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Gradient microstructure development and grain growth inhibition in high-entropy carbide ceramics prepared by reactive spark plasma sintering
Journal of the European Ceramic Society xxx (xxxx) xxx–xxx
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Original Article
Gradient microstructure development and grain growth inhibition in highentropy carbide ceramics prepared by reactive spark plasma sintering Xiao-Feng Wei, Yuan Qin, Ji-Xuan Liu*, Fei Li, Yong-Cheng Liang, Guo-Jun Zhang* State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Materials Science and Engineering, College of Science, Institute of Functional Materials, Donghua University, Shanghai 201620, China
A R T I C LE I N FO
A B S T R A C T
Keywords: High-entropy carbide High-entropy carbide based composites Gradient microstructure Grain growth Hardness
High-entropy carbide ceramics (Ti0.2Hf0.2Nb0.2Ta0.2W0.2)C is prepared from five transition metal oxides and graphite by reactive spark plasma sintering. X-ray diffraction indicates the synthesized ceramics with the singlephase face-centered cubic structure. The elemental distribution maps by energy dispersive spectroscopy demonstrate homogeneous distribution of the five metal elements in both central and circumferential regions of the sample. SEM and corresponding back scattered electron observations show the residual graphite particles locating at the grain boundaries of high-entropy carbide ceramics. Moreover, the content of the residual graphite decreases and the grain size of the high-entropy carbide phase increases from central to circumferential region of the sample. Thermodynamic calculation results indicate that gradient gas pressure inside the sample affects the carbothermal reduction reactions during sintering and consequently results in the existence of residual graphite with gradient distribution feature. This study points out an effective way to inhibit the grain growth of highentropy carbide phase during sintering process by the incorporation of graphite as the second phase particles acting as grain growth inhibitor.
1. Introduction High-entropy alloys (HEAs) have been widely investigated because of their excellent physical and chemical properties, unique compositions, adjustable properties and attractive potential industrial applications [1,2]. Recently, high-entropy ceramics including oxides [3–6], nitrides [7,8], carbides [9–11], diborides [12–14], silicides [15,16] and single-phase metal oxycarbonitride ceramics [17] have also been successfully manufactured and characterized. In general, four or five transition metal compounds such as metal oxides, carbides and diborides were usually used as the raw powders to synthesize the corresponding high-entropy ceramics. These are traditional and straightforward methods to prepare solid solution compounds [18]. Same as ZrC and HfC ceramics prepared by two-step method, dense high-entropy carbide ceramics can be fabricated by pressureless carbothermal reduction reaction in the first step [19] and followed by the second step of pressure-assisted sintering [13,20]. As for high-entropy carbides (HECs), it may "inherits" the high melting point, high hardness, high chemical inertness, high resistance to irradiation, etc. of carbide-based ultra-high temperature ceramics (UHTCs) to a certain extent or even exhibits new behaviors [21]. Accordingly, HECs is possible candidate materials for aerospace applications, nuclear reactors, cutting tools, etc.
⁎
SPS is often used to prepare high-entropy carbide and diboride ceramics thanks to its attractive advantages including high thermo-efficiency, improved sintering activity, fast sintering under low temperatures [13,20–24]. The rapid sintering process will effectively prevent the grain growth and refine material microstructures [25], resulting in strength improvement according to the Hall-Patch relationship [26,27]. In addition, reactive SPS that involves certain chemical reactions during sintering is an effective SPS approach to fabricate materials, such as monolithic carbides and HECs [27–30]. In our previous work, (Ti0.2Zr0.2Nb0.2Ta0.2W0.2)C ceramics were prepared by three different typical ceramic processing, including carbide process by SPS, element process and oxide process by reactive SPS [22]. The specimen synthesized by oxides and graphite has a two-phase structure, one is HEC phase and the other one is zirconium-rich phase. In addition, a small amount of residual carbon was observed in HEC-O sample, especially in the central area. However, it is considered that a number of complex factors resulted in the above microstructure features, including the complex formation process of HEC phase, high reduction temperature of ZrO2 as well as the gradient temperature and gas pressure in the specimen during SPS process. In this study, oxide process by reactive SPS was used to prepare high-entropy carbide ceramics, but for
Corresponding authors. E-mail addresses: [email protected] (J.-X. Liu), [email protected] (G.-J. Zhang).
https://doi.org/10.1016/j.jeurceramsoc.2019.12.034 Received 8 October 2019; Received in revised form 15 December 2019; Accepted 17 December 2019 0955-2219/ © 2019 Elsevier Ltd. All rights reserved.
Please cite this article as: Xiao-Feng Wei, et al., Journal of the European Ceramic Society, https://doi.org/10.1016/j.jeurceramsoc.2019.12.034
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for graphite (JCPDS NO: 41-1487). This indicates that the residual carbon in the central area of the specimen is graphite. While no graphite diffraction peak is observed in Fig. 1(2). It is considered that the graphite content in circumferential area of the specimen should be relatively lower, so diffraction peaks cannot be detected. The theoretical density of HEC (11.28 g/cm3) was calculated from the average mass of the atoms in a unit cell and the XRD measured lattice parameters [22]. The relative density of the sintered whole sample was 95.5 %. The tested residual oxygen content of the sintered ceramics (half of the specimen was cut and measured) was only 0.028 %, indicating the carbothermal reduction reactions between the oxides and graphite during SPS have been completed.
