Powder Metallurgy Processing and Characterization of the χ Phase Containing Multicomponent Al-Cr-Fe-Mn-Mo Alloy

: High entropy alloys present many promising properties, such as high hardness or thermal stability, and can be candidates for many applications. Powder metallurgy techniques enable the production of bulk alloys with ﬁne microstructures. This study aimed to investigate powder metal-lurgy preparation, i.e., mechanical alloying and sintering, non-equiatomic high entropy alloy from the Al-Cr-Fe-Mn-Mo system. The structural and microstructural investigations were performed on powders and the bulk sample. The indentation was carried out on the bulk sample. The mechanically alloyed powder consists of two bcc phases, one of which is signiﬁcantly predominant. The annealed powder and the sample sintered at 950 ◦ C for 1 h consist of a predominantly bcc phase (71 ± 2 vol.%), an intermetallic χ phase (26 ± 2 vol.%), and a small volume fraction of multielement carbides—M 6 C and M 23 C 6 . The presence of carbides results from carbon contamination from the balls and vial during mechanical alloying and the graphite die during sintering. The density of the sintered sample is 6.71 g/cm 3 (98.4% relative density). The alloy presents a very high hardness of 948 ± 34 HV 1N and Young’s modulus of 245 ± 8 GPa. This study showed the possibility of preparing ultra-hard multi-component material reinforced by the intermetallic χ phase. The research on this system presented new knowledge on phase formation in multicomponent systems. Moreover, strengthening the solid solution matrix via hard intermetallic phases could be interesting for many industrial applications.


Introduction
High entropy alloys (HEAs), also known as compositionally complex alloys (CCAs), are multicomponent materials consisting of at least five main elements, each of them with content between 5 and 35 at.% [1]. The research on HEAs started with the introduction of CoCrFeMnNi alloy, known as Cantor alloy, showing promising properties [2]. However, nowadays, HEAs consist of different elements and can be divided into different groups, such as 3d-transition metal, 4f-transition metal, refractory metal, and light metal [3]. Moreover, some HEAs contain elements from different groups. HEAs present a very wide range of promising properties depending on the chemical composition, processing, and microstructure. HEAs are under current scientific interest from different fields of materials science [1,4,5]. The most common properties are high strength and ductility at high and low temperatures, high hardness, creep resistance, thermal stability, irradiation resistance, good weldability, oxidation resistance, resistance to corrosion, and hydrogen embrittlement [6]. Many exceptional properties of HEAs could be explained, at least partially, by their multielement composition, and the four core effects resulting from that, i.e., high entropy effect (solid solution stabilization), cocktail effect (unexpected synergies of properties of constituent elements), sluggish diffusion (thermal stability, easy formation of nanograin structure), and severe lattice distortion (high mechanical strength) [7]. However, in the interesting to continue the studies of different compositions from the Al-Cr-Fe-Mn-Mo family prepared by PM due to a large difference in melting temperature between elements.
In this study, the main aim was to prepare a novel non-equiatomic HEA with very high hardness. Compared to the equiatomic alloy, a lower content of Mo was selected to improve homogenization during mechanical alloying and decrease the density of the alloy, while a higher content of Fe was chosen to reduce the cost. A non-equiatomic alloy from the Al-Cr-Fe-Mn-Mo system was selected and prepared by powder metallurgy techniques. The investigation focused on the structure and microstructure of mechanically alloyed, annealed powders, and the sintered bulk sample. The main focus was on the presence of the χ phase, which is very rarely reported in multicomponent alloys. Moreover, mechanical properties were evaluated by the indentation test. It should be noted that it is the first experimental study of this composition in bulk samples.

