Microstructure and Mechanical Properties of Co-Cr-Mo-Si-Y-Zr High Entropy Alloy

: Presented work was focused on obtaining new, up to our knowledge, non-described previously in the literature high entropy Co 15 Cr 15 Mo 25 Si 15 Y 15 Zr 15 alloy to ﬁll in the knowledge gap about the six-elemental alloys located in the adjacent to the center of phase diagrams. Material was obtained using vacuum arc melting. Phase analysis revealed the presence of a multi-phase structure. Scanning electron microscopy microstructure analysis revealed the existence of three di ﬀ erent phases with partially dendritic structures. Chemical analysis showed that all phases consist of all six principal elements—however, with di ﬀ erent proportions. Transmission electron microscopy microstructure analysis conﬁrmed the presence of amorphous and nanocrystalline areas, as well as their mixture. For the studied alloy, any phase transformation and solid-state crystallization were not revealed in the temperature range from room temperature up to 1350 ◦ C. Nanoindentation measurements revealed high nanohardness (13(2) GPa and 18(1) GPa for dendritic and interdendritic regions, respectively) and relatively low Young’s modulus (185(23) GPa and 194(9) GPa for dendritic and interdendritic regions, respectively) of the observed phases.


Introduction
High entropy alloys (HEAs) belong to the group of multi-principal-element materials composed of more than five chemical elements. They have been known in the literature since 2004; however, background works began much earlier-in the late 1970s [1,2]. HEAs have gained much attention in recent years due to their interesting and promising properties in comparison to conventional alloys; by 2018, over 900 papers describing high entropy alloys were reported [3]. However, only about 140 high entropy compositions exhibit a single-phase structure [3]. The first high entropy composition described in the literature by Cantor et al. [1] was a single-phase, faced-centered cubic (FCC), five elemental, equiatomic Co 20 Cr 20 Fe 20 Mn 20 Ni 20 alloy. In the same year, other studies on high entropy alloys were reported [4][5][6][7][8].
As it was mentioned, HEAs exhibit promising properties, which can promote their industrial applications. Zhang et al. proposed that the five elemental, high entropy alloy FeCoNi(AlSi) x multi-phase HEAs are mainly formed. Moreover, for Ω ≤ 1, intermetallic compounds and segregations are observed [26]. Further studies on solid-solution creations in HEAs indicated that the atomic packing fraction of a unit cell has also a great influence on the different type of structure formations after solidification. Such a study was performed by Ye et al. They proposed a thermodynamic parameter denoted as φ [27]. In calculations of the φ parameter, the ideal configurational entropy of mixing using Boltzmann's constant (S c ), mixing enthalpy (∆H mix ), empirical melting point of an alloy (T m ), and excess mixing entropy (S E ) is used. Excess mixing entropy was introduced as a new variable, which can be proposed as a supplement of the ideal configurational entropy of mixing (∆S mix ) [3]. S E describes the changes in mixing entropy depending on the atomic packing fraction (ξ). Atomic packing fraction mainly takes values of 0.68 and 0.74, which correspond to the atomic packing fractions in BCC/HCP (hexagonal closest packed) (74%) and FCC (68%) structures, respectively. Moreover, excess mixing entropy is always negative and corresponds to atomic sizes influencing the atomic packing fraction of a unit cell [27]. Geometrical description of the solid solution formation has also been performed. Atomic packing misfit and topological instability was described by Wang et al. and denoted as γ [28]. For binary alloys, it was shown that the critical value of an atom misfit is γ = 1.167, which corresponds to 15% atomic size differences between the alloying elements proposed by Hume-Rothery [28]. For multi-component alloys, including HEAs, γ < 1.175 determines the solubility of multiple elements. Amorphous high entropy alloys (high entropy bulk metallic glasses) also bring a lot of attention. Inoue in 2000 created empirical rules for the formation of amorphous phases for bulk metallic glasses, but the same rules have been also adopted for amorphous-phase high entropy alloy formations. Inoue's rules include: (1) the alloy system contains at least three elements, (2) the mixing enthalpy among the principal elements has a large and negative value, and (3) the atomic size difference among the major constituent elements is larger than 12% [29]. As it can be noticed, Inoue's rules are mainly focused on atomic size differences between alloying elements and mixing enthalpy, which correspond to the above-described δ and ∆H mix parameters, respectively.
