Fabrication,Microstructure And Properties Of(Ti,Zr,Nb,Ta,Me)C(Me=Mo,V,Cr,W) High-entropy Ceramics

With the increase requirements of carbides performance,the high-entropy ceramics have received extensive attention.In this dissertation,four different?TiZrNbTaMe?C?Me=Mo,V,Cr,W?high-entropy ceramics were prepared by carbothermal reduction-assisted hot pressing.The phases,microstructures,mechanical properties and phase formation mechanism of four high-entropy ceramics were compared;the effect of hot pressing parameter and Ti element mole ratio on the phases,microstructures and the solid solubility of Cr element in the grains of the?TiZrNbTaCr?C ceramics were studied.The oxidation behaviour of the?TiZrNbTaV?C ceramic wes also investigated.Using oxides and carbon black as starting materials can effectively prevent oxygen contamination.For the preparation of?TiZrNbTaMo?C high-entropy ceramic,thermodynamic calculations show that all the carbothermal reduction reactions can be carried out completely when the temperature is higher than 1135°C at 50Pa and keept it for a while.During the experiment,it is found that the carbothermal reduction temperature should be kept at 1500°C and the holding time should be kept as 1h to obtain the high purity carbide composite powders.The particle size of the carbide powders is between100³00nm.The face-centered cubic?TiZrNbTaMo?C high-entropy ceramic is fabricated by sinteting the as-synthesized carbide powders at 1800°C for 1h and then 2100°C for0.5h.The relative density of the?TiZrNbTaMo?C ceramic is 98.6%and the lattice parameter is 0.4867nm?obtained by Rietveld method?.The grain size is about8.80±3.00?m and the elements are uniformly and randomly distributed from microscale to nanoscale.The nanoindentation hardness,elasticity modulus,fracture toughness and flexural strength of the?TiZrNbTaMo?C ceramic are 31.3±2.5GPa,467±7.6GPa,446±24MPa and 3.28±0.12MPa·m^1/2,respectively.The analysis of the phase formation process of the?TiZrNbTaMo?C ceramic shows that the carbides powders form ZrC-based and TaC-based solid solutions firstly and then form the single-phase solid solution high-entropy ceramic.The other three?TiZrNbTaMe?C?Me=V,Cr,W?high-entropy ceramics were prepared in the same manner.The experiment results show that the three high-entropy ceramics are face-centered cubic structure.The relative density of them are 98.4%,99.1%and 94.8%,repectively.The measured grain sizes of the?TiZrNbTaV?C and?TiZrNbTaCr?C ceramics are 2.69±0.82?m and 5.39±2.41?m,but the grain size of the?TiZrNbTaW?C ceramic is not measured because of the pores and residual carbon.The five metal elements are distributed randomly and evenly in the?TiZrNbTaV?C and?TiZrNbTaW?C ceramics,but Cr element is segregated at grain boundaries in the?TiZrNbTaCr?C ceramic.The change of the hot-pressing sintering parameter makes little differences on the microstruture and Cr element solubility in grain in the?TiZrNbTaCr?C ceramics.But increasing the content of Ti element can improve the Cr element solubility,which shows that the solid solubility of Cr element is affected by the type of elements in the?TiZrNbTaCr?C ceramic.But because of the Cr₃C₂ decompositon and the limitation of the solid solution capability of the TaC lattice,the solubility of the Cr element fail to reach the ultimate solid solubility in the crystal lattice.The V,Mo and Cr elements are all favorable for promoting the sintering ability and inhibiting the grain growth.The lattice distortion of the four high-entropy ceramics are no more than 6%.The oxidation behavior of?TiZrNbTaV?C ceramic was studied.It was found that the weight gain of ceramics during the oxidation process at 600°C,800°C and 1000°C showed a linear change with the time,which means the oxidation mechanism is reactions control.The intense oxidation process occurred at between 600°C and 800°C.The undensified oxide layer and the productions of the gas lead to the continuous oxidation processing.The?TiZrNbTaV?C high-entropy ceramic have poor oxidation resistance and complex oxidation products.