Material Preparation
The samples were prepared by PM techniques, i.e., MA and hot press sintering. The elemental powders were purchased from the company Goodfellow. The purity and particle sizes were as follows: Al (>99.0%, <15 µm), Cr (>99.0%, 38-45 µm), Fe (>99.0%, <60 µm), Mn (>99.5%, <45 µm), and Mo (>99.9%, <350 µm). The raw powders were mechanically alloyed in a vibrational mill SPEX 8000 operating at 18 Hz. The composition of the powder mixture before milling was as follows: 19 at.% Al, 22 at.% Cr, 34 at.% Fe, 19 at.% Mn, 6 at.% Mo. This corresponds to the composition of the bcc phase defined in a previous paper dealing with the Al-Cr-Fe-Mn-Mo family [33]. The parameters of MA were selected based on our previous articles [32,33]. The MA was conducted for 32 h at close to room temperature under an argon atmosphere using a vial and balls made of hardened steel. Breaks in milling were applied to prevent the heating of the powder-30 min break after each hour of milling. The ball-powder ratio was 10:1. The powders were mechanically alloyed without adding a process control agent because frontal shocks in a vibrational mill cause less agglomeration than shocks in a planetary mill. The alloyed powder was annealed at 950 • C for 1 h under argon in a high-temperature furnace (Centorr Vacuum Industries). Before the consolidation, the MA powder was put into 10 mm-diameter graphite die, which is common for such processing, to produce a sample of a height of 5 mm. For easier sample removal after sintering, graphite foils were placed between the punches, powder, and die. All manipulations with the powder were conducted in a glove box filled with argon to avoid powder oxidation. The sintering was carried out under argon at 950 • C for 1 h in a Centorr Vacuum Industries furnace equipped with an Instron 4507 press under a load of 80 MPa. The heating and cooling rates were 10 • C per minute.

Material Characterization
The phase information of the mechanically alloyed powder, annealed powder, and bulk sample was obtained from the X-ray diffraction (XRD) investigations using an X'PERT Pro Philips-Panalytical equipped with a cobalt tube (λ K-alphaCo = 1.79021 Å) working at 20 mA and 40 kV. Scans were acquired at a rate of 0.03 • /min in the range of 2 theta between 20 • and 120 • . The XRD data were analyzed using DIFFRAC.EVA software by Bruker and compared with the files of existing phases from the PDF ICDD database. Mössbauer spectrometry (MS) was conducted at room temperature using a 57 Co (Rh) source and Aries-Wissel devices. Mössbauer spectra were deconvoluted using Lorentzian shape peaks and iron as reference. The following Mössbauer parameters were calculated: the peak width (W), hyperfine field (HF), quadrupole splitting (QS), isomer shift (IS), and relative amount of each iron environment (A). The scanning electron microscope (SEM) investigations were performed using a JEOL JSM-7800F with an energy-dispersive X-ray spectroscopy detector (EDX). The powder and bulk samples were prepared using a standard metallographic preparation method, i.e., hot mounting and polishing with grinding papers up to P4000. The final preparation was performed using diamond polishing pastes with a diamond size of 3.0, 1.0, and 0.25 µm. The volume fraction of phases in the annealed powder and the bulk sample was determined based on many SEM backscattered electrons (BSE) images using ImageJ software. The density measurements were performed according to Archimedes' principle in air and ethanol using a Mettler Toledo balance. The Vickers micro-hardness of the bulk sample was measured using a CSM Instruments Micro-Hardness Tester. For each load between 0.5 and 9 N, 15 measurements were performed at a dwell time of 10 s. The minimal space between indents was the distance of the five-time diameter of the biggest indent. Data were analyzed using the Oliver-Pharr method [36].

Structure
According to the XRD results (Figure 1), the MA powder consists of two bcc phases. The predominant phase is bcc#2 (a = 2.91 Å, lattice parameter close to chromium and iron, ICDD PDF 00-006-0694), while the minor phase is bcc#1 (a = 3.13 Å, lattice parameter close to molybdenum, ICDD PDF 00-004-0809). Similar lattice parameters were detected in other powders from the Al-Cr-Fe-Mn-Mo system [32]. The formation of the bcc phase after MA is not surprising because Cr, Fe, Mn, and Mo present the bcc structure as pure elements, while Al is known as a bcc phase stabilizer. The annealing of the powder at 950 • C causes the disappearance of the bcc#1 phase. The annealed powder consists of a predominant bcc#2 phase (a = 2.90 Å), intermetallic χ phase (a = 8.95 Å; space group: I-43m (217), ICDD PDF 01-081-3981), and very small fractions of carbides-M 6 C (a = 11.15 Å; space group: Fd-3m (227), ICDD PDF 01-083-3017) and M 23 C 6 (a = 10.66 Å; space group Fm-3m (225), ICDD PDF 00-035-0783). The structure of the bulk sample is similar to that of the annealed powder. The presence of carbides is related to the carbon contamination from balls and vial materials during MA and from graphite die during sintering. Carbon contamination during powder metallurgy processing is common and often results in the formation of carbides [14,15].
Alloys 2023, 2, FOR PEER REVIEW 4 spectroscopy detector (EDX). The powder and bulk samples were prepared using a standard metallographic preparation method, i.e., hot mounting and polishing with grinding papers up to P4000. The final preparation was performed using diamond polishing pastes with a diamond size of 3.0, 1.0, and 0.25 µ m. The volume fraction of phases in the annealed powder and the bulk sample was determined based on many SEM backscattered electrons (BSE) images using ImageJ software. The density measurements were performed according to Archimedes' principle in air and ethanol using a Mettler Toledo balance. The Vickers micro-hardness of the bulk sample was measured using a CSM Instruments Micro-Hardness Tester. For each load between 0.5 and 9 N, 15 measurements were performed at a dwell time of 10 s. The minimal space between indents was the distance of the five-time diameter of the biggest indent. Data were analyzed using the Oliver-Pharr method [36].

