Jia-Xiang Shang

Unexpected relationship between interlayer distances and surface/cleavage energies in γ-TiAl: density functional study

Density functional calculations were performed to study the γ-TiAl (001), (100), (110) and (111) surfaces. The (100) surface is the most stable under Ti-rich conditions, while the Al-termination (110) surface becomes the most stable with the increase of Al chemical potential. We calculate that in γ-TiAl intermetallic compound the larger the interlayer distance, the larger the surface energy and cleavage energy. This is different from the situation in a pure metal. This phenomenon can be explained by the analysis of the bonding characteristics in γ-TiAl. In particular there are both metallic and covalent bonds in γ-TiAl, and the strongest covalent bonds mainly focus on the center of three Ti-Al-Ti atoms. It is the covalent bonds that affect greatly the cleavage energy, the surface energy and the surface stability.

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

Oxidation of the two-phase Nb/Nb5Si3 composite: The role of energetics, thermodynamics, segregation, and interfaces

Oxidation behavior of the two-phase Nb/Nb(5)Si(3) composite is of significant importance for the potential applications of the composite at high-temperature conditions. We investigate the atomic-scale oxidation mechanism of the Nb/Nb(5)Si(3) composite with first-principles density-functional theory and thermodynamics analysis. In particular, the effects of energetics, thermodynamics, segregation, and interfaces are identified. The clean composite surface is found to be composed of both Nb(110) and Si-terminated Nb(5)Si(3)(001). Energetics and thermodynamics calculations show that, during the oxidation process, the Nb(110) surface is oxidized first, followed by the segregation of niobium of the Nb(5)Si(3)(001) surface and subsequent oxidation of the Nb element of Nb(5)Si(3). High coverage of oxygen results in dissolved oxygen in bulk Nb through the diffusion of oxygen in the surface and at the interface. The theoretical investigation also provides an explanation, at the atomic-scale, for the experimental observation that the oxidation layer is essentially composed of niobium oxide and almost free of silicon. Furthermore, the methodology of this work can be applied to investigations of the oxidation behavior of other two-phase and multi-phase composites.

The effects of alloying elements Al and In on Ni–Mn–Ga shape memory alloys, from first principles

The electronic structures and formation energies of the Ni(9)Mn(4)Ga(3-x)Al(x) and Ni(9)Mn(4)Ga(3-x)In(x) alloys have been investigated using the first-principles pseudopotential plane-wave method based on density functional theory. The results show that both the austenite and martensite phases of Ni(9)Mn(4)Ga(3) alloy are stabilized by Al alloying, while they become unstable with In alloying. According to the partial density of states and structural energy analysis, different effects of Al and In alloying on the phase stability are mainly attributed to their chemical effects. The formation energy difference between the austenite and martensite phases decreases with Al or In alloying, correlating with the experimentally reported changes in martensitic transformation temperature. The shape factor plays an important role in the decrease of the formation energy difference.

First-principles study on the incipient oxidization of Nb(110)

The incipient oxidization of Nb(110) has been investigated using the density functional theory method. We rationalize the well-known puzzle, i.e., Nb(110) is difficult to clean, by calculating the O dissolution, and the on-surface and subsurface adsorption at low concentration. It is found that the structure of on-surface O adsorption at 0.50 monolayer (ML) coverage has the largest binding energy and minimum work function, in agreement with experimental results. At 1.00 ML coverage, the inward diffusion of O atoms is promoted by O adatoms, attributed to the formation of a local electric field. Our theoretical results improve the understanding of the experiments showing that NbO(x) oxides on the surface can be formed and decomposed by treating samples at 1500-2000 K in vacuum. Furthermore, the thermodynamic analysis of the O/Nb(110) systems shows that bulk NbO is stable in vacuum, in agreement with the observed formation of NbO nanostructures on Nb(110).

Thickness dependence of structure stability of Co/Cu(100) superlattices

The electronic structures and stability of Co/Cu(100) superlattices have been investigated by a first-principles method based on density functional theory. The models 3Co/xCu (x = 1–8 monolayers) with different Cu layer thicknesses are investigated. The result shows that the stability increases with an increase in Cu layer thickness for odd (or even) Cu layer models. The charge transfer is prominent at the Co–Cu interface; the magnetic moment of atoms at the interfaces is larger than that of the interior Co layers, and the nonmagnetic Cu layer at the interface is slightly spin polarized under the influence of the ferromagnetic Co layers in the neighborhood. Finally, the Fermi energy, densities of states and structural energy are also discussed.

Co/Cu INTERFACE ELECTRONIC STRUCTURES STUDY FROM FIRST-PRINCIPLES

The electronic structures of Co/Cu interface have been calculated by first-principles method based on local spin density approximation. Models 3Co/xCu (x=1-8 monolayers) with different Cu layer thickness are investigated. The results show that the oscillation of the density of states near the Fermi surface with the Cu spacer thickness has been observed and the period of oscillation is about 6 atom layers, which has a good agreement with the corresponding experiments. We also discuss the spin polarization and magnetic resistance with the change of Cu layers thickness. Further analysis shows majority spin states near the Fermi surface played a key role in giant magnetoresistance (GMR).