The New Complex High-entropy Metal Boron Carbonitride: Microstructure and Mechanical Properties


 Dense (Ta, Nb, Hf, Zr, Ti)(BCN) ceramics (HEC-2) were successfully synthesized via hot-pressing sintering. Results show that sintering temperature and (BCN) addition have great impacts on the microstructure and mechanical properties of HEC-2 ceramics. The microstructure and phase of HEC-2 bulk ceramics were characterized by SEM, XRD and TEM. It was found that high-entropy phase, oxide phase and BN/C phase were precipitated when HEC-2119 powders were sintered at 1900 oC. In addition, the Vicker’s hardness, Bending strength, Bulk density and Open porosity of HEC-2319 ceramic are 24.54 GPa, 522.00 MPa, 9.07 g/cm3 and 0.05%, respectively, whose molar ratio of metal to (BCN) is 1: 0.334. In this work, BCN high-entropy ceramics were sintered on the basis of powder preparation, as provided that sintering temperatures and components have great influences on microstructure and mechanical properties of HEC-2 ceramics.


Introduction
It has been about 20 years since the rst article about the high-entropy alloys (HEAs) reported, and then the studies on preparations and properties of HEAs are increasing gradually [1][2][3]. On the basis of the research of HEAs, what prepared by doping different nonmetallic elements, such as Si, C, N, B, etc., has good mechanical properties on the advantage of light weight and great application prospects in the elds of aerospace, electronic parts, transportation and etc [4][5]. Under the studies of excellent properties of HEAs, many scholars introduced the concept of high entropy into the ceramics, expecting to design and synthesize high-performance high-entropy ceramics (HECs). In 2015, Rost et al. [6] prepared high-entropy oxide ceramics and proved the importance of entropy on phases stability for the rst time. Since then, the research upsurge of high-entropy ceramics started. Previous researches on HECs mainly focused on highentropy nitride coating [7][8], high-entropy carbides [9][10][11] and diborides ultra-high temperature ceramics [12][13][14][15][16], wear-resistant high-entropy silicides [17][18], high-entropy oxide functional materials [19][20] and other single-anion HECs [21][22]. Obviously, the hardness and creep resistance of HECs have been greatly improved than the unit and the binary ceramics, because more components are introduced into ceramics and the cation sizes are quite different which causes the solution strengthening. What's more, when solid solution is formed, it is easy to produce a lot of lattice distortion, which increases the resistance of dislocation movement and makes it very di cult for atoms to slip under external force and then limits the dislocation movement necessary for plastic deformation [23][24].
Castle E et al. [9] prepared (Hf-Ta-Zr-Ti)C and (Hf-Ta-Zr-Nb)C single-phase solid solutions by SPS sintering. It was found that lattice mismatch of monobasic carbide components was the key factor to form single-phase solid solution. Test results showed that the hardness of high-entropy carbide was 36.1 ± 1.6 GPa, which was higher than that of monobasic carbide and binary carbide of corresponding metal elements. Tallarita G et al. [14][15] prepared high-entropy diboride (Hf 0.2 Mo 0.2 Ta 0.2 Nb 0.2 Ti 0.2 )B 2 whose density was 92.5% by two-step method. XRD test showed that a high-entropy diboride single-phase solid solution was obtained, and the structure of diboride was similar to that proposed by Gild J [12], which was a layered structure with alternating boron atomic layers and metal layers. Performance test showed that the hardness of high-entropy phase was 22.9 GPa which was higher than that of single diboride. It was considered that the effects of preparing powder by self-propagating method was better than that by high-energy ball-milling. Mayrhofer PH et al. [25] prepared ZrB 2 , (Zr,Ti)B 2 and high-entropy diboride (Zr 0.23 Ti 0.20 Hf 0.19 V 0.14 Ta 0.24 )B 2 (HEB 2 ) by non-reactive magnetron sputtering physical vapor deposition.
It was found that not only the hardness was gradually improved, but also the thermal stability of structural rearrangement and decomposition into binary diboride was improved with the increase of the amount of metal elements. The hardness of HEB 2 was 47.2 ± 1.8 GPa which was harder than (Zr,Ti)B 2 (H = 45.8 ± 1.0 GPa) and ZrB 2 (H = 43.2 ± 1.0 GPa).
Recently, on the basis of single-anion high-entropy ceramics, researchers broke the limitation of singleanion high-entropy ceramics and prepared binary or ternary-anion high-entropy ceramics [26][27][28][29][30] [27] studied the multi-anion high-entropy aluminum-silicide (Mo 0.25 Nb 0.25 Ta 0.25 V 0.25 )(Al 0.5 Si 0.5 ) 2 . They analyzed the possibility of forming high-entropy aluminosilicate based on rst-principles calculation from two aspects of chemical reaction thermodynamics and lattice size difference, and then successfully prepared high-entropy alumino-silicate at 1573 K by solidstate reaction technology. The results showed that the high-entropy aluminum-silicate had the hexagonal crystal structure of single-metal aluminum-silicate, and the distribution of all constituent elements was highly uniform. Chen H et al. [31] prepared several high-entropy ceramics powders that larger lattice strain along with considerable amount of Al vacancies. And the Vickers hardness of the composite was measured to be 11.1 ± 1.1 GPa, but the fracture toughness was 3.7 ± 0.4 MPa·m 1/2 , which was lower than that of the single M 2 AlC phase. But the speci c mechanism of the atomic solid solution diffusion has not been studied.
In this study, under the research of single and binary anion high-entropy ceramics, we try to prepare BCN series high-entropy ceramics with ternary anions by high-energy ball-milling [35], and synthesis (Ta 0.2 Nb 0.2 Zr 0.2 Hf 0.2 Ti 0.2 )(BCN) (HEC-2) bulk ceramics by hot-pressing sintering. The effects of sintering temperature and molar ratio of metal to nonmetal on microstructure and mechanical properties have been studied. The preparation of powder by high-energy ball-milling is carried out in two steps, and the speci c ball milling process could refer to previous reports [35]. For different systems, what is xed is the ball-milling process, including the ball-milling time (24 h), rotation speed (600 rpm) and ball-material ratio (20: 1), and changes are corresponding metal types and nonmetal contents of the (BCN). Seven species high-entropy ceramics powders are prepared according to the Table 1.