Structure
According to the XRD results (Figure 1), the MA powder consists of two bcc phases. The predominant phase is bcc#2 (a = 2.91 Å , lattice parameter close to chromium and iron, ICDD PDF 00-006-0694), while the minor phase is bcc#1 (a = 3.13 Å , lattice parameter close to molybdenum, ICDD PDF 00-004-0809). Similar lattice parameters were detected in other powders from the Al-Cr-Fe-Mn-Mo system [32]. The formation of the bcc phase after MA is not surprising because Cr, Fe, Mn, and Mo present the bcc structure as pure elements, while Al is known as a bcc phase stabilizer. The annealing of the powder at 950 °C causes the disappearance of the bcc#1 phase. The annealed powder consists of a predominant bcc#2 phase (a = 2.90 Å), intermetallic χ phase (a = 8.95 Å ; space group: I-43m (217), ICDD PDF 01-081-3981), and very small fractions of carbides-=-M6C (a = 11.15 Å ; space group: Fd-3m (227), ICDD PDF 01-083-3017) and M23C6 (a = 10.66 Å ; space group Fm-3m (225), ICDD PDF 00-035-0783). The structure of the bulk sample is similar to that of the annealed powder. The presence of carbides is related to the carbon contamination from balls and vial materials during MA and from graphite die during sintering. Carbon contamination during powder metallurgy processing is common and often results in the formation of carbides [14,15].  MS results revealed that the MA powder is more homogeneous than the annealed powder (Figure 2a,b, and Table 1), for which the acquired Mössbauer spectra are less symmetrical and broader. Moreover, the number of different iron sites increases after the heat treatment. It is the result of the multiphase structure of annealed powder (mostly bcc and χ phases) compared to the largely predominant bcc#2 phase in MA powder.
MS results revealed that the MA powder is more homogeneous than the annealed powder (Figure 2a,b, and Table 1), for which the acquired Mössbauer spectra are less symmetrical and broader. Moreover, the number of different iron sites increases after the heat treatment. It is the result of the multiphase structure of annealed powder (mostly bcc and χ phases) compared to the largely predominant bcc#2 phase in MA powder.  Table 1. Mössbauer hyperfine parameters, such as the hyperfine field (H), isomer shift (IS), quadrupole splitting (QS), peak width (W), and relative amount of each iron environment (A) of iron environments in the mechanically alloyed powder and annealed powder.

Microstructure
SEM secondary electron observations of MA powder showed the agglomeration of small particles with a very rugged surface (Figure 3a). The particles present a rather elongated shape. Most of the powder particles are of a size between 3 and 5 µ m. It should be noted that powder agglomeration occurs, even though the frontal shocks in a vibrational mill cause less agglomeration than shocks in a planetary mill. Some agglomerates are of a size of up to a few tens of micrometers. The process control agent could probably minimize the agglomeration; however, it will introduce contamination, mostly carbon, to the powder. The significant carbon contamination could lead to the formation of a high volume fraction of carbides and significantly change the phase formation during annealing or sintering, which was the case in other studies on the Al-Cr-Fe-Mn-Mo system [34]. Therefore, a process control agent was not added in this study.  Table 1. Mössbauer hyperfine parameters, such as the hyperfine field (H), isomer shift (IS), quadrupole splitting (QS), peak width (W), and relative amount of each iron environment (A) of iron environments in the mechanically alloyed powder and annealed powder.