avoiding the formation of zirconium-rich phase, element zirconium was replaced by hafnium. The results showed that a single-phase high-entropy carbide ceramics (Ti0.2Hf0.2Nb0.2Ta0.2W0.2)C was prepared via the reactive SPS through the reaction between oxides and graphite. The microscopic structures and EDS mappings in different regions of this single-phase ceramics are carefully observed and analyzed. The results demonstrate that more homogeneous distributions of metallic elements in the synthesized HEC phase can be obtained by the replacement of zirconium with hafnium. In addition, an uncommon gradient microstructure is found, and the formation mechanism is discussed based on thermodynamic analysis. 2. Experimental
3.2. Gradient microstructural feature To prepare (Ti0.2Hf0.2Nb0.2Ta0.2W0.2)C ceramics, graphite (≥99 % pure, particle size ∼ 1.5 μm, Shanghai Graphite Company, China) and oxide powders including TiO2 (≥99.99 % pure, particle size ∼ 0.1 μm) from Shanghai Titan Technology Co., Ltd. China, HfO2 (≥99.5 % pure, particle size ∼ 0.2 μm), Ta2O5 (≥99. 5 % pure, particle size ∼ 1.5 μm) as well as Nb2O5 (≥99.8 % pure, particle size ∼ 2 μm) provided by Zhuzhou Cemented Carbide Group Co., Ltd. China and WO3 (≥99.8 % pure, particle size ∼ 0.1 μm, Yangzhou Sanhe Chemical Co., Ltd. China) were used as the raw materials. The processing of the (Ti0.2Hf0.2Nb0.2Ta0.2W0.2)C ceramics is the same as the oxides mothed described in our previous publication [22]. The starting powders were mixed by planetary ball mill at 560 r/min for 10 h with WC balls in ethanol milling media. The ball-to-powder ratio was 6:1 in weight. After drying and sieving, the powder mixtures were directly densified by reactive SPS (HP D 25, FCT, Germany). In order to reduce the impediment to the gas-releasing reaction between oxides and graphite, the samples were firstly heated to 1800 °C and held for 10 min in vacuum under 6.4 MPa, which is the minimum axial pressure of the machine. Then, the pressure was increased to 50 MPa and the temperature was raised to 2000 °C and held for 5 min to get dense HEC samples. The sample size was 20 mm in diameter and about 2.5 mm in thickness. Surface layers about 0.5 mm in thickness of the as-sintered samples were removed by grinding and polishing. The densities of the sintered samples were measured by Archimedes’ method. The phase composition of the mixed powders and the as-sintered ceramics were identified by X-ray diffraction (XRD, D/max-2550VB+/PC, Japan) using Cu-Kα radiation. Polycrystal silicon was used as the internal standard to calibrate the XRD peak positions [31]. The lattice parameters of the assintered samples were calculated based on the XRD data via Jade [11]. Microstructural characterization of the samples was observed by field emission scanning electron microscope (SEM) and corresponding back scattered electron (BSE) (MAIA3, TESCAN, Czech Republic) with energy dispersive spectroscopy (EDS). The residual oxygen content was analyzed by Oxygen/Nitrogen Analyzer (TC600C, Leco, USA). The Vickers hardness of the sample was measured by the Vickers Hardness Tester (HXD-1000TM/LCD, Taiming, China) with a load of 4.9 N and a dwell time of 10 s.
The SEM images of the fracture and polished surfaces of the sample are shown in Fig. 2. The fracture mode of the HEC ceramics is mainly transgranular dominated as shown in Fig. 2 (a) and (b), which is very similar to that in the previous reported (Ti0.2Zr0.2Nb0.2Ta0.2W0.2)C high-entropy ceramics [22]. Based on the SEM and BSE images, the following findings are needed to discuss. Firstly, graphite phase with dark contrast which generally located at the grain boundaries of HEC was observed in the sintered sample (marked by white arrows in Fig. 2 (c) and (e) and enlarged in Fig. 2 (g)). Besides, a small number of closed pores (marked by red arrows in Fig. 2 (c) and (e)) were also observed in the central area of the sintered sample. In contrast, the contents of graphite and closed pores in the circumferential region are less as shown in Fig. 2. This is in accordance with the results of XRD in Fig. 1. In addition, due to the different thermal contraction between graphite and HEC (here the thermal expansion coefficient of HEC is assumed to be approximate to those of the corresponding monolithic carbide ceramics), interfacial pores appears at the interfaces between graphite and HEC grains during cooling from the sintering temperature, as shown in the enlarged image in Fig. 2 (a) and (g). Secondly, the grain size of HEC in the central area (Fig. 2 (e)) is obviously smaller than that in circumferential area (Fig. 2 (f)). Previous investigations have revealed that the presence of the second phase particles can inhibit grain growth of the matrix phase. Based on Zener pinning theory, the average grain size of the matrix phase is closely related to the particle size, shape and content of the second phase [32,33]. In two-phase materials, the relationship can be summarized as Eq. (1) [34,35]:
D∝
d f
(1)
where D is the average grain size of the matrix phase, d is the average particle diameter of the second phase, and f is the volume friction of the second phase. The graphite content in the central area of the sample is significantly higher, and the grain size of HEC is smaller, which is consistent with Zener pinning theory. It is obvious that the presence of graphite in the grain boundaries severely limited the growth of HEC grains, similar to the phenomena reported in literature [36,37]. Moreover, a number of small-sized graphite particles are entrapped in the HEC grains in the circumferential area of the sample as shown in Fig. 2 (f). It is considered that a faster grain growth of the HEC phase in the circumferential area resulted in some small graphite particles entrapped in the HEC grains [13]. The compositional EDS mappings indicate the homogeneous distribution of five metal elements in both central and circumferential areas of the sample as shown in Fig. 3 (a) and (b). The carbon-rich areas marked by white arrows in Fig. 3 (a) are consistent with the graphite distribution in Fig. 2. The atomic percentages of elements in both central and circumferential areas of HEC are listed in Table 1. The atomic percentage data indicate the uniform distribution of the metallic elements in the material. The results further confirm that a single-phase high-entropy carbide with similar elemental composition in both central and circumferential areas of the sample is formed, which is in
3. Results and discussion 3.1. Phase analysis Fig. 1 shows the XRD pattern of the starting powder mixture and the sintered ceramics. The circumferential and central areas of the sintered sample show nearly the same diffraction patterns which are similar to the diffraction pattern of previous reported HEC phase [22]. This indicates that single-phase HEC with rock salt structure and a nominal composition of (Ti0.2Hf0.2Nb0.2Ta0.2W0.2)C was successfully synthesized by SPS and the phase composition of the circumferential and central areas of the sample is consistent. Moreover, a small diffraction peak is observed around 26.5° in Fig. 1 (a) (3). The XRD patterns of the 2θ from 26.2° to 27° are enlarged and the results indicate that the small peak is 2
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Fig. 1. Phase composition analysis. (a) XRD patterns of (1) mixed starting powders; (2) circumferential area of the specimen; and (3) central area of the specimen; (b) enlarged XRD patterns of the 2θ from 26.2° to 27°.
