IntroductionUltra-high temperature ceramics include a series of high melting point materials, such as transition metal carbides, nitrides and borides, especially transition metal carbides with excellent high-temperature mechanical properties, stable physico-chemical properties, corrosion resistance and radiation resistance, showing broad prospects for application in the hypersonic aircraft, rocket engines, fourth-generation nuclear reactor, and other extreme environments. However, the traditional single-component transition metal carbide ceramics have been unable to meet the emerging requirements under extreme environments, there is an urgent need to develop a new high-performance material under ultra-high temperature. In recent years, the concept of multi-component “high entropy” has greatly expanded the scope of material composition design and property optimization. Compared with single-component carbides, multi-component ceramics perform better in terms of overall properties, including hardness, creep resistance, oxidation resistance and radiation resistance. These improvements mainly result from their complex component composition, electronic structure and lattice distortion. At present, the influence of elemental species on the microstructure evolution and mechanical properties of carbide high-entropy ceramics is not reported. In this paper, (TiZrNbTaMe)C (Me=V, Cr, Mo, W) high-entropy ceramics are prepared by hot press sintering, and the effects of Me elemental species on the physical phase, microstructure evolution, and mechanical properties of (TiZrNbTaMe) high-entropy ceramics are investigated.MethodsIn this work, (TiZrNbTaMe)C (Me=V, Cr, Mo, W) high-entropy ceramics with equimolar ratio were prepared by carbothermal reduction-assisted hot pressing using transition metal oxides and carbon black as raw materials. The oxides and carbon black were mixed using a planetary ball mill (Fritsch, model P4, Germany). Carbide powders were obtained by carbothermal reduction under vacuum using a pressureless sintering furnace (WS0404, Ningxia Sincere Co. Ltd., China) with a process of 1500 ℃/1 h. The synthesized carbide powders were loaded into graphite molds, and the ceramic samples were prepared by a two-step hot pressing method (AVS, model 1540, USA). The samples were held at 1850 ℃ for 1 h, and then at 2100 ℃ for 0.5 h under pressure of 30 MPa or 10 Pa. Phase analysis was carried out by X-ray diffractometry (XRD; D/max-B, Rigaku, Japan) using Cu-K_α rays. Scanning electron microscopy (SEM; Quanta 200FEG, USA) was used to analyze the microstructure and elemental content and distribution. Transmission electron microscopy (TEM; Talos F200X, USA) was used to analyze the microstructure, elemental content and distribution, and grain boundary characteristics of the materials. The relative densities of the samples were measured using image analysis software (Photoshop, Adobe, USA) based on the pores in the SEM photographs. Vickers hardness was measured using a Vickers hardness tester (HVS-30) at a load of 9.8 N with a holding time of 15 s. Fracture toughness was also measured using the indentation method. The interaction parameters between the metal elements were also calculated using DFT calculation.Results and discussionThe high-entropy ceramics are all characterized by an FCC crystal structure. Except for the (TiZrNbTaW)C sample, the porosity of the other three high entropy ceramics is low, and their density exceeds 98% and the element distribution is relatively uniform. Compared with the (TiZrNbTaMo)C sample, the grain sizes of the (TiZrNbTaV)C and (TiZrNbTaCr)C samples are significantly reduced, which are 2.69 μm and 5.39 μm, respectively. This shows that the addition of V and Cr elements helps to improve the sintering properties of the (TiZrNbTaMe)C system and inhibit grain growth. The element content analysis results of (TiZrNbTaCr)C show that the Cr element segregates significantly at the grain boundaries, while the other four metal elements are distributed more evenly. The segregation of Cr at the grain boundaries may be related to chromium carbides with low melting points. For example, the melting point of Cr₃C₂ is about 1810 ℃, while the sintering temperature is as high as 2100 ℃, suggesting that chromium carbides may form a liquid phase and aggregate at the grain boundaries during sintering. In addition, the complex composition of high-entropy ceramics may also affect the solid solubility of Cr. The interaction coefficients between Cr and each metal element in transition metal carbides show that the interaction parameter values of Cr and other metal elements are high. It is difficult for Cr to form a solid solution with other metal elements, thus tending to be enriched at the grain boundaries. Therefore, (Ti_xZr_0.4–xNb_0.2Ta_0.2Cr_0.2)C ceramics with different Ti and Zr contents (x=0, 0.2, 0.3 and 0.4) were designed and prepared. The effect of Ti content on the solid solubility of Cr in the (TiZrNbTaCr)C sample was investigated. As the x value increases from 0 to 0.4, the solid solubility of Cr in the grains increases from 3.18% to 8.68%. It shows that the increase of Ti content is beneficial to improve the solid solubility of Cr. In addition, for the (TiZrNbTaCr)C sample, the high lattice distortion leads to solid solution strengthening in the grains and the high density of the system increases its hardness, which has the best mechanical properties. Its Vickers hardness and fracture toughness reach 29.8 GPa and 3.71 MPa·m^1/2, respectively.ConclusionsThe four (TiZrNbTaMe)C (Me=V, Cr, Mo, W) high-entropy ceramics are all face-centered cubic structures, and the elements are uniformly distributed in the high-entropy ceramic systems except for the (TiZrNbTaCr)C sample. The (TiZrNbTaCr)C sample is also characterized by the presence of a significant Cr segregation at grain boundaries, resulting in a low Cr content inside the grains. The (TiZrNbTaCr)C sample has the best overall mechanical properties, with the Vickers hardness and fracture toughness reaching 29.8 GPa and 3.71 MPa·m^1/2, respectively. In-depth studies show that the solid solubility of Cr element in high-entropy carbide ceramics is closely related to the species and content of metal elements. Enhancing the content of Ti element helps to improve the solid solubility of Cr element in high-entropy carbide ceramics, in which the solid solubility of Cr element in the grain of (Ti_0.4Nb_0.2Ta_0.2Cr_0.2)C system reaches the maximum value of about 8.68%.