Abstract
AbstractIntrinsic defects and Mo‐related defects in vanadium carbide VC, as well as the defect complexes between vacancies and Mo defects were investigated by means of first‐principles calculations within the framework of density functional theory. In addition, Mo diffusion in VC was also studied using LST/QST method. The formation energies of defects have clearly shown that except C vacancy (VC) all other point defects are not energetic favorable compared to perfect VC. VC can exist in the lattice forming nonstoichiometric carbide VCx (x < 1), and also can stabilize the Mo‐related defects (SMo‐V, SMo‐C, and TMo). Free Mo atoms have the strong tendency to enter the already formed VV and occupy the lattice position of V atoms. Meanwhile, Mo atom in C lattice (SMo‐C) and interstitial Mo (IMo) atom can also enter the VV position stabilizing the lattice structure. SMo‐C + VV will transform into SMo‐V + VC and IMo + VV will transform into SMo‐V during optimization, and large binding energy makes Mo atom tend to exist in the interstitial position. From the perspective of energy, Mo atom tends to diffuse through the interstitial position.
Highlights
Among all kinds of Transition-metal carbide (TMC), vanadium carbide VC has attracted more attention all over the world due to its relatively low price of element V compared with W/Mo and relatively low formation energy, which is beneficial to control the precipitation of carbides.[4,5]
While free Mo atom has high probability to enter into V vacancy (VV) position to form SMo-V, because the formation energy for SMo-V is significantly lower than that for VV, which indicates that VV in VC cell is occupied by free Mo atom
When extraneous Mo atom has already occupied V/C lattice or interstitial position, such as in the case of SMo-V, SMo-C, and IMo, it is suggested from formation energy values that Mo atom in C lattice and interstitial Mo atom can enter the VV position stabilizing the lattice structure
Summary
Transition-metal carbide (TMC) is a kind of material with high melting point, high hardness, high thermal stability, and excellent corrosion resistance as well as special electrical, magnetic, and catalytic properties, which has been widely used in the field of machinery, mining, high-temperature structural components, nuclear materials, and new catalytic materials.[1,2,3]. The researches about point defects in TMCs have been reported from time to time It is well-known that the point defects are the ones at the atomic scale, and it is difficult to study their effect in detail by conventional experimental methods. Carbide TiC was chosen by Sun et al[23] to calculate the self-diffusion of Ti interstitial-based point defects and complexes. The first-principles method is adopted in this work to calculate the VC systems with different intrinsic defects (vacancy, substitution, and tetrahedral interstitial) and analyze their formation energy and electronic structure. Element Mo is often used as the binder in the preparation of hard alloys,[30] and Mo content is always found relatively high in VC in a self-designed novel Febased alloy by our group.[31] So Mo-related defects and the influence of impurity Mo atoms on the defects in VC, as well as Mo diffusion are studied in this paper. Defect calculations were performed in a prerelaxed 64-atom supercell, consisting of 2 × 2 × 2 replica of VC unit cell
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