and the content of residual graphite. The residual graphite particles act as the second phase particles inhibiting the grain growth of the HEC matrix phase. The hardness of different regions in the gradient material is determined by the synergetic effects of the content of graphite phase and grain size of HEC. As shown in Fig. 4(b), the region containing ∼4 vol.% graphite phase exhibits the highest hardness, and this graphite content can be considered as the limit for the balance between the grain refinement and mechanical properties, which is similar to the literature reported monolithic carbide-graphite materials [38,41]. The results point out a new approach to design fine-grained HEC with improved mechanical properties. Besides, it is reported that high-entropy ceramics with severe lattice distortion demonstrate better irradiation properties, and the interfacial nano-sized pores between HEC and graphite would provide spaces for capturing irradiation gaseous products [11,42,43]. Hence HEC-graphite composites could be a candidate material for nuclear applications.
accordance with the results of XRD analysis. In order to further investigate the microstructure evolution along the radial direction of the sintered ceramics, the sample was analyzed by dividing into ten rings in the radial direction with 1 mm wide as shown in Fig. 4 (a). The amount of residual graphite was estimated based on the area fraction of graphite phase in each ring. More than two hundred HEC grains were measured in the polished surface of each ring to get the average grain size of the HEC phase. The measured amount of residual graphite and average grain size of HEC in each ring is shown in Fig. 4 (b). It can be seen that both the content of residual graphite and the average grain size of HEC phase exhibits gradient distributions along the radial direction in the sintered sample. From the central to the circumferential regions, the content of residual graphite decrease slowly while the average grain size of HEC phase increased gradually. The evolution of hardness along the radial direction of the sintered ceramics was also investigated. Vickers hardness in each ring was measured and the reported value was an average of 10 tests. As for the hardness, it is generally influenced by porosity, grain size and the hardness of the second phase [38,39]. Samples with higher density, finer grain size and harder second phase will have higher hardness [35,36]. The hardness of graphite is much lower than that of HECs [22]. Thus, the hardness of the specimen in the central area is remarkably reduced due to the existence of higher graphite content (as well as the interfacial pores between graphite and HEC phase). An increase tendency in hardness can be observed with decrease of residual amount of graphite and reaches a maximum hardness at the distance of 7 mm where the amount of residual graphite is about 4 vol.% and the grain size of HEC is about 12 μm. In these areas, the hardness of the material is mainly controlled by the content of graphite. However, the grain size of HEC increases rapidly when the distance is greater than 7 mm, which results in the slight decrease of the hardness when comparing with the maximum hardness as shown in Fig. 4(b). According to the Hall-Petch relationship, the hardness of ceramics generally decreases with increasing grain size, as showed in Eq. (2) [38,40]:
Hv= H0 + kD−1/2
3.3. Mechanism for the formation of gradient microstructures Thermodynamic calculation by thermodynamic software (HSC chemistry 6, Outotec, Finland) for the circumferential area and the central area of the specimen are discussed in order to understand the formation mechanism of the gradient microstructure. Carbothermal reduction reactions (3)∼(12) between oxides and graphite occurred during sintering, which resulted in the five carbide resultants and released CO and CO2 gases [29,43]. The synthesized transition metal carbides subsequently solidify to form a single-phase high-entropy carbide [19,44]. The Gibbs’ free energy of chemical reactions (3) to (12) under 10 Pa (vacuum degree in the furnace) were calculated as shown in Fig. 5 (a). Gibbs’ free energy of these reactions decrease with the increase of temperature, and all of them are less than zero before the temperature reaching to 1800 °C. Obviously, the ΔG values of reactions (3) to (7) are lower than those of reaction (8) to (12) at higher temperature. The generation of CO and CO2 gases should cause a rapid increase in gas pressure inside the sample. The generated gas was easily diffused along the open pores but could only escape along the gap between the sample and the mold. Therefore, a gradient gas pressure from central to circumferential region of the sample would be formed during the chemical reactions. A higher gas pressure appeared in the central region, while a lower gas pressure existed in the circumferential region. Complex gradient gas pressure in the mold would result in complex and different conditions, and then affect the chemical reactions and even densification in the sample at different positions. Reaction (3) to (7) can be concluded as reaction (13) and reaction (8) to (12) can be concluded as reaction (14). Reaction (15) is obtained by the subtraction
(2)
However, in the near circumferential area, factors that affect hardness include not only the larger grain size but also the lower content of graphite, closed pores and interfacial pores in the material. Therefore, when the distance is greater than 7 mm, the hardness of the material keeps almost unchanged. It should be noted that changing the size and geometry of the graphite die and sintering conditions, such specific distance value will be changed. However, a similar gradient microstructure is believed to exist. Based on the aforementioned discussions, the obtained ceramics in this work exhibits a gradient microstructure in grain size of HEC phase 3
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Fig. 2. SEM images of the sintered ceramics. (a) and (b) are the fracture surfaces of central area and circumferential area, respectively; (c) and (d) are polished surfaces of central area and circumferential area, respectively; (e) and (f) are the corresponding BSE images of (c) and (d); (g) and (h) are the SEM and BSE images of a graphite particle on the polished surface.
