Microstructure and mechanical properties of (TiZrNbTaMo)C high-entropy ceramic

doi:10.1016/j.jmst.2019.07.056

Abstract

Abstract:

A high-entropy (TiZrNbTaMo)C ceramic has been successfully fabricated by hot pressing the newly-synthesized quinary carbide powder to investigate its microstructure and mechanical properties. The carbothermal reduction process of equimolar quinary metallic oxides at 1500 ℃ for 1 h generates a carbide powder mixture, which consists mainly of TaC- and ZrC-based solid solutions. The as-synthesized powder was then sintered to form a single-phase high-entropy ceramic by a two-step hot pressing at 1850 ℃ for 1 h and 2100 ℃ for 0.5 h, respectively. The high-entropy ceramic exhibits a fine grain size of about 8.8 μm, a high compositional uniformity and a high relative density of 98.6% by adding Mo as the strategic main component. The measured nanohardness values of (TiZrNbTaMo)C ceramic are 25.3 GPa at 9.8 N and 31.3 GPa at 100 mN, respectively, which are clearly higher than those of other available high-entropy carbide ceramics.

Key words: High-entropy ceramic, (TiZrNbTaMo)C, Microstructure, Enhanced hardness

Kai Wang, Lei Chen, Chenguang Xu, Wen Zhang, Zhanguo Liu, Yujin Wang, Jiahu Ouyang, Xinghong Zhang, Yudong Fu, Yu Zhou. Microstructure and mechanical properties of (TiZrNbTaMo)C high-entropy ceramic[J]. J. Mater. Sci. Technol., 2020, 39: 99-105.

Figures/Tables 7

Table 1 Relationship between the difference in Gibbs free energy (ΔG) and reaction temperature (T).

Reactions	Relationship between ΔG (kJ/mol) and T (℃)	Critical reaction temperature (℃)
ZrO₂ + 3C = ZrC + 2CO	ΔG=-471.9 × 10^-3T+535.757	1135.3
TiO₂ + 3C = TiC + 2CO	ΔG=-461.9 × 10^-3T+402.028	870.4
Ta₂O₅ + 7C = 2TaC + 5CO	ΔG=-1153.6 × 10^-3T+844.281	731.9
Nb₂O₅ + 7C = 2NbC + 5CO	ΔG= -1173.1 × 10^-3T+741.056	631.7
MoO₃(s) + 7/2C(s) = 1/2Mo₂C(s) + 3CO	ΔG=-704.6 × 10^-3T+185.780	263.7

Fig. 1. Characterization of the intermediate products powders: (a) XRD patterns of ball-milled powder mixture and as-synthesized powder at different temperatures; (b) enlarged XRD patterns of as-synthesized powder; (c) SEM micrographs of as-synthesized powder at 1500 ℃.

Fig. 2. Characterization of phase formation process: (a) XRD patterns of TaC, ZrC, as-synthesized carbide powder at 1500 ℃ and high-entropy ceramic; (b) Rietveld refinement of (TiZrNbTaMo)C (Fullprof, Rwp = 8.88); (c) schematic illustration showing the formation process of high-entropy ceramic (different colors represent different types of metallic atoms).

Fig. 3. XRD patterns of carbide powders and ceramics: (a) as-synthesized (Ta0.4Nb0.2Ti0.2Mo0.2)C powder; (b) (Ta0.4Nb0.2Ti0.2Mo0.2)C ceramic; (c) as-synthesized (Zr0.4Nb0.2Ti0.2Mo0.2)C powder; (d) (Zr0.4Nb0.2Ti0.2Mo0.2)C ceramic.

Fig. 4. SEM and EDS analysis of (TiZrNbTaMo)C ceramic: (a) SEM image of the polished surface, with the corresponding EDS mapping of Ti, Zr, Nb, Ta and Mo element; (b) elemental component of point as marked in (a); (c) statistical chart of grain size distribution.

Fig. 5. TEM analysis of the (TiZrNbTaMo)C ceramic: (a) HRTEM image; (b) SAED pattern; (c) the HADDF image and the corresponding EDS compositional maps.

Table 2 Hardness and fracture toughness of high-entropy (TiZrNbTaMo)C carbide ceramic for a comparative study.

Ceramic	Fracture toughness (MPa m^1/2)	Hardness at different loads (GPa)	Reference
(TiZrNbTaMo)C	3.28 ± 0.12	16.8 ± 1.2 (98 N) 17.9 ± 1.0 (49 N) 25.3 ± 0.3 (9.8 N) 31.3 ± 2.5 (100 mN)	This work
(TiZrNbTaHf)C	--	27.5 (0.5 N)	[18]
(TiZrNbTaHf)C	--	32 (0.3 N)	[16]
(TiZrNbTaHf)C	--	15 (9.8 N)	[20]
(TiZrNbTaHf)C	3.0	18.8 (9.8 N) 22.5 (0.98 N) 40 (8 mN)	[19]
(TiZrNbV)C	4.7	19 (49 N) 22.5 (0.98 N)	[25]

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