Study Of High-entropy Transition Metal Carbide Ceramics

Transition metal carbides（TMCs）have many excellent physical and chemical properties including ultra-high hardness,low thermal conductivity,and corrosion resistance and have broad application prospects under the extreme conditions in aerospace,nuclear power,and high-speed machining technique.However,with the increasing requirements of the national major strategy including aerospace and national defense military on the properties of the TMCs,traditional TMCs cannot satisfy the usage requirements.To develop novel TMCs materials which can be applied under the extreme conditions in the future,inspired by the design concept of high-entropy alloy,this thesis performed comprehensive and in-depth studies on the high-entropy transition metal carbide ceramics for the first time in the world.First,on the basis of the combination of first-principles calculations,the formation possibility of a series of high-entropy carbides（HECs）were theoretically analyzed,and then a series of HECs were designed and fabricated by hot-pressing sintering technique.The phase composition,microstructure,and element distribution uniformity were comprehensively characterized and the mechanical,thermophysical,and the oxidation resistance properties were investigated in-depth with the revealing of the mechanism of thermal conductivity,strengthening,and oxidation resistance.The main research contents and results are as follows:Based on the theoretical analysis of the formation possibility of the(Hf_0.2Zr_0.2Ta_0.2Nb_0.2Ti_0.2)C（HEC-1）system by first-principles calculation,a novel single-phase rock-salt structural HEC-1 ceramic was successfully fabricated by hot-pressing sintering technique（2073 K,30 MPa）.The theoretical analysis indicated that the mixing enthalpy of HEC-1 system was-0.869±0.29 k J/mol and the mixing entropy of HEC-1 system was 0.805R.However,the lattice size difference of this system is 2.98%,which indicates that the formation possibility of HEC-1 high-entropy ceramics is considerably high.The experimental results showed that the fabricated HEC-1 has single-phase rock-salt structure with element distribution uniformity from microscale to nanoscale.Due to the solid solution strengthening effect,HEC-1 has relatively higher hardness（40.6±0.6 GPa）and elastic modulus（518±10 GPa）and inherits the brittleness of TMC with the fracture toughness of 3.0±0.2 MPa·m^1/2.In addition,the oxidation behavior of HEC-1 from 1073-1773 K in air was studied in-depth and the oxidation process is a weight gain process.Its oxidation kinetics all followed the parabolic law and its oxidation rate was controlled by the diffusion rate of oxygen in the oxide layer.Inspired by the design concept of tuning the alloy composition to get high-performance high-entropy alloys,this thesis designed four-component(Zr_0.25Nb_0.25Ti_0.25V_0.25)C（HEC-2）high-entropy carbide ceramics through component control.Based on the theoretical study of the formation possibility of HEC-2 in combination with first-principles calculation,HEC-2 was prepared by hot-press sintering technique（2373 K,30 MPa）.In combination with the first-principles calculation,the results showed that the mixing enthalpy of HEC-2 was 5.526 k J/mol and the mixing entropy of HEC-2 was 0.693R.However,the lattice size difference of this system is 4.59%,which indicates that the formation possibility of HEC-1 high-entropy ceramics is considerably high.The experimental results showed that the as-fabricated HEC-2 exhibits single-phase rock-salt structure and all the elements distribute uniformly from microscale to nanoscale.The study found out that some interesting nanoplates can be observed on its cross section,and the element distribution from nanoscale to micrometer scale is uniform.Due to the solid-solution effect and the presence of nanoplates and pores in HEC-2 ceramic,HEC-2exhibited a lower thermal conductivity at room temperature（15.3±0.3 W/（m×K））compared to that of the individual TMCs.Because of the effects of solid-solution strengthening,nanoplate extraction and crack deflection,HEC-2 exhibits excellent comprehensive mechanical properties.The nanohardness（30.3±0.7 GPa）,elastic modulus（460.4±19.2 GPa）and high fracture toughness(4.7±0.5 MPa×m^1/2)of HEC-2 were higher than those of the individual component systems simultaneously.To further increase the configurational entropy of HEC system,this thesis improved the chemical disorder of the anionic sublattice by increasing the number of principal anions and designed a novel multi-anionic high-entropy carbides(Hf_0.2Zr_0.2Ta_0.2Nb_0.2Ti_0.2)N_0.5C_0.5（HENC-1）.In combination with the theoretical analysis of the formation possibility,HENC-1 was fabricated by hot-press sintering technique at a lower temperature（1773 K,30 MPa）.First,we used first-principles calculation to compare the Gibbs free energy（?,）of(Hf_0.2Zr_0.2Ta_0.2Nb_0.2Ti_0.2)N_0.5C_0.5（HENC-1）,(Hf_0.2Zr_0.2Ta_0.2Nb_0.2Ti_0.2)N（HEN-1）,and HEC-1.Due to the contribution of the configuration entropy(HENC-1 has the highest?"_#$%),HENC-1 will become the most stable system among the three（HENC-1 has the lowest?,）above 600K.The experimental results showed that the fabricated HENC-1 had single-phase rock-salt structure with elements distributing uniformly from microscale to nanoscale.The results showed that due to the presence of mass disorder,lattice distortion and small grains in the HENC-1 sample,the HENC-1 sample exhibited the highest hardness（33.4±0.5 GPa）and Modulus（429 GPa）.Between 300 and 800 K,due to the presence of the solid-solution effect,the HENC-1 sample has lower thermal conductivity compared with the average value of the“mixing rule”.This research enriches the types of high-entropy ceramics and gives reference for exploiting high-entropy ceramics with multi-cationic and multi-anionic crystal structures.