The selection of the appropriate alloying elements is also critical in the formation of amorphous phases. It was reported that glass-forming ability (GFA) elements can be categorized into three groups based on their atomic radii: (1) small metalloids (C, B, and Si); (2) intermediate transition metals (Fe, Ni, Co, Cu, Mo, Zn, Nb, Ta, and Ti); and (3) large elements (Zr, Sn, Sc, Sb, Y, La, and Ca) [30]. Si addition as a small metalloid element is effective in glass formation for refractory alloys such as Cu, Fe, and Ni alloys [31][32][33]. The amorphization ability of Si can be visible for the Cu 47 Ti 34 Zr 11 Ni 8 alloy, where a substitution of 1% Ti by Si increased the amorphous volume fraction [31]. On the other hand, silicon additions in Zr-based alloys are detrimental, due to the formation of silicates [30]. The amorphous ability of Si were also reported for Co-Fe-Ta-B bulk metallic glass, where the Si addition favors a glassy formation and, also, improves the mechanical properties [34]. It was also reported that the addition of large elements such as Y also increased the amorphous volume fraction. For instance, in the case of the (Cu 60 Zr 30 Ti 10 ) 100−x M x alloy, an addition of only 2% of Y increased the amorphous volume fraction [35]. The same phenomenon was also reported for Fe 63−x Zr 8 Co 6 Al 1 Mo 7 B 15 M x , where x = 1.5-2% of Y significantly increased the amorphous volume fraction [36,37]. Improvement of the glassy formation ability by Y was also reported for (Fe 72 Nb 4 B 20 Si 4 ) 100−x Y x (x = 0-5) [38]. Other independent studies also revealed a high level of glass formability for Cu-Hf-Ti alloys by a small addition of Si and Y (from 0.1 to 1 at.%) [39].
HEAs compositions can be described in the N-elemental composition space within a (N-1)-dimensional simplex (N-1 dimensional solids) [3]. Conventional alloys (with two or more alloying elements) are mainly located at the corners of phase diagrams in comparison to equiatomic HEAs located at the center of the phase diagram. Non-equiatomic HEAs ale mainly located adjacent to the center of the phase diagram [22]. It can be calculated that there can be 7059052 possible high entropy compositions containing six alloying elements [3]. The presented work was focused on obtaining a new, up to our knowledge, non-described previously in the literature multi-component Co 15  Additionally, the main aim of the performed studies was also to fill in the knowledge gap about the six-elemental alloys located adjacent to the center of the phase diagrams. The material was obtained using the vacuum arc melting method and characterized on various scale levels-from a macro-to nanoscale. In the presented work, we wanted to learn whether, based only on the proposed thermodynamic parameters, it is possible to obtain an amorphous alloy in a bulk form immediately after the arc melting process (after solidification). The detailed characterization of the new Co 15 Cr 15 Mo 25 Si 15 Y 15 Zr 15 high entropy alloy could contribute to the development of amorphous high entropy alloys.

Materials and Methods
The composition of the proposed high entropy alloy (Co 15 Cr 15 Mo 25 Si 15 Y 15 Zr 15 ) was determined by the performed thermodynamical calculation. The alloying elements were chosen by taking into account their different atomic radii and different melting points (see Table 1). Thermodynamic calculations were carried out in order to fulfil all criteria for HEAs (Table 2). Additionally, silicon (Si) and yttrium (Y) were chosen as amorphization ability elements [30]. For the studied alloy, all the above-described thermodynamic parameters for the solid solution formation were also calculated. The obtained values are presented in Table 2. The results, especially atomic size mismatch parameter (δ) and mixing enthalpy (∆H mix ), confirmed that of all the above-described, Inoue's conditions of amorphous phase formation were met.