Sintering process
The sintered powders were put in a glove box in argon atmosphere lled into a graphite mold with a diameter of 30 mm. After pre-pressing forming, the ceramic block was sintered by hot-pressing sintering. The speci c sintering process is as follows: maintaining temperature and pressure for 60 min at preset temperature after heating at 20 K/min in Ar atmosphere, then releasing pressure slowly and taking it out after furnace cooling to room temperature. The speci c sintering curve is shown in Fig. 1 below. We chose HEC-2 as the research object and analyze its sintering process, microstructure and mechanical characters and the speci c sintering curve is shown in Table 2. What's more, in order to study the effects of BCN content on ceramics, we synthesized three ceramics with different molar ratio between metal and (BCN) as 1: 1, 1:0.5 and 1: 0.334 named HEC-21, HEC-22 and HEC-23, respectively.
The high-temperature thermal stability of the powder was measured by STA449C comprehensive thermal analyzer (TG-DSC) produced by NETCHI, Germany (heating rate 10 ~ 20K, heating to 1400 o C, argon atmosphere). Density is determined by Archimedes drainage method, and the sample mass is determined by DV314C analytical balance produced by Ohaus Company of America, and its density is determined according to Eq. (1): In which, ρ v is the bulk density of ceramic samples, g/cm 3 ; ρ H2O is the density of deionized water at room temperature, g/cm 3 ; m air is the mass of ceramic sample in air, g; m H2O is the mass of ceramic sample in deionized water. The open porosity is calculated by Eq. (2): Where, π α is the open porosity, %; m wet is the weight of ceramic sample after absorbing water, g. In terms of mechanical properties test, the Vickers hardness was tested by HVS-5 microhardness tester produced by Laizhou Testing Machine Factory, Shandong Province (the load was 1 kg and the holding time was 15 s), and the Bending strength and Elastic modulus were tested by Instron-5569 electronic universal material testing machine produced by Instron Company, USA (the sample was cut into strips with a cross section of 3 mm × 4 mm and the platform span was 20 mm).