Microstructure
SEM secondary electron observations of MA powder showed the agglomeration of small particles with a very rugged surface (Figure 3a). The particles present a rather elongated shape. Most of the powder particles are of a size between 3 and 5 µm. It should be noted that powder agglomeration occurs, even though the frontal shocks in a vibrational mill cause less agglomeration than shocks in a planetary mill. Some agglomerates are of a size of up to a few tens of micrometers. The process control agent could probably minimize the agglomeration; however, it will introduce contamination, mostly carbon, to the powder. The significant carbon contamination could lead to the formation of a high volume fraction of carbides and significantly change the phase formation during annealing or sintering, which was the case in other studies on the Al-Cr-Fe-Mn-Mo system [34]. Therefore, a process control agent was not added in this study.
The SEM microstructural analysis of MA powder was conducted to verify the homogeneity of the powder before sintering. The powder is homogeneous in the SEM-BSE images (Figure 3b), which means it is not possible to distinguish the bcc phases at this scale of analysis. It is related to a nanometric grain size which is common for powder metallurgy samples [13]. However, slight contamination by iron was detected by EDX, which is related to the erosion of the balls and vial during MA. The final composition is 18 at.% Al, 21 at.% Cr, 37 at.% Fe, 18 at.% Mn, 6 at.% Mo, close to the starting composition. The analysis of the annealed powder revealed a significantly different microstructure (Figure 3c). Nevertheless, the particle size and morphology of the annealed powder did not significantly change compared to mechanically alloyed powder. Based on the comparison between SEM, EDX, and XRD results, the dark grey matrix phase was associated with the bcc#2 and the light grey contrast corresponds to the intermetallic χ phase. The grey contrast was associated with M 23 C 6 carbide, while white spots correspond to M 6 C carbide. The microstructure of the bulk sample (Figure 3d) is similar to that of the annealed powder. The analysis of the annealed powder and the bulk sample showed that the resulting structure is purely the effect of the temperature, and the pressure during sintering did not impact the phase formation. Moreover, it should be noted that the phases are distributed homogenously, which confirms further uniform distribution of elements in MA powder. The MA process seems crucial in obtaining a homogeneous distribution of phases in the bulk sample. The SEM microstructural analysis of MA powder was conducted to verify the homogeneity of the powder before sintering. The powder is homogeneous in the SEM-BSE images (Figure 3b), which means it is not possible to distinguish the bcc phases at this scale of analysis. It is related to a nanometric grain size which is common for powder metallurgy samples [13]. However, slight contamination by iron was detected by EDX, which is related to the erosion of the balls and vial during MA. The final composition is 18 at.% Al, 21 at.% Cr, 37 at.% Fe, 18 at.% Mn, 6 at.% Mo, close to the starting composition. The analysis of the annealed powder revealed a significantly different microstructure ( Figure  3c). Nevertheless, the particle size and morphology of the annealed powder did not significantly change compared to mechanically alloyed powder. Based on the comparison between SEM, EDX, and XRD results, the dark grey matrix phase was associated with the bcc#2 and the light grey contrast corresponds to the intermetallic χ phase. The grey contrast was associated with M23C6 carbide, while white spots correspond to M6C carbide. The microstructure of the bulk sample (Figure 3d) is similar to that of the annealed powder. The analysis of the annealed powder and the bulk sample showed that the resulting structure is purely the effect of the temperature, and the pressure during sintering did not impact the phase formation. Moreover, it should be noted that the phases are distributed homogenously, which confirms further uniform distribution of elements in MA powder. The MA process seems crucial in obtaining a homogeneous distribution of phases in the bulk sample.
The SEM-EDX analyses ( Figure 4 and Table 2) showed that the bcc phase has a composition relatively close to the overall powder composition, and the χ phase is slightly enriched in Mo (approximately 11 at.%). The M23C6 carbide is chromium-rich, while M6C is rich in molybdenum, which is common for these carbides. The volume fraction of phases in the center of the sample (shown in Table 2) revealed a predominance of the The SEM-EDX analyses ( Figure 4 and Table 2) showed that the bcc phase has a composition relatively close to the overall powder composition, and the χ phase is slightly enriched in Mo (approximately 11 at.%). The M 23 C 6 carbide is chromium-rich, while M 6 C is rich in molybdenum, which is common for these carbides. The volume fraction of phases in the center of the sample (shown in Table 2) revealed a predominance of the bcc#2 phase and high content of the χ phase. Some small volume fractions of carbides (M 6 C and M 23 C 6 ) were also detected. The volume fraction of phase in annealed powder is similar to that of the bulk sample, i.e., 71 ± 2% of bcc#2 phase, 27 ± 2% of χ phase, 1 ± 1% of M 6 C, and 1 ± 1% of M 23 C 6 . The results concerning phase content are in agreement with the XRD results, which revealed a predominance of bcc#2 phase, a high-volume fraction of χ phase, and a small content of multielement carbides. of M6C, and 1 ± 1% of M23C6. The results concerning phase content are in agreement with the XRD results, which revealed a predominance of bcc#2 phase, a high-volume fraction of χ phase, and a small content of multielement carbides.  The formation of multicomponent carbides is not surprising because similar carbides form in many HEAs prepared by powder metallurgy due to contamination by carbon in such processing from vial and ball materials and from graphite die during consolidation [14,15]. The contamination during MA is more homogenously distributed, while the contamination during sintering is rather close to the surface of the sample. Similarly to this study, M6C is usually molybdenum-rich [37,38], while M23C6 is usually chromium-rich [39][40][41] according to the literature. However, the formation of the rarely detected intermetallic χ phase is very surprising. The presence of the χ phase could be due to the significant presence of Fe, Cr, and Mo in the alloy, which favors the formation of this phase. In addition, it should be noted that in the literature, a close link between the χ phase and M6C carbide is noted. It seems that both phases can exist in equilibrium or substitute each other depending on the heat treatment performed [42]. It is worth mentioning that the χ phase is a part of the Cr-Fe-Mo ternary system [43] and, in high molybdenum stainless steels, can be described as Fe36Cr12Mo10 according to Xu et al. [44]. In addition, from the structural point of view, the χ phase is considered to be an ordered α-Mn phase [21].
A different microstructure was detected in the area near the surface (a few tens of micrometers), which was in close contact with the graphite die during sintering ( Figure  5). The significant contamination in this area due to the more pronounced diffusion of carbon from the graphite die led to a substantial decrease in the χ phase volume fraction and the formation of many M6C and M23C6 carbides. The bcc#2 phase is still the predominant phase in this area. The formation of carbides needs significant amounts of molybdenum and chromium; therefore, the content of these elements seems to be insufficient to form the χ phase. Moreover, the carbon could dissolve (into the holes in the crystal structure) in the χ phase (sometimes classified as M18C), although in a significantly lower amount than in carbides [42]; thus, higher carbon contamination will favor the formation of carbides instead of the χ phase. In our previous study of a slightly different nonequiatomic chemical composition, the presence of the χ phase was not detected; however,  The formation of multicomponent carbides is not surprising because similar carbides form in many HEAs prepared by powder metallurgy due to contamination by carbon in such processing from vial and ball materials and from graphite die during consolidation [14,15]. The contamination during MA is more homogenously distributed, while the contamination during sintering is rather close to the surface of the sample. Similarly to this study, M 6 C is usually molybdenum-rich [37,38], while M 23 C 6 is usually chromiumrich [39][40][41] according to the literature. However, the formation of the rarely detected intermetallic χ phase is very surprising. The presence of the χ phase could be due to the significant presence of Fe, Cr, and Mo in the alloy, which favors the formation of this phase. In addition, it should be noted that in the literature, a close link between the χ phase and M 6 C carbide is noted. It seems that both phases can exist in equilibrium or substitute each other depending on the heat treatment performed [42]. It is worth mentioning that the χ phase is a part of the Cr-Fe-Mo ternary system [43] and, in high molybdenum stainless steels, can be described as Fe 36 Cr 12 Mo 10 according to Xu et al. [44]. In addition, from the structural point of view, the χ phase is considered to be an ordered α-Mn phase [21].
A different microstructure was detected in the area near the surface (a few tens of micrometers), which was in close contact with the graphite die during sintering ( Figure 5). The significant contamination in this area due to the more pronounced diffusion of carbon from the graphite die led to a substantial decrease in the χ phase volume fraction and the formation of many M 6 C and M 23 C 6 carbides. The bcc#2 phase is still the predominant phase in this area. The formation of carbides needs significant amounts of molybdenum and chromium; therefore, the content of these elements seems to be insufficient to form the χ phase. Moreover, the carbon could dissolve (into the holes in the crystal structure) in the χ phase (sometimes classified as M 18 C), although in a significantly lower amount than in carbides [42]; thus, higher carbon contamination will favor the formation of carbides instead of the χ phase. In our previous study of a slightly different non-equiatomic chemical composition, the presence of the χ phase was not detected; however, the volume fraction of carbides seemed to be slightly higher [33]. Therefore, it could be concluded that very small changes in the content of elements, especially molybdenum, aluminum (bcc stabilizer), and carbon coming from contamination could significantly influence the phase formation in the investigated system. The density of the bulk sample is 6.71 g/cm 3 , which corresponds to 98.4% of the theoretical density calculated based on the density of pure elements. It highlights the fact that the sintering parameters were correctly chosen to obtain a near-fulldensity bulk sample. Moreover, it shows that mechanical parameters were appropriately selected to not have internal porosity in powder particles. In addition, some agglomerates detected in mechanically alloyed powder did not significantly reduce the density of the bulk sample. the volume fraction of carbides seemed to be slightly higher [33]. Therefore, it could be concluded that very small changes in the content of elements, especially molybdenum, aluminum (bcc stabilizer), and carbon coming from contamination could significantly influence the phase formation in the investigated system. The density of the bulk sample is 6.71 g/cm 3 , which corresponds to 98.4% of the theoretical density calculated based on the density of pure elements. It highlights the fact that the sintering parameters were correctly chosen to obtain a near-full-density bulk sample. Moreover, it shows that mechanical parameters were appropriately selected to not have internal porosity in powder particles. In addition, some agglomerates detected in mechanically alloyed powder did not significantly reduce the density of the bulk sample.