of reaction (13) and (14).
0.5Nb2O5 + 2.25C = NbC + 1.25CO2(g)
(10)
TiO2 + 3C = TiC + 2CO (g)
(3)
0.5Ta2O5 + 2.25C = TaC + 1.25CO2(g)
(11)
HfO2 + 3C = HfC + 2CO(g)
(4)
WO3 + 2.5C = WC + 1.5CO2(g)
(12)
0.5Nb2O5 + 3.5C = NbC + 2.5CO(g)
(5)
MeO2 + 3C = MeC + 2CO(g)
(13)
0.5Ta2O5 + 3.5C = TaC + 2.5CO(g)
(6)
MeO2 + 2C = MeC + CO2(g)
(14)
WO3 + 4C = WC + 3CO(g)
(7)
CO2(g) + C = 2CO(g)
(15)
TiO2 + 2C = TiC + CO2(g)
(8)
HfO2 + 2C = HfC + CO2(g)
(9)
It is obvious that reaction (15) is significantly affected by the gas pressure. The balance between CO and CO2 gases during sintering can be calculated by Eq. (16). 4
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Fig. 3. EDS images of the high-entropy carbide ceramics in (a) central area and (b) circumferential area.
constant, T is temperature (K), PCO and PCO2 are the partial gas pressures of CO and CO2, respectively. The amounts of C, CO2 and CO as a function of the gas pressure from 0 to 70 bar under 1800 °C were calculated to explore the effect of gas pressure on reaction (15). As shown in Fig. 5(b), the amount of residual carbon (graphite in this work) is negligible under low gas pressure, but it increases rapidly with an increase in gas pressure, the same trend as the amount of CO2. Based on Eq. (16) and Fig. 5(b), the increased internal gas pressure in the central region of the specimen during sintering would suppress the reaction (13) and result in the appearance of residual graphite in the central region of the sample as shown in Fig. 2. The above discussions reveal that the generated CO and CO2 gases during heating caused a gradient distribution of inner gas pressure and
Table 1 Atomic percentage of elements in both center and circumferential areas of HEC phase from point EDS. Atomic percentage%
Center Circumferential
ΔGT0 = −RT *( where
ΔGT0
2 PCO ) PCO2
Elements Ti
Hf
Nb
Ta
W
C
O
10.51 10.62
7.72 7.51
12.15 12.51
7.27 7.47
7.54 7.76
54.58 53.80
0.23 0.33
(16)
is the Gibbs’ free energy of reaction (15), R is the ideal gas
Fig. 4. Characterization of the gradient sample prepared by reactive SPS; (a) model diagram of gradient change in the sample; (b) the evolution of microstructure and mechanical property along the radial direction of the sintered ceramics. 5
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Fig. 5. Thermodynamic analysis during the SPS process; (a) Gibbs’ free energy changes for reaction (3) to (12) and reaction (15) as a function of the temperature under 50 Pa; (b) the amount of C, CO2 and CO for reaction (15) as a function of the gas pressure under 1800 °C.
University (No. 19ZK0113).
consequently favored the formation of CO2, which resulted in the existence of residual carbon and its gradient distribution. The gradient content of residual graphite which located at the grain boundaries of HEC matrix eventually caused the formation of gradient microstructure during densification.