A six-elemental composition alloy (Co 15 Cr 15 Mo 25 Si 15 Y 15 Zr 15 ) was prepared from bulk elements (purity ≥ 99%). Elemental pieces were mixed and melted using a vacuum arc melting (VAM) technique (Institute of Materials Engineering, Chorzów, Poland) in an argon protective gas atmosphere. The appropriate cooling rate was achieved by a water-cooled copper crucible. High-purity Ti was used as a Ti-getter for residual gases in a vacuum chamber (chamber pressure level was 1.2 bar). The obtained alloy was prepared in the form of a button of~10 mm thick and~6.5 mm in diameter. Weight of the button was~6.5 g. In order to achieve a homogeneous distribution of elements, the obtained button was remelted ten times for about 180 s and flipped over for each remelting stage. No further thermal treatment was performed. Subsequently, the sample was embedded into resin and ground using SiC grinding papers with different grain sizes (grit from 320 to 4000). After grinding, the sample Metals 2020, 10, 1456 5 of 16 was polished using diamond suspensions with grain sizes from 6 µm to 1 µm. Final polishing was performed using colloidal silica oxide (SiO 2 ) suspension with 0.04-µm grain size.
The X-ray powder diffraction (XRD) measurements were carried out using a Panalytical Empyrean diffractometer (Malvern Instruments, Malvern, UK) with Cu anode with a wavelength of 1.54056 Å working at an electric current of 30 mA and voltage of 40 kV and equipped with a PIXcell 3D detector (Malvern Instruments, Malvern, UK). The XRD patterns were collected in the angular range of 2θ = 15-145 • with 0.02 • steps. The phase analysis was performed using reference standards from the International Centre for Diffraction Data (ICDD) PDF-4 database. The microstructure analysis was performed using the JEOL JSM-6480 (JEOL Ltd., Tokyo, Japan) scanning electron microscope (SEM) working with the accelerating voltage of 20 kV equipped with the energy-dispersive X-ray spectroscopy (EDS) detector (IXRF, Austin, TX, USA) from IXRF. The structure of the alloy was also studied using JEOL JEM-3010 high-resolution transmission electron microscope (HR TEM, JEOL Ltd., Tokyo, Japan) with 300-kV acceleration voltage, equipped with a Gatan 2k × 2k Orius™ 833 SC200D CCD camera (Gatan Inc. Pleasanton, CA, USA) and the energy-dispersive X-ray spectroscopy (EDS) detector from Oxford Instruments Company (Oxford Instruments Company, Abingdon, UK). Thin foil samples for TEM measurements were cut with a diameter d = 3 mm and~100-µm thickness from the central part of the VAM bullet. Next, the samples were ground on both sides and Ar-ion polished on the Gatan Precision Ion Polishing System (PIPS) (Gatan Inc. Pleasanton, CA, USA). Beam energy was changed in the range of 4 to 2.5 keV in order to reduce the risk of an amorphous layer forming on the surface of a thin foil sample. In addition, the angular range of the ion guns varied from 4 • to 1 • with the beam energy changes. The recorded TEM selected area electron diffraction (SAED) patterns were indexed using the ElDyf computer software (version 2.1, Institute of Materials Engineering, Chorzów, Poland). High-temperature differential scanning calorimetry (HT DSC) measurements were performed using Netzsch Pegasus 404 F1 (NETZSCH Holding, Selb, Germany) in the temperature range from room temperature (RT) up to 1350 • C, with a 5-K/min heating/cooling rate. HT DSC measurements were carried out under an argon protective gas atmosphere, with 20-mL/min gas flow. The studied and reference samples were measured in Al 2 O 3 crucibles. Nanohardness test was performed using Hysitron TI 950 Triboindenter (Bruker, Billerica, MA, USA) equipped in Berkovich tip with a total included angle~142 • under the static load 500 µN.