Mechanical Properties
The hardness of the bulk sample is very high, between 900 and 960 HV depending on the indentation load ( Figure 6). A slight hardness decrease is observed with increasing load, which is related to the indentation size effect (ISE) [45]. The hardness is very high compared to other alloys prepared by powder metallurgy, e.g., 762 HV of AlCoCrFeNiTi0.5 consisting of fcc, B2, sigma, and TiC phases [46] or 670 HV of AlCoCrFeNi consisting of a mixture of fcc and bcc phases [47]. The high hardness of the alloy could be beneficial in tribological and high wear-resistant applications. The high hardness is the effect of different strengthening mechanisms, such as solid solution strengthening (matrix bcc multielement solid solution), strengthening by very hard intermetallic phase (χ phase), and grain boundary strengthening (fine grain size, in order between 200 and 400 nm based on our previous studies on this system [33]). The hardness of the χ phase measured by Gwalani et al. [19] was 1090 HV ± 14; therefore, it is obvious that the χ phase has an impact on the hardness of the produced sample. It should be noted that the χ phase is harder than many other intermetallic phases, e.g., a Huesler phase with L21 ordering in studies by Gwalani et al. [19] or Laves phase in studies by Zhao et al. [18]. The bulk sample also presents a very high Young's modulus, e.g., 245 ± 8 GPa for the measurement under a load of 1 N. The value is higher than the value of 213 GPa predicted by CALPHAD simulation of equiatomic AlCrFeMnMo alloy consisting of a bcc phase and AlMo3 compound.