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4. Conclusions A single-phase high entropy carbide ceramics (Ti0.2Hf0.2Nb0.2Ta0.2W0.2)C was successfully synthesized by oxides and graphite via one-step reactive SPS. The obtained ceramics has an interesting microstructure with gradient distributions of grain size of the HEC phase, content of the residual graphite, as well as hardness. Residual oxygen measurement and EDS mappings prove the completion of the carbothermal reduction reactions of oxides in different parts of the sample. Thermodynamic calculations demonstrate that not only CO but also CO2 can be generated during reaction process and the generated gas causes a gradient gas pressure inside the specimen. The higher inner gas pressure suppresses the formation of CO and favors the formation of CO2, results in the formation of residual graphite in the central region of the sample. The gradient distribution of residual graphite eventually inhibited the grain growth of HEC phase during densification and resulted in the formation of a gradient microstructure. The results indicate that HEC ceramics with fine-grained microstructures can be prepared by incorporating second phase particles such as graphite particles. Declaration of Competing Interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments The present work was financially supported by the National Natural Science Foundation of China (No. 51872045, 51532009 and 51671126), the Fundamental Research Funds for the Central Universities (No. 19D110904), Science and Technology Commission of Shanghai Municipality (No. 18ZR1401400) and State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, Donghua 6
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Original Article
Gradient microstructure development and grain growth inhibition in highentropy carbide ceramics prepared by reactive spark plasma sintering Xiao-Feng Wei, Yuan Qin, Ji-Xuan Liu*, Fei Li, Yong-Cheng Liang, Guo-Jun Zhang* State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Materials Science and Engineering, College of Science, Institute of Functional Materials, Donghua University, Shanghai 201620, China
A R T I C LE I N FO
A B S T R A C T
Keywords: High-entropy carbide High-entropy carbide based composites Gradient microstructure Grain growth Hardness
High-entropy carbide ceramics (Ti0.2Hf0.2Nb0.2Ta0.2W0.2)C is prepared from five transition metal oxides and graphite by reactive spark plasma sintering. X-ray diffraction indicates the synthesized ceramics with the singlephase face-centered cubic structure. The elemental distribution maps by energy dispersive spectroscopy demonstrate homogeneous distribution of the five metal elements in both central and circumferential regions of the sample. SEM and corresponding back scattered electron observations show the residual graphite particles locating at the grain boundaries of high-entropy carbide ceramics. Moreover, the content of the residual graphite decreases and the grain size of the high-entropy carbide phase increases from central to circumferential region of the sample. Thermodynamic calculation results indicate that gradient gas pressure inside the sample affects the carbothermal reduction reactions during sintering and consequently results in the existence of residual graphite with gradient distribution feature. This study points out an effective way to inhibit the grain growth of highentropy carbide phase during sintering process by the incorporation of graphite as the second phase particles acting as grain growth inhibitor.
1. Introduction High-entropy alloys (HEAs) have been widely investigated because of their excellent physical and chemical properties, unique compositions, adjustable properties and attractive potential industrial applications [1,2]. Recently, high-entropy ceramics including oxides [3–6], nitrides [7,8], carbides [9–11], diborides [12–14], silicides [15,16] and single-phase metal oxycarbonitride ceramics [17] have also been successfully manufactured and characterized. In general, four or five transition metal compounds such as metal oxides, carbides and diborides were usually used as the raw powders to synthesize the corresponding high-entropy ceramics. These are traditional and straightforward methods to prepare solid solution compounds [18]. Same as ZrC and HfC ceramics prepared by two-step method, dense high-entropy carbide ceramics can be fabricated by pressureless carbothermal reduction reaction in the first step [19] and followed by the second step of pressure-assisted sintering [13,20]. As for high-entropy carbides (HECs), it may "inherits" the high melting point, high hardness, high chemical inertness, high resistance to irradiation, etc. of carbide-based ultra-high temperature ceramics (UHTCs) to a certain extent or even exhibits new behaviors [21]. Accordingly, HECs is possible candidate materials for aerospace applications, nuclear reactors, cutting tools, etc.
⁎
SPS is often used to prepare high-entropy carbide and diboride ceramics thanks to its attractive advantages including high thermo-efficiency, improved sintering activity, fast sintering under low temperatures [13,20–24]. The rapid sintering process will effectively prevent the grain growth and refine material microstructures [25], resulting in strength improvement according to the Hall-Patch relationship [26,27]. In addition, reactive SPS that involves certain chemical reactions during sintering is an effective SPS approach to fabricate materials, such as monolithic carbides and HECs [27–30]. In our previous work, (Ti0.2Zr0.2Nb0.2Ta0.2W0.2)C ceramics were prepared by three different typical ceramic processing, including carbide process by SPS, element process and oxide process by reactive SPS [22]. The specimen synthesized by oxides and graphite has a two-phase structure, one is HEC phase and the other one is zirconium-rich phase. In addition, a small amount of residual carbon was observed in HEC-O sample, especially in the central area. However, it is considered that a number of complex factors resulted in the above microstructure features, including the complex formation process of HEC phase, high reduction temperature of ZrO2 as well as the gradient temperature and gas pressure in the specimen during SPS process. In this study, oxide process by reactive SPS was used to prepare high-entropy carbide ceramics, but for
Corresponding authors. E-mail addresses: [email protected] (J.-X. Liu), [email protected] (G.-J. Zhang).
https://doi.org/10.1016/j.jeurceramsoc.2019.12.034 Received 8 October 2019; Received in revised form 15 December 2019; Accepted 17 December 2019 0955-2219/ © 2019 Elsevier Ltd. All rights reserved.
Please cite this article as: Xiao-Feng Wei, et al., Journal of the European Ceramic Society, https://doi.org/10.1016/j.jeurceramsoc.2019.12.034
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for graphite (JCPDS NO: 41-1487). This indicates that the residual carbon in the central area of the specimen is graphite. While no graphite diffraction peak is observed in Fig. 1(2). It is considered that the graphite content in circumferential area of the specimen should be relatively lower, so diffraction peaks cannot be detected. The theoretical density of HEC (11.28 g/cm3) was calculated from the average mass of the atoms in a unit cell and the XRD measured lattice parameters [22]. The relative density of the sintered whole sample was 95.5 %. The tested residual oxygen content of the sintered ceramics (half of the specimen was cut and measured) was only 0.028 %, indicating the carbothermal reduction reactions between the oxides and graphite during SPS have been completed.
