Synthesis And Properties Of Group ⅣB-ⅤB Carbide Solid Solutions

Transition metal carbides（TMC）solid solution powders with superior comprehensive performance can effectively overcome the application bottleneck of mono-component TMC powders.Thereby,they are widely used as the second phase of composites,structural materials of aircraft,thermal/environmental coatings,electromagnetic materials,etc.Nevertheless,the strong covalent interaction between the transition metal and carbon atoms in TMC makes it difficult to fabricate TMC solid solutions,which results in a limited design of the composition.At present,the research of TMC solid solutions generally focuses on the preparation of materials with equimolar composition and their mechanical and thermal properties,while that of solid solution powders with other components is still restricted,and therefore it is impossible to establish the relationship between the components and properties of materials systematically.In light of this,this thesis proposes a new strategy for fabricating TMC solid solution powders by pressureless heat-treating TMC and Co mixed powders at a relatively low temperature based on the dissolution-precipitation mechanism.On the basis of the thermodynamic calculation,ⅤB-ⅤB and ⅣB-ⅤB group TMC solid solution powders were prepared at 1500℃,and their composition,morphology,formation mechanism,and properties were comprehensively explored.The research contents and results are as follows:（1）Thermodynamic calculation confirmed that the dissolution-precipitation process of(Nb_xTa_1-x)C powders prepared by this method can proceed spontaneously at 1300～1900℃.Despite this,experimental results verified that(Nb_xTa_1-x)C prepared at 1300℃were heterogeneous solid solutions.Only at a temperature above 1500℃can the single-phase solid solutions with uniform composition be formed.Compared with the traditional solid inter-diffusion that needs to overcome the strong covalent bond between the transition metal and carbon atoms,Co metal acting as flux can form a molten liquid phase at high temperatures,which accelerates the mass transfer process of the reaction,making it possible to successfully prepare homogeneous TMC solid solutions via the pressureless low-temperature heat treatment.The solid solution powders synthesized according to this route at 1500℃were microcuboids with smooth surfaces,distinct edges and corners,and exposed the{100}facet.Besides,the chemical composition was homogeneous and can be continuously regulated by adjusting the ratio of pristine carbide powders,and the carbon vacancy concentration was 0.08.The annealing temperature influenced the phase and morphology of(Nb_xTa_1-x)C.Specifically,(Nb_xTa_1-x)C powders were transferred into cubes with rounded corners or even a near-spherical morphology at 1900℃,and the carbon vacancy concentration was reduced from 0.08 to 0.02.As-fabricated(Nb_xTa_1-x)C powders exhibited good thermal stability,which still remained their phase and morphology at 1500℃.Besides,the Nb/Ta molar ratio can significantly affect the properties of(Nb_xTa_1-x)C.On the one hand,the increasing Ta C content helped to improve the oxidation resistance due to the enhanced temperature and activation energy of oxidation,as well as the dense structure of as-formed oxides.On the other hand,the relationship between the hardness and Young’s modulus of(Nb_xTa_1-x)C with Nb C molar fraction simply obeyed the rule of mixture,and the presence of carbon vacancy will further improve the hardness of(Nb_xTa_1-x)C.（2）The dissolution-precipitation strategy was extended to the preparation of TMC solid solutions composed of carbides in different groups.(Ti_xTa_1-x)C and(Ti_xNb_1-x)C powders with full-spectrum composition were prepared at 1500℃.The research demonstrated that the molar fraction of Ti C and carbon vacancy concentration can significantly change the electronic structure and bonding properties of carbides,and therefore the density,mechanical and mechanical performance.Both(Ti_xTa_1-x)C and(Ti_xNb_1-x)C exhibited obvious solid solution hardening,and the p-dσorbital between the transition metal and carbon was completely filled when the Ti C content was 60%（namely VEC=8.4）,which maximized the covalent properties and shear deformation resistance of the carbides and hence led to the maximum value of hardness.In addition,(Ti_xTa_1-x)C exhibited low dielectric loss and hence satisfactory microwave transparent performance as a consequence of the combination of low electrical conductivity,low carbon vacancy concentration,and smooth cuboid morphology,among which solid solution with equimolar composition possessed the highest transmittance（above 90%）at a specific thickness of 1.5 mm.（3）Equimolar（Hf,Nb,Ta）C binary and ternary solid solution powders were prepared through the dissolution-precipitation reaction.The results indicated that the formed molten Co-Hf-Nb-Ta alloys can make it possible to successfully prepare ternary TMC solid solutions at1500℃,which is expected to be extended to the synthesis of carbide solid solution powders with more components（like high-entrpoy carbides）.Studies on the oxidation behavior and kinetics of（Hf,Nb,Ta）C showed that the effect between various metal elements can significantly influence the phase of oxidation products of solid solutions;besides,the dense structure of oxides will inhibit further oxidation process,inducing better oxidation resistance than their mono-component TMC.Surprisingly,(Hf_0.5Ta_0.5)C with the oxidation activation energy up to261.0±7.8 k J/mol and a temperature of 707.5℃corresponding to the oxidation conversion rate of 10%exhibited the best oxidation resistance performance among all carbides.