Mechanical Properties
The hardness of the bulk sample is very high, between 900 and 960 HV depending on the indentation load ( Figure 6). A slight hardness decrease is observed with increasing load, which is related to the indentation size effect (ISE) [45]. The hardness is very high compared to other alloys prepared by powder metallurgy, e.g., 762 HV of AlCoCrFeNiTi 0.5 consisting of fcc, B2, sigma, and TiC phases [46] or 670 HV of AlCoCrFeNi consisting of a mixture of fcc and bcc phases [47]. The high hardness of the alloy could be beneficial in tribological and high wear-resistant applications. The high hardness is the effect of different strengthening mechanisms, such as solid solution strengthening (matrix bcc multielement solid solution), strengthening by very hard intermetallic phase (χ phase), and grain boundary strengthening (fine grain size, in order between 200 and 400 nm based on our previous studies on this system [33]). The hardness of the χ phase measured by Gwalani et al. [19] was 1090 HV ± 14; therefore, it is obvious that the χ phase has an impact on the hardness of the produced sample. It should be noted that the χ phase is harder than many other intermetallic phases, e.g., a Huesler phase with L2 1 ordering in studies by Gwalani et al. [19] or Laves phase in studies by Zhao et al. [18]. The bulk sample also presents a very high Young's modulus, e.g., 245 ± 8 GPa for the measurement under a load of 1 N. The value is higher than the value of 213 GPa predicted by CALPHAD simulation of equiatomic AlCrFeMnMo alloy consisting of a bcc phase and AlMo 3 compound. The SEM observations revealed the presence of circumferential cracks in the vicinity of indentation sites (Figure 7). It shows the brittleness of the alloy, which is caused primarily by the presence of the intermetallic χ phase, which is known to be very brittle [48]. The brittleness increase due to the χ phase was reported in other HEAs [18,19]. However, there are no radial cracks, which are typical for brittle material [49]. Therefore, some plasticity of this sample due to the less brittle matrix bcc phase is revealed. The SEM observations revealed the presence of circumferential cracks in the vicinity of indentation sites (Figure 7). It shows the brittleness of the alloy, which is caused primarily by the presence of the intermetallic χ phase, which is known to be very brittle [48]. The brittleness increase due to the χ phase was reported in other HEAs [18,19]. However, there are no radial cracks, which are typical for brittle material [49]. Therefore, some plasticity of this sample due to the less brittle matrix bcc phase is revealed. The SEM observations revealed the presence of circumferential cracks in the vicinity of indentation sites (Figure 7). It shows the brittleness of the alloy, which is caused primarily by the presence of the intermetallic χ phase, which is known to be very brittle [48]. The brittleness increase due to the χ phase was reported in other HEAs [18,19]. However, there are no radial cracks, which are typical for brittle material [49]. Therefore, some plasticity of this sample due to the less brittle matrix bcc phase is revealed.