Phase Analysis
The performed phase analysis revealed the presence of four crystalline phases-two hexagonal and two cubic phases. They were identified by X-ray diffraction phase analysis ( Figure 1) based on the standards from ICDD database.

Phase Analysis
The performed phase analysis revealed the presence of four crystalline phases-two hexagonal and two cubic phases. They were identified by X-ray diffraction phase analysis (Figure 1) based on the standards from ICDD database.  The first identified phase was the hexagonal phase with a structure similar to Si 3 Y 5 (ICDD 04-001-2096) with space group P6 3 /mcm and lattice parameters a 0 = 8.44 Å and c 0 = 6.35 Å. The highest intensity peak was obtained for 2θ = 35 • and represents {112} planes, with ICCD interplanar distance d hkl = 2.53 Å. The second identified phase was a hexagonal CoCrZr-like structure (ICDD 04-003-7686) with space group P6 3 /mmc and lattice parameters from ICCD are a 0 = 5.00 Å and c 0 = 8.20 Å. The highest intensity peak for that structure was revealed for 2θ = 39 • , which corresponds to {200} planes, with ICDD interplanar distance d hkl = 2.17 Å. The third of the identified phases was the cubic phase with a structure similar to Cr 0.4 Mo 0.6 (ICDD 04-018-6044), with space group Im3m and a 0 = 3.07 Å ICDD lattice parameter. The highest intensity peak was obtained for 2θ = 59 • , which also corresponds to {200} planes with ICDD interplanar distance d hkl = 1.54 Å. The fourth was the cubic phase structure similar to CrMoZr (ICDD 04-015-0011) with space group Fm3m and lattice parameters a 0 = 3.39 Å. The highest intensity peak was obtained for 2θ = 42 • , which corresponds to {222} planes with interplanar distance d hkl = 2.13 Å.
For the Cr 0.4 Mo 0.6 -like phase, the X-ray-diffraction pattern revealed that peaks were slightly shifted towards lower diffraction angles. It means that there is the increase of the lattice parameter comparing what can be associated with the unit cell expansion due to the presence of higher atomic radii atoms like Y(~1.80 Å). Additionally, due to the highest intensity for peak 2θ = 60 • in comparison to other peaks, the presence of texture is highly probable. No significant diffraction peak shifts were observed for the CrMoZr phase. Only for peak 2θ = 72 • representing the {440} plane peak was widening observed. It may correspond to a strain in the crystal structure. For the CoCrZr-like phase and Si 3 Y 5 -like phase, no significant peak shifts and/or widening were observed. Additionally, the XRD pattern revealed raised backgrounds in the 2θ = 15 • -25 • and 2θ = 30 • -47 • regions, which could correspond to the presence of amorphous fractions. In order to confirm the observation, detailed HR TEM studies were performed.
Although the Inoue rules of bulk metallic glasses were fulfilled, the obtained material is mostly crystalline. It is known that the cooling rate has a great influence on the alloy structure formation. A glassy structure forms due to freezing the undercooled liquid to the glass transition temperature T g . To prevent the transition from a liquid to a crystalline phase, a critical cooling rate has to be exceeded. High cooling rates of 10 5 -10 6 K·s −1 are required to obtain the amorphous phase [42]. Additionally, up to now, amorphous alloys were obtained by fabrication methods, which ensure reaching a critical cooling rate such as: suction casting (e.g., Ti 16 [43]) or melt spinning (e.g., Pd 20 Pt 20 Cu 20 Ni 20 P 20 [44]).