Microstructures of HEC powders
After high-energy ball-milling, all of seven system powders became Face-centered cubic (Fcc) solid solution as seen in Fig. 2, and the related studies on the microstructure about HEC-1, HEC-2 and HEC-3 powders have been introduced in our previous reports [35]. As far as we know, the formation of solid solution is closely related to size factor and atomic electronegativity difference. Because the radius ratios of B, C, N to metal atoms (r x /r M ) (X = B, C, N; M = Ta, Nb, Hf, Zr, Ti, …) are all less than 0.59, it is possible for B, C, N atoms to form interstitial compound (MX) with transition metals. Moreover, the interstitial compounds with the same structure are promoted to form in nite solid solution, because the radius differences of metal atoms are less than 15%. Specially, the solid solution process of atoms still needs further study, which is one of the key points to our next work.
According to the mixing rule and Bragg's law, HEC-6 system has the minimum average metal atom size, the minimum interplanar distance (the speci c diffraction angle can be seen in Table 3) and the largest crystal plane diffraction angle, which is consistent with the XRD results in Fig. 2. The diffraction angle of (111) crystal plane decreases in the wake of the average metal atom size increasing, which can be seen from the line chart intuitively in Fig. 3. Obviously, after high-energy ball-milling, the mixed powders agglomerated, and all kinds of powders are similar to sphere as seen in Fig. 4  Compared with the values of weight loss from HEC-1 to HEC-7, HEC-2 has the lest weight loss and the values are shown in Table 4. Therefore, we choose HEC-2 system as a specimen to study the subsequent sintering process. The speci c sintering process has been introduced in Sect. 2.2.

Microstructures of HEC bulk ceramics
After hot-pressing sintering at 1900 o C, the HEC-2 mainly has a Fcc phase, followed by a small amount of oxide impurities and BN(C) phase (Fig. 6 (c)). XRD pattern shows that the crystallinity of Fcc phase increases with sintering temperature increasing, at the same time the FWHM of XRD diffraction peak decreases and the diffraction intensity increases (Fig. 6 (a), (b) and (c)). What's more, the amorphous BCN phase crystallized forming BN/C phase when heating to1900 o C. In Ref.
[35], it was found that oxides formed when the powders heated at a lower temperature (1100 o C) in argon atmosphere, because oxygen impurities were inevitably brought into the powder preparation process by adopting high-energy ball-milling. Besides, the use of high-activity metal elemental as raw material powders also made it easier to introduce oxygen impurities. With the increase of sintering temperature (> 1700 o C), the oxides have a phase transformation from monoclinic phase to high-temperature stable tetragonal phase which is consistent with the existing researches [36-37]. In addition, when the sintering temperature reached 1900 o C, a small amount of BN/C precipitated.
The microcracks and holes on the ceramic surfaces indicate that the ceramics are not completely dense (Fig. 7). It can be seen from the fracture morphology of the ceramics that when the sintering temperature is 1700 o C, the ceramics are composed of nano-sized small particles, and no obvious cracks or holes appear on the fracture section and particles are intact on section in Fig. 8. When the sintering temperature is up to 1800 o C, small particles connect and there are clearly cracks on section. When sintering temperature is 1900 o C, there is obvious lamellar phase in ceramics. According to XRD analysis, the lamellar substance is BN/C phase, and the fracture surface has obvious traces of lamellar tearing and large particles pulling out with sintering temperature increasing.
In order to further analyze the microstructures characteristics, HEC-2119 ceramic after hot-pressing sintering was tested by TEM (Fig. 9). Based on the electron diffraction spots, the position 1, 2, 3 and 4 are the Fcc phase, the oxide phase, the mixture of amorphous phase and nanocrystalline and the BN/C phase, respectively. It can be seen from the EDS patterns that all elements in the Fcc high-entropy phase are uniformly distributed, but the solid solubility of N in the high-entropy phase is far less than others, whose atomic size is the smallest and which mainly concentrates in BN phase. One of the reasons for that situation is probably that besides atomic size, electronegativities of atoms also have a great in uence on solubility. Compared with C and B, the electronegativity difference between N and transition metal is the largest, causing the smallest solubility.
We speculate that oxides are solid solutions composed of hafnium, niobium, zirconium and oxide based on the EDS results, which can be expressed as (Hf, Nb x , Zr y )O z . By compared the content of metal atoms between oxide phase and high-entropy phase, it can be found that the hafnium and zirconium contents of former are higher than latter, while it is opposite for Nb elements. By studying the oxidation sequence of elements in subgroup and under the condition of high entropy, Backman L et al [38] found that the predatory ability to oxygen of metal element is Hf > Zr > Ti > Ta > Nb, so Hf and Zr atoms are oxidized rst. However, Nb atoms with the least oxidation tendency are oxidized, which may be due to the great solubility of oxygen. Due to the limitation of oxygen content, Nb atoms are not oxidized into the most stable Nb 2 O 5 , but exist in the form of NbO 2 . According to the principle of similar compatibility, HfO 2 , ZrO 2 and NbO 2 are dissolved into single solid-solution oxides because of the same crystal structure. It is worth noting that ZrO 2 has a phase transformation from monoclinic-phase to tetragonal-phase when temperature is higher than 1170 o C [39], and changes to monoclinic-phase back when the temperature decreases. But that phenomenon is not observed during the sintering process of HEC-21 powders, we speculate that the formation of oxide phase is based on HfO 2 structure. It proves once again that HEC-21 is a multiphase ceramic mixed with high-entropy phase, oxides and BN/C phase sintered at 1900 o C, 60 MPa in argon atmosphere.

