievable through DED. This method allowed the
cooling rate to be varied from 25-6,400 K/s. Their findings showed that the lowest Al-containing
compositions exhibited a dual-phase FCC + BCC structure which transitioned to a pure BCC/B2 at near
equiatomic compositions. Additionally, compositions with low Al content showed a primary FCC phase
with a cellular microstructure. The cellular microstructure followed a power law of the form
where λ is the cell size, A is a fitted parameter, and is the cooling rate. The microstructure refinement
resulted in hardening following the Hall-Petch relationship. This work illustrates the potential for laser-
based AM methods to rapidly and simultaneously explore the effects of composition and cooling rate on the
phase evolution and mechanical properties of HEAs.