SEM Microstructure Studies
The microstructure analysis performed using SEM confirmed the presence of a multiphase microstructure. The backscattered electrons contrast image (BSE) (Figure 2) revealed the presence of three phases with different contrasts correlated with the different chemical compositions. The phases were denoted as follows: dendrites, "light-grey", and "dark-grey". The fourth phase revealed by X-ray phase analysis was not visible in the BSE image. It is probably due to the fact that there is only a slight difference in the contrast between this phase and one of the others present in the sample. It should be also noted that the alloy microstructure also revealed the presence of pores. A chemical composition analysis was performed using the EDS technique. For all identified phases, the EDS studies confirmed the presence of all six principal elements. The chemical composition results obtained for all phases are presented in Table 3.  (7) Dark-grey phase 1.6(7) 1.8(7) 1.9(7) 33.2(8) 59.7(6) 1.8 (9) Additionally, using the EDS technique, element distribution maps were obtained (Figure 3). The results confirmed the above-described analysis of the quantitative chemical composition for all phases. A chemical composition analysis was performed using the EDS technique. For all identified phases, the EDS studies confirmed the presence of all six principal elements. The chemical composition results obtained for all phases are presented in Table 3. Additionally, using the EDS technique, element distribution maps were obtained (Figure 3). The results confirmed the above-described analysis of the quantitative chemical composition for all phases.
Light-grey phase 22.6(6) 24.5(8) 14.4(8) 12.0(3) 3.3(3) 23.2(7) Dark-grey phase 1.6(7) 1.8 (7) 1.9(7) 33.2(8) 59.7(6) 1.8 (9) Additionally, using the EDS technique, element distribution maps were obtained (Figure 3). The results confirmed the above-described analysis of the quantitative chemical composition for all phases.  The EDS chemical composition analysis revealed that, for "dendrites" rich in Cr and Mo, they could correspond to the cubic Cr 0.4 Mo 0.6 phase. This is confirmed by the above-described XRD phase analysis. The "light-grey" phase shows enrichment in Co, Cr, Mo, Si, and Zr, which corresponds to the hexagonal CoCrZr and the cubic CrMoZr phases, as is given by the X-ray phase analysis. The last "dark-grey" phase reveals enrichment in Si and Y, which could correspond to the hexagonal Si 3 Y 5 phase.
During solidification from a liquid state, Cr-and Mo-rich dendrites formed primarily due to the highest melting points in comparison to other alloying elements (Cr 0.4 Mo 0.6 -like phase). Co, Cr, Zr, and Si segregated in the interdendritic phases due to lower melting points and low miscibility, especially Si and Y with other alloying elements (CoCrZr, CrMoZr, and Si 3 Y 5 phases). The "light-gray" phase can be considered as a five-elemental high entropy alloy. Presence of the Si 3 Y 5 phases observed as "dark-grey" areas can be considered as precipitates. The described elemental segregation of all elements stays in good agreement with the EDS elemental distribution maps (Figure 3).

TEM Microstructure Analysis
A detailed structure analysis was performed using HR TEM in order to examine the possible presence of the amorphous phase in the structure. Additionally, chemical composition using the EDS method was also examined. TEM observations confirmed the existence of amorphous areas revealed by the X-ray diffraction measurements. The wide amorphous regions are present in many regions of the specimen (Figure 4a,b). TEM observations revealed also a mixture of amorphous and nanocrystalline phases (denoted further as the "mixture area"; see Figure 4c-blue circle and Figure 4e) and the presence of purely nanocrystalline areas (Figure 4h,i). The mutual coexistence of the amorphous and nanocrystalline areas in the material could be caused by the nonhomogeneous distribution of elements in the amorphous phase, which results in the faster crystallization of some areas. Crystallization could be also a result of ion beam polishing, which was reported for severely deformed NiTi-shape memory alloy [45]. Nevertheless, the ion beam energy was reduced in order to prevent heating of the specimen during ion polishing, but a crystallization of the amorphous phases could not be excluded. Such a claim can be supported by the fact that the nanocrystalline phase was not observed in the X-ray diffraction measurements. All the observed diffraction peaks were narrow, and only broad amorphous bumps were observed. From the nanocrystalline area, the high-resolution (HR) images were recorded. The observations