Chong-Yu Wang

First-principles study of the electronic structures of icosahedral TiN (N=13,19,43,55) clusters

We have studied the electronic structures of icosahedral Ti(N) clusters (N=13, 19, 43, and 55) by using a real-space first-principles cluster method with generalized gradient approximation for exchange-correlation potential. The hexagonal close-packed and fcc close-packed clusters have been studied additionally for comparisons. It is found that the icosahedral structures are the most stable ones except for Ti(43), where fcc close-packed structure is favorable in energy. We present and discuss the variation of bond length, the features of the highest occupied molecular orbitals and the lowest unoccupied molecular orbital, the evolution of density of states, and the magnetic moment in detail. The results are in good agreement with the predictions from the collision-induced dissociation and size-selected anion photoelectron spectroscopy experiments.

First-principles study of diffusion of oxygen vacancies and interstitials in ZnO

A comprehensive investigation of oxygen vacancy and interstitial diffusion in ZnO has been performed using ab initio total energy calculations with both the local density approximation (LDA) and the generalized gradient approximation (GGA). Based on our calculation results, oxygen octahedral interstitials are fast diffusers, contributing to annealing processes, as well as being responsible for the self-diffusion of oxygen for n-type ZnO, and oxygen vacancies are responsible for the self-diffusion of oxygen for p-type ZnO.

Detailed check of the LDA + U and GGA + U corrected method for defect calculations in wurtzite ZnO

Based on a detailed check of the LDA + U and GGA + U corrected methods, we found that the transition energy levels depend almost linearly on the effective U parameter. GGA + U seems to be better than LDA + U, with effective U parameter of about 5.0 eV. However, though the results between LDA and GGA are very different before correction, the corrected transition energy levels spread less than 0.3 eV. These more or less consistent results indicate the necessity and validity of LDA + U and GGA + U correction

The electronic effect in the (100) edge dislocation core system with a carbon atom in α-iron: a first-principles study

Using the DMol molecular cluster method and the self-consistent discrete variational method based on density functional theory, we investigated the electronic effect in the edge dislocation core system with a C atom in cr-iron. A cluster model containing 96 atoms was used to simulate the local environment of the Fe edge dislocation, and the optimization results show that the C atom moves away from the compression side to the dilated region and falls into a flat tetrahedral interstice composed of four adjacent Fe atoms. We present the characteristic parameters including the structural energy, the interatomic energy, the partial density of states and the charge-density difference of the dislocation core system. The results suggest that the C atom stays steadily at a favourable site in the tetrahedron and forms strong covalent-like bonds with its adjacent Fe atoms. Moreover, the remarkable charge redistribution and the large binding energy drop in the dislocation core system indicate the formation of a C impurity-Fe edge dislocation complex which implies an effect of trapping of the dislocation core on the C atom.

Effects of Cr, Mn on the cohesion of the γ-iron grain boundary

Abstract The effect of alloying elements Cr and Mn on the cohesion of the γ-iron Σ11[1 1 0]/(11 3 ) grain boundary (GB) is investigated based on the thermodynamic model of Rice–Wang by using the first-principles density functional theory. The electronic properties are studied for Cr/Fe and Mn/Fe systems. In these systems, the chemical effect of Cr and Mn is in favor of enhancing the cohesion of the grain boundary due to the anisotropic bonding which weakens the bonds in the grain boundary plane, but strengthens those in planes perpendicular to the grain boundary. However, the structural relaxation effect is detrimental to the cohesion of the grain boundary. After synthesizing these two effects, Cr can act as a cohesion enhancer and Mn is an embrittler.