avoiding the formation of zirconium-rich phase, element zirconium was replaced by hafnium. The results showed that a single-phase high-entropy carbide ceramics (Ti0.2Hf0.2Nb0.2Ta0.2W0.2)C was prepared via the reactive SPS through the reaction between oxides and graphite. The microscopic structures and EDS mappings in different regions of this single-phase ceramics are carefully observed and analyzed. The results demonstrate that more homogeneous distributions of metallic elements in the synthesized HEC phase can be obtained by the replacement of zirconium with hafnium. In addition, an uncommon gradient microstructure is found, and the formation mechanism is discussed based on thermodynamic analysis. 2. Experimental
3.2. Gradient microstructural feature To prepare (Ti0.2Hf0.2Nb0.2Ta0.2W0.2)C ceramics, graphite (≥99 % pure, particle size ∼ 1.5 μm, Shanghai Graphite Company, China) and oxide powders including TiO2 (≥99.99 % pure, particle size ∼ 0.1 μm) from Shanghai Titan Technology Co., Ltd. China, HfO2 (≥99.5 % pure, particle size ∼ 0.2 μm), Ta2O5 (≥99. 5 % pure, particle size ∼ 1.5 μm) as well as Nb2O5 (≥99.8 % pure, particle size ∼ 2 μm) provided by Zhuzhou Cemented Carbide Group Co., Ltd. China and WO3 (≥99.8 % pure, particle size ∼ 0.1 μm, Yangzhou Sanhe Chemical Co., Ltd. China) were used as the raw materials. The processing of the (Ti0.2Hf0.2Nb0.2Ta0.2W0.2)C ceramics is the same as the oxides mothed described in our previous publication [22]. The starting powders were mixed by planetary ball mill at 560 r/min for 10 h with WC balls in ethanol milling media. The ball-to-powder ratio was 6:1 in weight. After drying and sieving, the powder mixtures were directly densified by reactive SPS (HP D 25, FCT, Germany). In order to reduce the impediment to the gas-releasing reaction between oxides and graphite, the samples were firstly heated to 1800 °C and held for 10 min in vacuum under 6.4 MPa, which is the minimum axial pressure of the machine. Then, the pressure was increased to 50 MPa and the temperature was raised to 2000 °C and held for 5 min to get dense HEC samples. The sample size was 20 mm in diameter and about 2.5 mm in thickness. Surface layers about 0.5 mm in thickness of the as-sintered samples were removed by grinding and polishing. The densities of the sintered samples were measured by Archimedes’ method. The phase composition of the mixed powders and the as-sintered ceramics were identified by X-ray diffraction (XRD, D/max-2550VB+/PC, Japan) using Cu-Kα radiation. Polycrystal silicon was used as the internal standard to calibrate the XRD peak positions [31]. The lattice parameters of the assintered samples were calculated based on the XRD data via Jade [11]. Microstructural characterization of the samples was observed by field emission scanning electron microscope (SEM) and corresponding back scattered electron (BSE) (MAIA3, TESCAN, Czech Republic) with energy dispersive spectroscopy (EDS). The residual oxygen content was analyzed by Oxygen/Nitrogen Analyzer (TC600C, Leco, USA). The Vickers hardness of the sample was measured by the Vickers Hardness Tester (HXD-1000TM/LCD, Taiming, China) with a load of 4.9 N and a dwell time of 10 s.
The SEM images of the fracture and polished surfaces of the sample are shown in Fig. 2. The fracture mode of the HEC ceramics is mainly transgranular dominated as shown in Fig. 2 (a) and (b), which is very similar to that in the previous reported (Ti0.2Zr0.2Nb0.2Ta0.2W0.2)C high-entropy ceramics [22]. Based on the SEM and BSE images, the following findings are needed to discuss. Firstly, graphite phase with dark contrast which generally located at the grain boundaries of HEC was observed in the sintered sample (marked by white arrows in Fig. 2 (c) and (e) and enlarged in Fig. 2 (g)). Besides, a small number of closed pores (marked by red arrows in Fig. 2 (c) and (e)) were also observed in the central area of the sintered sample. In contrast, the contents of graphite and closed pores in the circumferential region are less as shown in Fig. 2. This is in accordance with the results of XRD in Fig. 1. In addition, due to the different thermal contraction between graphite and HEC (here the thermal expansion coefficient of HEC is assumed to be approximate to those of the corresponding monolithic carbide ceramics), interfacial pores appears at the interfaces between graphite and HEC grains during cooling from the sintering temperature, as shown in the enlarged image in Fig. 2 (a) and (g). Secondly, the grain size of HEC in the central area (Fig. 2 (e)) is obviously smaller than that in circumferential area (Fig. 2 (f)). Previous investigations have revealed that the presence of the second phase particles can inhibit grain growth of the matrix phase. Based on Zener pinning theory, the average grain size of the matrix phase is closely related to the particle size, shape and content of the second phase [32,33]. In two-phase materials, the relationship can be summarized as Eq. (1) [34,35]:
D∝
d f
(1)
where D is the average grain size of the matrix phase, d is the average particle diameter of the second phase, and f is the volume friction of the second phase. The graphite content in the central area of the sample is significantly higher, and the grain size of HEC is smaller, which is consistent with Zener pinning theory. It is obvious that the presence of graphite in the grain boundaries severely limited the growth of HEC grains, similar to the phenomena reported in literature [36,37]. Moreover, a number of small-sized graphite particles are entrapped in the HEC grains in the circumferential area of the sample as shown in Fig. 2 (f). It is considered that a faster grain growth of the HEC phase in the circumferential area resulted in some small graphite particles entrapped in the HEC grains [13]. The compositional EDS mappings indicate the homogeneous distribution of five metal elements in both central and circumferential areas of the sample as shown in Fig. 3 (a) and (b). The carbon-rich areas marked by white arrows in Fig. 3 (a) are consistent with the graphite distribution in Fig. 2. The atomic percentages of elements in both central and circumferential areas of HEC are listed in Table 1. The atomic percentage data indicate the uniform distribution of the metallic elements in the material. The results further confirm that a single-phase high-entropy carbide with similar elemental composition in both central and circumferential areas of the sample is formed, which is in
3. Results and discussion 3.1. Phase analysis Fig. 1 shows the XRD pattern of the starting powder mixture and the sintered ceramics. The circumferential and central areas of the sintered sample show nearly the same diffraction patterns which are similar to the diffraction pattern of previous reported HEC phase [22]. This indicates that single-phase HEC with rock salt structure and a nominal composition of (Ti0.2Hf0.2Nb0.2Ta0.2W0.2)C was successfully synthesized by SPS and the phase composition of the circumferential and central areas of the sample is consistent. Moreover, a small diffraction peak is observed around 26.5° in Fig. 1 (a) (3). The XRD patterns of the 2θ from 26.2° to 27° are enlarged and the results indicate that the small peak is 2
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Fig. 1. Phase composition analysis. (a) XRD patterns of (1) mixed starting powders; (2) circumferential area of the specimen; and (3) central area of the specimen; (b) enlarged XRD patterns of the 2θ from 26.2° to 27°.