confirmed presence of nanocrystallites of mixed phases in this area (Figure 4f). The average nanocrystallite size was approximately 23(11) nm. A phase analysis based on the selected area electron diffraction patterns (SAED) indicated that the nanocrystalline phases present in both the nanocrystalline, as well as the mixture areas, are the cubic CrMoZr and hexagonal Si 3 Y 5 (Figure 4e). The interplanar distances obtained from the fast Fourier-transformation image (FFT) (Figure 4g) confirmed the presence of those two phases. Additionally, the SAED patterns taken from the nanocrystalline area (Figure 4i) confirmed the presence of CoCrZr and CrMoZr phases with a small amount of the Si 3 Y 5 phase. The phase analysis based on the FFT image ( Figure 4g) and ring SAED pattern (Figure 4i) is in good agreement with TEM EDS chemical composition analysis (see Table 4). TEM EDS chemical composition measurements confirmed the presence of all alloying elements in all the studied areas.   It is highly probable that the presence of nanocrystallites, both in the nanocrystalline areas and mixture areas, could be the result of crystallization of places depleted in Si and Y, since Si and Y exhibit a strong amorphization ability and inhibit crystallization and grain growth. This can be also supported by the fact that amorphous areas are slightly enriched in those elements (TEM EDS analysis). The presence of nanometric sizes of crystallites could be also a result of Zr atoms distribution, which reduced the grain growth, as it was reported in the literature for CrMoZr alloy  It is highly probable that the presence of nanocrystallites, both in the nanocrystalline areas and mixture areas, could be the result of crystallization of places depleted in Si and Y, since Si and Y exhibit a strong amorphization ability and inhibit crystallization and grain growth. This can be also supported by the fact that amorphous areas are slightly enriched in those elements (TEM EDS analysis). The presence of nanometric sizes of crystallites could be also a result of Zr atoms distribution, which reduced the grain growth, as it was reported in the literature for CrMoZr alloy [46]. In these alloys, the addition of only 2% of Zr significantly reduced the grain size. The chemical composition of the "mixture area" exhibits also high amounts of Si and Y, which means that Si 3 Y 5 crystallites are present there.

HT DSC Measurements
In order to check the stability of phases in the studied alloy, DSC measurements at high temperatures were performed. Heating and cooling curves in the temperature range from RT up to 1350 • C are shown in Figure 5. For better visualization of the possible phase transformations in the studied alloy also, derivatives versus temperature are shown (inset- Figure 5).

HT DSC Measurements
In order to check the stability of phases in the studied alloy, DSC measurements at high temperatures were performed. Heating and cooling curves in the temperature range from RT up to 1350 °C are shown in Figure 5. For better visualization of the possible phase transformations in the studied alloy also, derivatives versus temperature are shown (inset- Figure 5). It could be seen that the heating/cooling curves showed no phase transformation nor other thermal effects in the studied temperature range. The obtained results suggest the structural stability of the studied HEA alloy from RT up to 1350 °C. The reversible slope change (also visible on the derivative) on the DSC signal above 600 °C may suggest a change in the heat capacity of the material. This could be a result of a small amount of the amorphous regions in the material and small enthalpy of crystallization.

Mechanical Properties
Nanohardness tests were performed by the nanoindentation technique using a three-sided pyramid Berkovich tip. Sample polishing by SiO2 suspension caused a slight etching of the sample surface and revealed the "dendrites" and "light-grey" phases. Topography images (10-μm × 10-μm area for the dendritic-like phase and 40 μm × 40 μm for the "light-grey" phase) were recorded using scanning mode. The nanoindentation points are labeled in the topography images ( Figure 6). One It could be seen that the heating/cooling curves showed no phase transformation nor other thermal effects in the studied temperature range. The obtained results suggest the structural stability of the studied HEA alloy from RT up to 1350 • C. The reversible slope change (also visible on the derivative) on the DSC signal above 600 • C may suggest a change in the heat capacity of the material. This could be a result of a small amount of the amorphous regions in the material and small enthalpy of crystallization.