Conclusions
Non-equiatomic HEA from the Al-Cr-Fe-Mn-Mo family was successfully prepared via powder metallurgy. The mechanically alloyed powder consists of two bcc phases. Annealing at 950 °C for 1 h led to the phase transformations. As a result, it was determined that the powder consists of a predominant bcc phase (71 ± 2%), intermetallic χ phase (27 ± 2%), and small volume fraction (1 ± 1%) of M6C and M23C6. The bulk sample presents a similar microstructure with a higher volume fraction of carbides in the area close to the surface due to the diffusion of carbon from the graphite die during sintering. The main finding of this paper is the presence of the χ phase in the bcc matrix in the multicomponent alloy. The intermetallic χ phase is very rarely reported in HEA, but it can significantly improve the hardness. It can open new perspectives on research into multicomponent alloys with a hard χ phase. The very high hardness and Young's modulus of the investigated bulk sample 948 ± 34 HV1N and 245 ± 8 GPa, respectively, show the need to investigate this HEA system further. Future studies should focus on the investigation of the wear

Conclusions
Non-equiatomic HEA from the Al-Cr-Fe-Mn-Mo family was successfully prepared via powder metallurgy. The mechanically alloyed powder consists of two bcc phases. Annealing at 950 • C for 1 h led to the phase transformations. As a result, it was determined that the powder consists of a predominant bcc phase (71 ± 2%), intermetallic χ phase (27 ± 2%), and small volume fraction (1 ± 1%) of M 6 C and M 23 C 6 . The bulk sample presents a similar microstructure with a higher volume fraction of carbides in the area close to the surface due to the diffusion of carbon from the graphite die during sintering. The main finding of this paper is the presence of the χ phase in the bcc matrix in the multicomponent alloy. The intermetallic χ phase is very rarely reported in HEA, but it can significantly improve the hardness. It can open new perspectives on research into multicomponent alloys with a hard χ phase. The very high hardness and Young's modulus of the investigated bulk sample 948 ± 34 HV 1N and 245 ± 8 GPa, respectively, show the need to investigate this HEA system further. Future studies should focus on the investigation of the wear resistance of HEA containing a high-volume fraction of the χ phase. Moreover, the mechanisms behind the formation of the χ phase in multicomponent alloys need to be better understood. Funding: The author T.S. sincerely thanks the University of Lille and the Region "Hauts-de-France" for financially supporting his graduate doctoral program. The SEM and TEM national facility in Lille (France) is financially supported by the Region "Hauts-de-France", the European Regional Development Fund (ERDF).

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.