Effect Of Vacancy On Cell Volume And Mechanical Properties Of Transition Metal Carbides And Nitrides

Transition metal carbides and nitrides have great research value in the field of technology and industrial production due to their high strength,high hardness,high melting point,excellent high temperature stability,good wear resistance,and electrical and thermal conductivity.Among them,IVB group transition metal carbide and nitride ceramics（TiC,ZrC,TiN and ZrN）also have low neutron absorption cross section and excellent irradiation resistance,which are considered to be promising candidates for TRISO fuel particles,fuel cladding tubes and inert matrix fuels in Generation IV nuclear reactors.However,they are prone to generate vacancies in the nonmetal sublattice resulting in the formation of non-stoichiometric carbides and nitrides,especially in extreme environments such as nuclear reactors and high temperatures.The formation of vacancies can change the lattice constant or cell volume of the material and thus reduce the service performance of the material.Therefore,it is of great importance to design and prepare a transition metal carbide and nitride materials whose cell volume does not change with the concentration of vacancies,which will provide a wider choice of materials for the design of new nuclear energy systems.Aiming at the above problems,this paper designs and prepares a material with cell volume independent of vacancies through a combination of first-principles calculations and experimental methods.The main research contents and results are as follows.（1）The variation laws of cell volume of four transition metal carbides and nitrides（TiC,ZrC,TiN,and ZrN）with the nonmetallic vacancy concentration were predicted by the first-principles calculations based on density functional theory.Based on their cell volume variation laws,a new material Ti_0.19Zr_0.81N_1-x with TiN and ZrN as the basic solid solution elements was designed.By calculating the variation relationship between their cell volume and N-vacancy concentration,it is found that the cell volume of this material remains basically constant in the range of vacancy concentration x<0.3.The theoretical analysis indicates that this phenomenon of the cell volume not varying with vacancy is the result of the competition between the vacancy substitution mechanism and the atomic bonding mechanism.（2）In order to verify the above theoretical calculations,TiN,ZrN and ZrH₂ were used as raw material powders,and the non-stoichiometric Ti_0.19Zr_0.81N_1-x（x=0-0.3）was successfully synthesized by the reactive spark plasma sintering technique at 1700°C for 10 min.The samples had small amounts of Zr₇O₁₁N₂ and m-ZrO₂ in addition to the main phase.The densities of all samples exceeded 97%,and the more nitrogen vacancies,the more dense the samples were.The lattice constants and cell volumes of the samples were calculated based on XRD patterns,and the lattice constants or cell volumes of the samples were almost constant in the range of nitrogen vacancy concentration x=0-0.3,which confirmed the accuracy of the theoretical calculation.With the increase of vacancy concentration,the grain size of the sample keeps increasing and the oxygen content in the matrix also increases,which is because the nitrogen vacancy can promote the material diffusion during the sintering process and reduce the energy barrier of oxygen solid solution into the main phase lattice.（3）The relationship between the mechanical properties of Ti_0.19Zr_0.81N_1-x and the vacancy concentration was investigated by first-principles calculations,and the relationship between the mechanical properties of TiC_1-x,ZrC_1-x,TiN_1-x and ZrN_1-x and the vacancy concentration was studied comparatively.The calculated results show that the Young’s modulus and hardness of Ti_0.19Zr_0.81N_1-x are between TiN_1-x and ZrN_1-x and are closer to the mechanical property values of ZrN_1-x,which are in accordance with the mixing law.In addition,the elastic constants,Young’s modulus and Vickers hardness of these transition metal carbides and nitrides show a decreasing trend with increasing vacancy concentration.The decrease of mechanical properties is due to the decrease of covalent bonding concentration and increase of metal bonding concentration due to the lack of carbon or nitrogen.The Vickers hardness value of Ti_0.19Zr_0.81N_1-x is in basic agreement with the hardness value calculated by using Chen’s model,and the difference is only～5%.In addition,with the increase of N-vacancy concentration in Ti_0.19Zr_0.81N_1-x,its average grain size becomes larger and the material fracture toughness gradually decreases.