High entropy alloys (HEAs), consisting of five or more elements with a concentration between 5 to 35 at. pct, are new alloy classes, which have attracted increasing attention from material scientists because of their excellent corrosion resistance, high strength, and fatigue resistance. HEAs show excellent mechanical properties even at elevated temperatures, promising potential for high temperature-applications.[1,2,3,4,5] However, HEAs may show a stable miscibility gap in the liquid, and liquid phase separation (LPS),[6,7,8,9,10] where these could lead to a heterogeneous microstructure and mechanical properties. Several studies have investigated the LPS of HEAs and reported inhomogeneity in the microstructure.[10,11,12] Munitz et al.[13] examined the effect of additions (Co, Cr, Ni, V, Al, Nb, and Ti) on LPS in HEAs, where the addition of sufficient amounts of the elements (Co, Al, Ti, and Ni) lowers the miscibility gap temperature, and consequently eliminates stable LPS. Nevertheless, it could be challenging to visualize the phase diagram space of HEAs and estimate the LPS formation in the liquid.[10] Among various HEAs, AlCoCrFeNi is one of the most investigated HEA systems because of its relative abundance, low-cost constituent elements, and excellent mechanical properties. However, these alloys show a dual-phase with different mechanical properties depending on the Al content.[14] Lu et al.[15] studied the microstructure of AlCoCrFeNi HEAs manufactured using arc melting and reported an A2 phase, which was Al- (~ 3.9 at. pct) and Ni-depleted (~ 7.1 at. pct).

Selective laser melting (SLM) is one of the most widely used additive manufacturing (AM) processes that can fabricate the most complex-shaped object.[16,17,18] It can manufacture parts from HEAs for harsh environments in various industries, such as defense, manufacturing, and energy. A vast majority of SLM parts are produced from atomized powder, which produces almost spherical particles with relatively smooth surfaces using a very high cooling rate.[19] The powder particles properties play a vital role in the properties of SLM parts. The inhomogeneity in the microstructure or chemical composition of powder particles could lead to inhomogeneity in the microstructure and mechanical properties of the SLM parts.[14] However, only a few studies have carried out the microstructure and mechanical properties of HEA powder particles.[20,21] Nonetheless, there is a lack of analysis of the inhomogeneity in chemical composition, microstructure, and mechanical properties of gas-atomized HEAs powder. Accordingly, the present study investigated the microstructure, chemical composition, and mechanical properties of gas-atomized equiatomic AlCoCrFeNi powder. Inhomogeneity in the microstructure and mechanical properties of SLM parts from this powder were investigated.

AM parts were fabricated using a Realizer SLM50 equipped with a 120 W Yb-YAG laser (spot size of ~ 39 μm) from gas-atomized equiatomic AlCoCrFeNi powder particles. A hatch distance of 60 µm, laser scanning speed of 2 m/s, layer thickness of 25 µm, and laser power of 40 W were used for the part production. For the production of gas-atomized HEAs powder particles, HEA ingots were melted by vacuum arc melting in a high-purity argon atmosphere. A high-pressure and high-velocity argon jet was used to a stream of the molten HEAs in a gas atomizer device to produce powder particles. In addition, the properties of gas-atomized Ti6Al4V and CoCrFeMnNi powder particles were investigated. The gas-atomized Ti6Al4V powder particles produced by Realizer GmbH were used in the present study. Particle size distribution was measured using a laser particle size analyzer (HORIBA LA-950). The flowability of the powder was determined using a hall flowmeter funnel. A Zeiss FEG scanning electron microscope with energy-dispersive X-ray spectroscopy (EDS) and a light microscope (Zeiss Axiovert 25) were used for the characterization and observation. Elemental distribution was assessed over several zones of each sample with different magnifications by the EDS. The microstructural analysis was conducted using X-ray diffractometer (Rigaku SmartLab SE) with a D/teX Ultra 250 1D detector equipped with Cu-Kα radiation with a wavelength of 0.1542 nm. The HEAs powder particles are embedded with a cold mount, and Aqua regia was used as an etchant. The hardness of the powders was measured on the polished surface of the particles. The microhardness measurements of the powder particles were conducted over at least one hundred indents on a MICROMET 2001 machine with 50 g loads for powders with 10 seconds dwell time. The microhardness tests were carried out over at least 15 indents of five SLM samples with 200 g loads.

