Processing and Characterization of Refractory Quaternary and Quinary High-Entropy Carbide Composite

Abstract

Quaternary high-entropy ceramic (HEC) composite was synthesized from HfC, Mo2C, TaC, and TiC in pulsed current processing. A high-entropy solid solution that contained all principal elements along with a minor amount of a Ta-rich phase was observed in the microstructure. The high entropy phase and Ta-rich phase displayed a face-centered cubic (FCC) crystal structure with similar lattice parameters, suggesting that TaC acted as a solvent carbide during phase evolution. The addition of B4C to the quaternary carbide system induced the formation of two high-entropy solid solutions with different elemental compositions. With the increase in the number of principal elements, on the addition of B4C, the crystal structure of the HEC phase transformed from FCC to a hexagonal structure. The study on the effect of starting particle sizes on the phase composition and properties of the HEC composites showed that reducing the size of solute carbide components HfC, Mo2C, and TiC could effectively promote the interdiffusion process, resulting in a higher fraction of a hexagonal structured HEC phase in the material. On the other hand, tuning the particle size of solvent carbide, TaC, showed a negligible effect on the composition of the final product. However, reducing the TaC size from −325 mesh down to <1 µm resulted in an improvement of the nanohardness of the HEC composite from 21 GPa to 23 GPa. These findings suggested the possibility of forming a high-entropy ceramic phase despite the vast difference in the precursor crystal structures, provided a clearer understanding of the phase transformation process which could be applied for the designing of HEC materials.

Effect of SiC on Microstructure, Phase Evolution, and Mechanical Properties of Spark-Plasma-Sintered High-Entropy Ceramic Composite

“…The strongest diffraction peak of the hexagonal (191) structure at 2θ = 44° (marked with the red arrow) started to be visible at 1800 °C, suggesting the initial formation of hexagonal phase at 1800 °C. The composition of HEC-(SiC)x composites at 2000 °C is similar to what was achieved in our previous study on the B4(HfMo2TaTi)C/SiC composite, however at lower temperature (1800 °C) [17], attributed to a higher heating rate applied in this study (200 °C/min compare to 100 °C/min). Similar dependence of the sintering temperature on the heating rate in SPS has been reported [52,53].…”

“…The hexagonal (191) phase shows lattice parameters of a = 0.3051 nm and c = 0.3237 nm, while the FCC phase in all composites shows a lattice constant of approximately 0.447 nm, which is close to the lattice parameter of the constituent metal carbides TaC (0.446 nm, ICDD 03-065-8792, 50.3 at% C). The phase formation mechanism discussed in our previous work on high-entropy boron-carbides [16,17] suggested that TaC was the host FCC lattice in the current carbide system so that other atoms migrate in and occupy the corresponding cationic and anionic sites in the TaC rocksalt structure, due to the lowest metal vacancy formation energy of TaC among the FCC carbides.…”

“…Ceramics 2020, 3 FOR PEER REVIEW 8 discussed in our previous work on high-entropy boron-carbides [16,17] suggested that TaC was the host FCC lattice in the current carbide system so that other atoms migrate in and occupy the corresponding cationic and anionic sites in the TaC rocksalt structure, due to the lowest metal vacancy formation energy of TaC among the FCC carbides. Figure 6 shows the comparison of the XRD patterns of the as-mixed precursors and the HEC-(SiC)x composites sintered at different temperatures.…”

“…Our previous studies on HECs-SiC composite showed that a SiC-reinforced high-entropy B 4 (HfMo 2 TaTi)C/SiC ceramic composite reveals superior mechanical properties and remarkable oxidation resistance under different temperature conditions. In addition, SiC was found to play an essential role in the phase formation during the sintering of high-entropy boron-carbide/SiC ceramic composite [17]. In this study, high-entropy boron-carbide ceramic composites with different SiC content (from 0 to 37 vol.…”

New Advances in High-Entropy Alloys

“…The development and preparation of composites have realized the complementary advantages of a variety of different component materials, which can further improve the overall performance of alloys. Zhang et al [ 39 ] prepared a high-entropy ceramic composite by using HfC, Mo 2 C, TaC, and TiC in pulsed-current processing. Furthermore, Li et al [ 40 ] used a mixture of carbides and oxides in the preparation of an NbMoCrTiAl HEA.…”