and the content of residual graphite. The residual graphite particles act as the second phase particles inhibiting the grain growth of the HEC matrix phase. The hardness of different regions in the gradient material is determined by the synergetic effects of the content of graphite phase and grain size of HEC. As shown in Fig. 4(b), the region containing ∼4 vol.% graphite phase exhibits the highest hardness, and this graphite content can be considered as the limit for the balance between the grain refinement and mechanical properties, which is similar to the literature reported monolithic carbide-graphite materials [38,41]. The results point out a new approach to design fine-grained HEC with improved mechanical properties. Besides, it is reported that high-entropy ceramics with severe lattice distortion demonstrate better irradiation properties, and the interfacial nano-sized pores between HEC and graphite would provide spaces for capturing irradiation gaseous products [11,42,43]. Hence HEC-graphite composites could be a candidate material for nuclear applications.
accordance with the results of XRD analysis. In order to further investigate the microstructure evolution along the radial direction of the sintered ceramics, the sample was analyzed by dividing into ten rings in the radial direction with 1 mm wide as shown in Fig. 4 (a). The amount of residual graphite was estimated based on the area fraction of graphite phase in each ring. More than two hundred HEC grains were measured in the polished surface of each ring to get the average grain size of the HEC phase. The measured amount of residual graphite and average grain size of HEC in each ring is shown in Fig. 4 (b). It can be seen that both the content of residual graphite and the average grain size of HEC phase exhibits gradient distributions along the radial direction in the sintered sample. From the central to the circumferential regions, the content of residual graphite decrease slowly while the average grain size of HEC phase increased gradually. The evolution of hardness along the radial direction of the sintered ceramics was also investigated. Vickers hardness in each ring was measured and the reported value was an average of 10 tests. As for the hardness, it is generally influenced by porosity, grain size and the hardness of the second phase [38,39]. Samples with higher density, finer grain size and harder second phase will have higher hardness [35,36]. The hardness of graphite is much lower than that of HECs [22]. Thus, the hardness of the specimen in the central area is remarkably reduced due to the existence of higher graphite content (as well as the interfacial pores between graphite and HEC phase). An increase tendency in hardness can be observed with decrease of residual amount of graphite and reaches a maximum hardness at the distance of 7 mm where the amount of residual graphite is about 4 vol.% and the grain size of HEC is about 12 μm. In these areas, the hardness of the material is mainly controlled by the content of graphite. However, the grain size of HEC increases rapidly when the distance is greater than 7 mm, which results in the slight decrease of the hardness when comparing with the maximum hardness as shown in Fig. 4(b). According to the Hall-Petch relationship, the hardness of ceramics generally decreases with increasing grain size, as showed in Eq. (2) [38,40]:
Hv= H0 + kD−1/2
3.3. Mechanism for the formation of gradient microstructures Thermodynamic calculation by thermodynamic software (HSC chemistry 6, Outotec, Finland) for the circumferential area and the central area of the specimen are discussed in order to understand the formation mechanism of the gradient microstructure. Carbothermal reduction reactions (3)∼(12) between oxides and graphite occurred during sintering, which resulted in the five carbide resultants and released CO and CO2 gases [29,43]. The synthesized transition metal carbides subsequently solidify to form a single-phase high-entropy carbide [19,44]. The Gibbs’ free energy of chemical reactions (3) to (12) under 10 Pa (vacuum degree in the furnace) were calculated as shown in Fig. 5 (a). Gibbs’ free energy of these reactions decrease with the increase of temperature, and all of them are less than zero before the temperature reaching to 1800 °C. Obviously, the ΔG values of reactions (3) to (7) are lower than those of reaction (8) to (12) at higher temperature. The generation of CO and CO2 gases should cause a rapid increase in gas pressure inside the sample. The generated gas was easily diffused along the open pores but could only escape along the gap between the sample and the mold. Therefore, a gradient gas pressure from central to circumferential region of the sample would be formed during the chemical reactions. A higher gas pressure appeared in the central region, while a lower gas pressure existed in the circumferential region. Complex gradient gas pressure in the mold would result in complex and different conditions, and then affect the chemical reactions and even densification in the sample at different positions. Reaction (3) to (7) can be concluded as reaction (13) and reaction (8) to (12) can be concluded as reaction (14). Reaction (15) is obtained by the subtraction
(2)
However, in the near circumferential area, factors that affect hardness include not only the larger grain size but also the lower content of graphite, closed pores and interfacial pores in the material. Therefore, when the distance is greater than 7 mm, the hardness of the material keeps almost unchanged. It should be noted that changing the size and geometry of the graphite die and sintering conditions, such specific distance value will be changed. However, a similar gradient microstructure is believed to exist. Based on the aforementioned discussions, the obtained ceramics in this work exhibits a gradient microstructure in grain size of HEC phase 3
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Fig. 2. SEM images of the sintered ceramics. (a) and (b) are the fracture surfaces of central area and circumferential area, respectively; (c) and (d) are polished surfaces of central area and circumferential area, respectively; (e) and (f) are the corresponding BSE images of (c) and (d); (g) and (h) are the SEM and BSE images of a graphite particle on the polished surface.