The morphology of gas-atomized AlCoCrFeNi HEAs powder is shown in Figure 1(a), where it can be observed that some powder particles show irregular shapes. The powder showed poor flowability and it was observed to be 36 seconds (50 g)− 1, which is due to the irregular particles and satellite spheres. The frequency vs particle diameter plot is shown in Figure 1(b), where it can be observed that the powder particles show a narrow distribution with D10 ~ 28 µm and D90 ~ 56 µm, and the diameter of most particles is equivalent to or below ~ 39 µm (D50). The X-ray diffraction (XRD) of powder indicates a mixture of FCC and BCC phases with percentages of ~ 5 and ~ 95 pct, respectively (Figure 1(c)). The lattice parameters (determined by Bragg’s law, \(\lambda =2d\mathrm{sin}\theta \)) were observed to be 0.288 nm for BCC and 0.358 nm for FCC, similar to the results reported by Chou et al.[22] for the prepared alloys using vacuum arc-smelter. On the other hand, several studies have reported a single BCC phase for the gas-atomized AlCoCrFeNi HEAs powders.[1,20,23,24,25,26] Recently, Lehtonen et al.[27] reported a duplex phase structure, FCC + BCC, for gas-atomized CrFeNiMn HEAs powders and concluded that the BCC phase decreased with increasing the powder particle class size and with decreasing cooling rate. The two-phase structures (FCC + BCC) could be attributed to the LPS or phases mixture before the atomization process, which could appear in the powder because of the rapid solidification (104 °C/s to 106 °C/s[28,29]). Karlsson et al.[25] investigated equilibrium phase fractions using the CALPHAD, and indicated that gas-atomized equiatomic AlCoCrFeNi HEAs showed a mixture of phases in the temperatures close to melting point temperature, including sigma, BCC, and FCC. In addition, the EDS was used to reveal the chemical composition differences of the powder, which indicated that some powder particles were Al-depleted as compared to the nominal composition of equiatomic AlCoCrFeNi HEAs. The influences of Al addition on microstructure and mechanical properties of the bulk AlxCoCrFeNi HEAs have been studied extensively, and it was reported the presence of FCC phase for x ≥  ~ 0.3 at. pct.[30,31,32,33,34,35,36] Moreover, elemental mapping analysis of the AlCoCrFeNi HEAs powder was carried out to investigate the elemental distribution, where it showed an inhomogeneous distribution of Al. Such inhomogeneous distribution of Al could be attributed to the area that consisted of the FCC phase.

Fig. 1
figure 1

(a) Scanning electron microscopy (SEM) micrograph and the corresponding EDS maps, (b) particle size distribution, and (c) XRD analysis of gas-atomized equiatomic AlCoCrFeNi HEAs powder

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The Vickers method was used to evaluate the hardness of the powder particles, and the average microhardness and its deviation of AlCoCrFeNi HEA powder were observed to be 343 ± 201 HV. The average microhardness of other classes of alloys was measured to evaluate the reliability of this analysis, where they were found to be 293 ± 23 HV for Ti6Al4V and 184 ± 24 HV for CoCrFeMnNi HEAs powders (the SEM images, size distribution, and chemical composition of these powder particles are shown in the appendix). It can be seen that the average Vickers hardness of the gas-atomized equiatomic AlCoCrFeNi HEAs powder particles indicates an extremely large deviation from the average value, which can be attributed to the different phases. On the other hand, the microhardness results of the Ti6Al4V (Figure 2(c)) and CoCrFeMnNi HE (Figure 2(d)) alloys showed a low deviation, and both alloys showed a single phase obtained from the XRD analysis. The hardness distribution of the powder particles' surfaces is shown in Figures 2(e) through (g). The hardness values of the AlCoCrFeNi HEAs powder showed significant hardness variation, where it was either ~ 160 or ~ 530 HV (Figure 2(e)). On the other hand, the CoCrFeMnNi HEAs (Figure 2(f)) and Ti6Al4V (Figure 2(g)) alloys powder particles exhibited slight hardness variation. In addition, the chemical composition of the AlCoCrFeNi HEA powder particles with different hardness values was analyzed using the EDS (Figure 2(b)). It may be observed that some of the powder particles exhibited larger hardness indentation size and significantly lower Al content (~ 1 pct). The results here are in agreement with the report by Wang et al.,[34] where they reported a hardness of ~ 150 HV for Al0.1CoCrFeNi HEAs (0.1 at. pct for Al) and ~ 550 for equiatomic AlCoCrFeNi HEAs.