The effects of boron and hydrogen on the embrittlement of polycrystalline Ni3Al

The discrete-variational method within the framework of density functional theory is used to study the effects of both boron and hydrogen on the embrittlement of polycrystalline Ni3Al. The calculated results show that there are strong repulsive interaction between the boron and the hydrogen atoms, if they occupy the nearest interstitial sites, respectively, in the Ni3Al grain boundaries. It indicates that the boron atoms inhibit the diffusion of hydrogen atoms along the grain boundary. It may be the main reason why boron call suppress the moisture induced hydrogen embrittlement. Our results also show that the attractive interactions between boron and some substrate atoms are weakened, but the attractive interactions between boron and other substrate atoms are enhanced, when hydrogen atoms are forced into the grain boundary and occupy the nearest interstitial sites to boron atoms. As a result, the bonding states are polarized in the local region of the grain boundary. It may suppress the movement of slips across the grain boundary. Furthermore, the weakening effects of hydrogen to the grain boundary are hardly affected by the boron atoms, even though they are very near to each other. It can be concluded that hydrogen embrittlement takes place when the boron-doped polycrystalline Ni3Al are charged with hydrogen

Electronic effects of alloying elements Nb and V on body-centred-cubic Fe grain boundary cohesion

The segregation effects of Nb and V on bcc FeSigma3[110](111) grain boundary cohesion are studied by the first-principles DMol method within the framework of density functional theory. The calculated segregation energy difference between the grain boundary and the corresponding free surface is -0.51 eV (-0.58 eV) for solute Nb (V), which indicates that both Nb and V could enhance the grain boundary cohesion in body-centred-cubic Fe. We found that the chemical effect and the geometry effect of Nb (V) play crucial but opposite roles in determining whether a material is brittle or ductile. The chemical effect is dominant and advantageous for grain boundary cohesion. Also, Nb and V show very different behaviours: in chemical effect, Nb is more conducive to ductility than V; while in the geometry effect, Nb is less conducive to ductility and more conducive to embrittlement than V.

The effects of 3d alloying elements on grain boundary cohesion in γ-iron: a first principles study on interface embrittlement due to the segregation

By the use of a first principles density functional theory, two kinds of models, namely the Rice-Wang thermodynamics model and the Seah quasi-chemical model, are employed to evaluate the embrittling tendency of a grain boundary (GB) due to the 3d element segregation. The first principles method based on those two models is appropriate for calculating the chemical and structural relaxation contributions to the changes of GB cohesion with the 3d segregants. The effects of the 3d transition elements, such as Ti, V, Cr and Mn, on a stable fcc Fe Sigma 11 [1 (1) over bar0]/(11 (3) over bar) GB are studied and the difference between these two models is interpreted. When the chemical and the structural relaxation effects are taken into account, the calculated results for these two models are coincident for most of the elements studied, except for chromium. After analysing their chemical bonding in detail, we find that this discrepancy may be attributable to a lower susceptibility of the Seah model to the bonding anisotropy caused by Cr in the GB. It is proposed that the Seah model should be prudently used for some elements, especially those lying in the middle of a transition period.

First-principles study of the segregation effects on the cohesion of F.C.C. grain boundary

A scheme to evaluate the Griffith work of interfacial separation, 2 gamma (int), is proposed based on first principles to investigate the segregation effects on a Sigma 11[1 (1) over bar0]/(11 (3) over bar) gamma -iron grain boundary. The chemical interaction of substitutional segregants Cr and Mn is able to enhance the cohesion of the grain boundary by anisotropic bonding, which weakens the bondings in the grain boundary plane and strengthens those in the vertical plane. However, their structural relaxation contributions are both detrimental to the cohesion of the grain boundary. After combining these two contributions, Cr acts as a cohesion enhancer but Mn as an embrittler. The interstitial segregants carbon and nitrogen can strengthen the cohesion of the grain boundary by forming strong bonding with their neighbour Fe atoms and restraining their surrounding Fe atom relaxation. The ability of carbon and nitrogen to improve the property of the grain boundary is relative to the environment of their segregation sites. The consistency between the present work and the previous reports gives evidence for the correctness of the scheme.

First-principles study on the effects of co-segregation of Ti, B and O on the cohesion of the α-Fe grain boundary

The effects of Ti, B and O co-segregating on the ?-Fe ?5?[001]/(010) grain boundary are studied by a first-principles method, DMol. We found that (Ti+B) acts as an enhancer, but O can completely offset the beneficial effect of Ti. Based on the segregation energy analysis, it is also found that Ti can effectively prohibit O from segregating to the grain boundary and therefore eliminate the embrittlement of grain boundary induced by O. Thus, Ti is a kind of desirable addition in ?-Fe to improve the mechanical properties.