Effect Of Carbon(Nitrogen)vacancies On The Formation And Properties Of Entropy-Stabilized Compounds

Vacancies are the empty sites left by the atoms or ions after leaving their original lattice sites,and vacancy defects can affect the sintering activities and properties of materials.Carbon and nitrogen have high solid solubility in refractory metals,and thus refractory metal carbides and nitrides have a wide range of stoichiometry and belong to vacant solid solution.The presence of vacancy defects can accelerate mass transfer at high temperatures and reduce the sintering temperature of the materials,and moreover improve the intrinsic brittleness.In this paper,vacancy defects are introduced to improve the sintering activities and fracture toughness of refractory transition metal carbides and high entropy transition metal carbides(carbonitrides).VC_1-x(0.4?1-x?0.6)with high-concentration carbon vacancies were synthesized through mechanical alloying and spark plasma sintering.VC_1-x(0.5?1-x?0.6)with a quasi-monophasic structure are composed of carbon-rich matrix and carbon-poor intergranular/intragranular precipitates.The matrix and precipitates have the same rock-salt structure and the similar lattice parameters of 4.155?and 4.138?,respectively,which are smaller than that of VC(4.175?),caused by high-concentration carbon vacancies.The coherent interfaces and high-concentration carbon vacancies in VC_1-x can remarkably improve the toughness.The coherently interfaces strengthens phase boundaries and make a transgranular fracture feature and crack deflection near coherent nanoprecipitates.Due to vacancy defects,numerous amorphous bridges spanning the cracks appeared in VC_1-x.Attributed to the coherent toughening and amorphous bridging toughening,the VC_0.5sintered at 1400°C shows high hardness of 20.5 GPa and fracture toughness of 6.3 MPa·m^1/2.The fracture toughness of VC_0.5 increases by 103%,compared to that of VC(3.1 MPa·m^1/2).Using VC_0.5 as the source of vacancies,WC-VC_0.5 composites were synthesized.Due to the solid solution reaction between WC and VC_0.5,the densification of composites is efficiently facilitated and the fracture mode transition from intergranular type to transgranular type.Vacancies lead to the formation of numbers of stacking faults in composites.The WC-VC_0.5 composite sintered at 1750°C(WV5)shows high hardness of26.2 GPa and fracture toughness of 9.2 MPa·m^1/2,and the hardness of WV5 at 700°C is more than 20 GPa.Using non-stoichiometric compounds as the source of vacancies,a series of multi-principal entropy-stabilized compounds were synthesized at a relatively low temperature,including ternary Ti C_0.4/WC/0.5Mo₂C and Ti N_0.3/VC/Nb C,quaternary Ti N_0.3/VC/Nb C/Ti C,quintuple Ti N_0.3/VC/Nb C/Ti C/Ta C,hexatomic Ti C_0.4/VC/Nb C/Ti N/Mo₂C/WC and heptatomic Ti N_0.3/VC/Nb C/Ti C/Ta C/Mo₂C/WC.These entropy-stabilized compounds all have a face-centered-cubic(Fm3?m)structure with highly compositional uniformity.The existence of carbon(nitrogen)vacancies increases the mixing entropy and improves the thermodynamic stability of the system.Furthermore,vacancy defects can reduce the diffusion activation energy of metal atoms and promote the interdiffusion of atoms.Due to vacancy defects and solid solution strengthening,entropy-stabilized compounds show excellent hardness-toughness synergy.The hardness of entropy-stabilized compounds is significantly higher than that predicted from the rule of mixture.The hardness of quaternary Ti N_0.3/VC/Nb C/Ti C ranges from 23.0 to 26.0 GPa,and the toughness of Ti N_0.3/VC/Nb C/Ti C/Ta C/Mo₂C/WC sintered at 1300°C is 8.4 MPa m^1/2.Moreover,the thermal diffusivities of entropy-stabilized compounds are obviously lower than those of binary RTMCs,which is attributed to the vacancy defects and lattice distortion.