Fig. 2
figure 2

(a) Vickers hardness indentation of embedded AlCoCrFeNi HEAs powder and (b) chemical composition. Extra images of Vickers hardness indentation of AlCoCrFeNi HEAs are shown in the appendix. Vickers hardness indentation of the powders: (c) CoCrFeMnNi, and (d) Ti6Al4V. Hardness distribution for the powders: (e) AlCoCrFeNi, (f) CoCrFeMnNi, and (g) Ti6Al4V

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The microstructure of the AlCoCrFeNi powder particles showed a dendritic morphology (Figure 3(a)) because of rapid solidification, as also noted by Kunce et al.[35] The microstructure of as-built SLM AlCoCrFeNi HEAs is exhibited in Figure 3(b) (and its higher magnification in Figure 3(c)), in which a mixture of equiaxed dendritic morphology and fine microstructure can be observed, as also reported by Qiao et al.[36] In addition, Figure 3(f) indicates the XRD of the SLM sample, where it shows the presence of a single BCC phase, corroborating the HEAs structure. The crystallite size (determined by Scherrer’s Equation, \(D=\frac{k\lambda }{\beta \mathrm{cos}\theta }\)) and lattice parameter of the SLM samples were observed to be ~ 26 and ~ 0.2876 nm, respectively. The chemical composition of the SLM sample was analyzed (marked area in Figure 3(d)), and the detailed results are summarized in Table I. It can be seen that some regions are Al-depleted as compared to the nominal composition of equiatomic AlCoCrFeNi HEAs, which such inhomogeneity in the microstructure of the SLM parts may reflect the inhomogeneity inherited from the starting powder material. However, a large number of studies have reported the Al-depletion phase because of oxidation[37,38,39,40] or the melting loss of Al element.[41] This phenomenon could affect the mechanical properties, such as hardness, of the fabricated parts. Hence, hardness mapping was used to reveal the variation in hardness distribution of the SLM part (Figure 3(e)). It can be observed from the hardness distribution maps that the AlCoCrFeNi HEAs sample showed a degree of inhomogeneous distribution of hardness, where hardness ranges from ~ 500 to ~ 700 HV. The average microhardness of the SLM AlCoCrFeNi HEAs samples was found to be 517 ± 52 HV. In addition, the theoretical density of the as-built SLM was found to be ~ 95 pct.

Fig. 3
figure 3

(a) SEM micrograph of etched AlCoCrFeNi HEAs powder particle. SEM images of SLM AlCoCrFeNi HEAs in (b) and its higher magnification in (c). Note that the yellow and green arrows show the dendritic and fine microstructure, respectively. (d) Backscattered electron SEM images of SLM sample. (e) Hardness map and (f) XRD of the SLM sample (Color figure online)

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Table I Chemical Composition (At. Pct) of the Marked Area in Fig. 3(d)
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In the rapid solidification AM processes like the SLM process, the inhomogeneity in the powder particles could lead to a non-uniform structure, such as inhomogeneity in the composition.[42] The inhomogeneity microstructure and chemical composition of AlCoCrFeNi HEA affect the physical and mechanical properties, such as Charpy impact energy,[43] grain size,[34,44] tensile properties,[45] tribological behavior,[46] and lattice constant.[34] It is worth to mention that the post-processing or remelting strategies may eliminate inhomogeneity in the microstructural and mechanical properties of the SLM part.[14] Here, the results indicated that equiatomic AlCoCrFeNi HEAs powder showed a biphasic structure probably because of LPS, which in turn influences the final composition and structure of additively manufactured parts. The inhomogeneity in the microstructure and mechanical properties of the SLM parts will affect the performance of these AM-fabricated parts. The present research was an attempt to advance the narrow knowledge in the additive manufacturing processes of equiatomic AlCoCrFeNi HEAs from gas-atomized powder. This finding would be beneficial for improving printability and reducing the inhomogeneity in the chemical composition of the SLM parts.

In summary, the present study explored the phase structures, microstructure, and elemental distribution, hardness of the gas-atomized equiatomic AlCoCrFeNi HEAs powder feedstock, and revealed the inhomogeneity in the microstructure and chemical composition of these particles. The gas-atomized AlCoCrFeNi powder particles showed the presence of biphasic structure, FCC + BCC phases, and a large deviation in the hardness value. The microstructure of the SLM parts showed a significant inhomogeneity in chemical composition that may reflect the inhomogeneity inherited from the powder feedstock. These findings provide new insights into the gas-atomized HEAs powder particles, in particular for high cooling rate additive manufacturing processes such as SLM, which may improve the printability of these alloys, and consequently, could improve the technology readiness level of the AM HEAs for various industrial high-temperature applications.