Effects of nonmetal content on phase and microstructure
The addition amount of BCN has a certain in uence on phase and microstructure. Besides high-entropy phase and oxides phase, hexagonal phase M i B 2 was precipitated with the decrease of BCN, while the diffraction peak of BN/C phase disappeared (Fig. 6 (c), (d) and (e)). It can be seen from the fracture morphology that the lamellar BN/C gradually decreases with the decrease of BCN, and there is basically no lamellar phase in HEC-23, which is consistent with the results of XRD patterns (Fig. 6 (e)). Although there is a small amount of layered phase in HEC-22, it is not shown in Fig. 6 (d) because of its little content.
From the fracture section (Fig. 10), the fracture morphology of HEC-21 ceramic is composed of a large number of randomly oriented lamellar BN/C and a small number of intact particles covered by lamellar.
The fracture morphology of HEC-22 ceramic is mainly composed of intact particles and a small number of lamellar BN/C. The fracture morphology of HEC-23 ceramic with the least BCN content is composed of a large number of intact grains and a small number of grains with tearing characteristic, and there is substantially no lamellar phase. Compared with the grain size, it is found that with the decrease of BCN content, the grain size increases gradually. The main reason is that BN/C distribution among particles can effectively hinder grain growth.

Mechanical properties of HEC-2 bulk ceramics
With the sintering temperature increasing, the density and exural strength increase gradually, and the surface porosity decreases. The speci c values are in Table 5. After sintering at 1900 o C, the exural strength of HEC-2119 is as high as 495.82 MPa. In addition, the Vickers hardness increases rst and then decreases with the increase of sintering temperature (Fig. 11). The Vickers hardness of HEC-2118 ceramic is 7.57 GPa, and the hardness of HEC-2119 is 5.56 GPa. The reason for this variation tendency is that the ceramic form lamellar BN/C at 1900 o C that is easy to slip between lamellae, leading to the decrease of Vickers hardness. With the decrease of BCN content, the density, hardness and bending strength increase, but the surface porosity decreases. To a certain extent, the decrease of porosity indicates the increase of density, which is due to the high BCN content in HEC-21 ceramic, which effectively hinders the grain growth. The decrease of BCN content in HEC-22 ceramic makes the grain size larger than the former, and the increased grain squeezes lamellar to increase its density. There is no lamellar BN/C phase in HEC-23 ceramic to hinder the grain growth, which makes the grain size of this system the largest. The Vickers hardness, bending strength and open porosity of HEC-2319 are 24.54 GPa, 522.00 MPa and 0.05%, respectively. When BCN content decreases, the precipitation amount of lamellar BN/C phase decreases, and the proportion of high-entropy phase increases, which effectively improves its Vickers hardness and bulk density, and the speci c change of hardness is shown in Fig. 12.
From the fracture section ( Fig. 8 and Fig. 10), the change of phase and structure leads to the change of fracture mode. The fracture modes of HEC-21 ceramics are mainly lamellar tearing and intergranular fracture. The fracture modes of HEC-22 ceramics are lamellar pull-out and intergranular fracture. The fracture modes of HEC-23 ceramics with the least BCN content are intergranular fracture and transgranular fracture. In addition, compared with sintering temperature, BCN content has less in uence on the bending strength.

Conclusions
Compact high-entropy metal boron carbonitride HEC-21 ceramics were prepared at 1700 o C, 1800 o C and 1900 o C, respectively. Besides the face-centered cubic high-entropy phase, there are oxides and lamellar BN/C phases in HEC-2119. As the temperature increases, the particle size, bending strength and density increase, but the hardness increases at rst and then decreases. The mechanical properties of HEC-2 ceramic sintered at 1900 o C and 60 MPa show that the hardness, density and bending strength increase as the BCN content decreases.  Tables   Table 1 Ingredients of HEC-1~7 powders.   Table 3 Diffraction angles of (111) crystal plane.   Table 5 Mechanical properties of HEC-2 bulk ceramics sintered at different temperatures.     Vicker's Hardness of HEC-21 ceramics sintered at different temperatures.