of reaction (13) and (14).
0.5Nb2O5 + 2.25C = NbC + 1.25CO2(g)
(10)
TiO2 + 3C = TiC + 2CO (g)
(3)
0.5Ta2O5 + 2.25C = TaC + 1.25CO2(g)
(11)
HfO2 + 3C = HfC + 2CO(g)
(4)
WO3 + 2.5C = WC + 1.5CO2(g)
(12)
0.5Nb2O5 + 3.5C = NbC + 2.5CO(g)
(5)
MeO2 + 3C = MeC + 2CO(g)
(13)
0.5Ta2O5 + 3.5C = TaC + 2.5CO(g)
(6)
MeO2 + 2C = MeC + CO2(g)
(14)
WO3 + 4C = WC + 3CO(g)
(7)
CO2(g) + C = 2CO(g)
(15)
TiO2 + 2C = TiC + CO2(g)
(8)
HfO2 + 2C = HfC + CO2(g)
(9)
It is obvious that reaction (15) is significantly affected by the gas pressure. The balance between CO and CO2 gases during sintering can be calculated by Eq. (16). 4
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Fig. 3. EDS images of the high-entropy carbide ceramics in (a) central area and (b) circumferential area.
constant, T is temperature (K), PCO and PCO2 are the partial gas pressures of CO and CO2, respectively. The amounts of C, CO2 and CO as a function of the gas pressure from 0 to 70 bar under 1800 °C were calculated to explore the effect of gas pressure on reaction (15). As shown in Fig. 5(b), the amount of residual carbon (graphite in this work) is negligible under low gas pressure, but it increases rapidly with an increase in gas pressure, the same trend as the amount of CO2. Based on Eq. (16) and Fig. 5(b), the increased internal gas pressure in the central region of the specimen during sintering would suppress the reaction (13) and result in the appearance of residual graphite in the central region of the sample as shown in Fig. 2. The above discussions reveal that the generated CO and CO2 gases during heating caused a gradient distribution of inner gas pressure and
Table 1 Atomic percentage of elements in both center and circumferential areas of HEC phase from point EDS. Atomic percentage%
Center Circumferential
ΔGT0 = −RT *( where
ΔGT0
2 PCO ) PCO2
Elements Ti
Hf
Nb
Ta
W
C
O
10.51 10.62
7.72 7.51
12.15 12.51
7.27 7.47
7.54 7.76
54.58 53.80
0.23 0.33
(16)
is the Gibbs’ free energy of reaction (15), R is the ideal gas
Fig. 4. Characterization of the gradient sample prepared by reactive SPS; (a) model diagram of gradient change in the sample; (b) the evolution of microstructure and mechanical property along the radial direction of the sintered ceramics. 5
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Fig. 5. Thermodynamic analysis during the SPS process; (a) Gibbs’ free energy changes for reaction (3) to (12) and reaction (15) as a function of the temperature under 50 Pa; (b) the amount of C, CO2 and CO for reaction (15) as a function of the gas pressure under 1800 °C.
University (No. 19ZK0113).
consequently favored the formation of CO2, which resulted in the existence of residual carbon and its gradient distribution. The gradient content of residual graphite which located at the grain boundaries of HEC matrix eventually caused the formation of gradient microstructure during densification.
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4. Conclusions A single-phase high entropy carbide ceramics (Ti0.2Hf0.2Nb0.2Ta0.2W0.2)C was successfully synthesized by oxides and graphite via one-step reactive SPS. The obtained ceramics has an interesting microstructure with gradient distributions of grain size of the HEC phase, content of the residual graphite, as well as hardness. Residual oxygen measurement and EDS mappings prove the completion of the carbothermal reduction reactions of oxides in different parts of the sample. Thermodynamic calculations demonstrate that not only CO but also CO2 can be generated during reaction process and the generated gas causes a gradient gas pressure inside the specimen. The higher inner gas pressure suppresses the formation of CO and favors the formation of CO2, results in the formation of residual graphite in the central region of the sample. The gradient distribution of residual graphite eventually inhibited the grain growth of HEC phase during densification and resulted in the formation of a gradient microstructure. The results indicate that HEC ceramics with fine-grained microstructures can be prepared by incorporating second phase particles such as graphite particles. Declaration of Competing Interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments The present work was financially supported by the National Natural Science Foundation of China (No. 51872045, 51532009 and 51671126), the Fundamental Research Funds for the Central Universities (No. 19D110904), Science and Technology Commission of Shanghai Municipality (No. 18ZR1401400) and State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, Donghua 6
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