Mechanical Properties
Nanohardness tests were performed by the nanoindentation technique using a three-sided pyramid Berkovich tip. Sample polishing by SiO 2 suspension caused a slight etching of the sample surface and revealed the "dendrites" and "light-grey" phases. Topography images (10-µm × 10-µm area for the dendritic-like phase and 40 µm × 40 µm for the "light-grey" phase) were recorded using scanning mode. The nanoindentation points are labeled in the topography images ( Figure 6). One can see that, for both observed phases, there are the traces of the tip inside the circles (Figure 6). For the microhardness and reduced Young's modulus calculations, only unambiguous indents belonging were taken into account. The obtained values of nanohardness for the dendrites and "light-grey" phase are HDendrites = 13(2) GPa and HLight-grey = 18(1) GPa, respectively. The reduced Young's modulus for the dendriticlike and "light-grey" phases are ErDendrites = 185(23) GPa and ErLight-grey = 194(9) GPa, respectively. Both the studied phases differed distinctly in the nanohardness values, whereas the obtained values of the reduced Young's modulus are similar within the standard deviation. The obtained nanohardness and reduced Young's modulus values were compared with the literature data and presented in Table 5.  The obtained values of nanohardness for the dendrites and "light-grey" phase are H Dendrites = 13(2) GPa and H Light-grey = 18(1) GPa, respectively. The reduced Young's modulus for the dendritic-like and "light-grey" phases are Er Dendrites = 185(23) GPa and Er Light-grey = 194(9) GPa, respectively. Both the studied phases differed distinctly in the nanohardness values, whereas the obtained values of the reduced Young's modulus are similar within the standard deviation. The obtained nanohardness and reduced Young's modulus values were compared with the literature data and presented in Table 5.  The reduced Young's modulus is comparable to the other high entropy alloys reported in the literature, like CoCrFeNiAl 0.3 [49]. Nevertheless, the silicon and yttrium are present in all the phases observed in the microstructure and can cause a significant increase in hardness of the obtained Co 15 Cr 15 Mo 25 Si 15 Y 15 Zr 15 high entropy alloy. Such a phenomenon was also reported for the CoCrFeNiY 0.3 alloy [51], where a 3% addition of Y increased both the nanohardness and Young's modulus. For Si containing a Tribaloy (Co-Cr-Mo-Si) [47], it was revealed that a 1% Si addition also significantly increased the nanohardness and Young's modulus.
Due to the presence of a multiphase structure and amorphous regions, the studied material is brittle. This can be a drawback for their potential applications. However, the amorphous phase could be achieved by changes in the chemical composition, such as: increasing the concentration of Si and Y and their amorphization abilities in comparison to other alloying elements. On the other hand, modification of the production method in order to reach the critical cooling rate could increase the probability of the formation of the amorphous phase.

Conclusions
The six-elemental high entropy Co 15 Cr 15 Mo 25 Si 15 Y 15 Zr 15 alloy was obtained using the vacuum arc melting technique. The XRD phase analysis revealed the multiphase structure of the alloy, including the presence of the amorphous phase. SEM EDS and TEM EDS chemical analysis revealed that all six alloying elements are present in all the observed phases. The existence of amorphous regions was confirmed by TEM analysis. The TEM phase analysis of ring electron diffraction patterns confirmed XRD phase analysis. HT DSC measurements did not reveal any additional phase transformation during the heating (and cooling) up to 1350 • C. The solid-state crystallization process was also not observed due to a relatively low amount of the amorphous phase. The mechanical tests revealed an increase of the reduced Young's modulus and nanohardness, which were caused by the Si and Y additions. The obtained results are comparable to data reported for the other high entropy alloys.
Nonetheless, the presented results showed that the examined alloy is a good candidate for metal glass material.