On the atomic structure of {001}W

Results of a field ion microscope study of the clean surface structure of the {001} surface plane of tungsten are presented. The major conclusions are that {001}W is reconstructed over the temperature range 15–580 K, and that the reconstructed surface contains an alternating vertical component to the displacements of the W surface atoms. There are two magnitudes of verticaldisplacement, each arrayed with p(2 × 2) symmetry, and if the differences in the magnitude are neglected, the vertical displacements are arrayed in the well known (√2 × √2)R45° configuration. Details of this newly developed experimental approach for the study of surface reconstruction are reported, along with a number of control experiments which exclude the possibility that these results are artifacts due to the experimental technique. The discussion includes a comparison of the present results with those drawn from other experimental techniques, primarily low energy electron diffraction.     

Properties of mono-vacancy in L12-type Ni3Al ordered alloy

The relaxed structures and the formation and migration energies of the mono-vacancy in L1₂-type Ni₃Al ordered alloy have been investigated by combining the modified analytical embedded-atom method (MAEAM) with molecular dynamics (MD) simulation. The movements of the atoms in the vicinity of the mono-vacancy are toward the vacancy, except for the second-nearest-neighbor Al atoms and the fifth-nearest-neighbor Ni atoms around an Al vacancy and the third-nearest-neighbor Al atoms and the fourth-nearest-neighbor Ni atoms around a Ni vacancy that move outward from the vacancy. Furthermore, each of the atoms in the vicinity of either an Al or a Ni vacancy moves approximately along a line connected between the atom and the vacancy. From energy minimization, it is easy for a Ni vacancy to form and to migrate in one nearest-neighbor jump.     

Effect of alkali leaching on the surface structure of Ni3Al catalyst

The surface structure of the alkali-leached single-phase Ni₃Al powder was investigated by X-ray diffraction, BET (Brunauer-Emmett-Teller) surface area analysis, electron microscopy, X-ray photoelectron spectroscopy, and temperature-programmed reduction. It was found that fine Ni particles of several nm in diameter were formed on the outer surface layer of the Ni₃Al powder after the alkali leaching process. The surface of the Ni particles was covered with a thin layer of Ni oxides and hydroxide, Ni₂O₃, NiO and Ni(OH)₂, and these Ni oxides and hydroxide can be easily reduced by hydrogen to the metallic nickel that is catalytically active. The inside of the Ni₃Al powder remained as the original Ni₃Al ordered structure after alkali leaching. Having heat resistant properties, the Ni₃Al phase can serve as a support of the fine Ni particles and provide the structural and thermal stabilities to the fine Ni particles.     

Direct observation of the atomic structure of W {100} surfaces

Fully resolved atomic images of the W {100} plane are obtained with the field ion microscope at 21°K. The image structure is consistent with the (1 × 1) structure. If shifts of atoms in the 〈110〉 directions parallel to the plane have occurred as derived from LEED investigations in the temperature range of 100–370°K, then the shifts at 21°K are too small to be observed in the FIM.     

Ni/Ni3Al interface: A density functional theory study

The optimal geometries, mechanical and thermal properties, and electronic structures of the three low index (0 0 1), (1 1 0), (1 1 1) Ni/Ni₃Al thin film were studied using first principle calculations. Simulated results indicated that Ni and Al atoms in γ^′ phase preferred to place in the hollow site of Ni atoms in γ phase. In hollow site models, electronic states affected by interface localize within 2 atomic layers. While the top site model, electronic states localize within 3 atomic layers. It is also found that hollow site (1 1 0) interface has the best mechanical properties. Hollow site (0 0 1) interface is the most easily formed interface, which has the best thermodynamic properties.     

Diffusion research between Ni3Al coating and titanium alloy produced by plasma spraying process

A Ni₃Al coating was prepared by plasma spraying technique on the surface of titanium alloy. Ni-Al mixed powders, coatings and reaction products were investigated by scanning electron microscope, EDS, DSC and XRD. A tight bonding between the coating and the substrate was formed. The X-ray diffraction analysis of the patterns showed that the coating not only had Ni₃Al phase, but also had NiO and Al₂O₃ phase microcontent. Comparing Ni coated Al to Ni₃Al at 900 °C, the diffusion was stronger and the diffusion layer was thicker. A minute pore structure was formed at 1200 °C in the front edge of solid-state reaction layer. So Ni₃Al restrained the solid-state reaction of the coating with the substrate, and as a whole weakened the entry of oxygen atoms into the substrate and quenched the out-diffusion of titanium.     

The effect of Pt on Ni3Al surface oxidation at low-pressures

The fully-oxidized surface that forms on (1 1 1) oriented Ni₃Al single crystals, with and without Pt addition, at 300-900 K under oxygen pressures of ca. 10⁻⁷ Torr was studied using XPS, AES, and LEIS. Two main types of surfaces form, depending upon oxidation temperature. At low-temperature, the predominant oxide is NiO, capped by a thin layer of aluminum oxide, which we refer to generically as Al_xO_y. At high-temperature (i.e., 700-800 K), NiO is replaced by a thick layer of Al_xO_y. By comparing samples that contain 0, 10 and 20 at.% Pt in the bulk, we find that the effect of Pt is to: (1) reduce the maximum amount of both NiO and Al_xO_y; and (2) shift the establishment of the thick Al_xO_y layer to lower temperatures. Platinum also decreases the adsorption probability of oxygen on the clean surface.     

液态金属Ni3Al凝固过程中结构转变的分子动力学模拟

采用分子动力学模拟方法对液态Ni3Al凝固过程进行了研究,考察了不同冷却速度下液态Ni3Al结构变化特点,原子间相互作用势采用F－S多体势,结构分析采用键取向序和对分析技术,计算结果表明,冷却速度对液态Ni3Al结构转变有重要影响,给出了不同冷却速度下液态Ni3Al结构转变的微观信息。     

Pressure dependence of the Curie temperature of some Ni3Al compounds

Pressure experiments on four ferromagnetic Ni₃Al compounds with Curie temperatures between 30 K and 72 K show a decrease of T_c with increasing pressure. Values for the parameter α(= -T_c?T_c/?p) are between 17 and 26.     

LEED structure analysis of a Co{001} surface

A {001} surface of face-centered-cubic cobalt was cleaned to the point of elimination of all impurities except carbon and oxygen, which were reduced to minimum terminal amounts. A LEED structure analysis of this surface, using 12 intensity spectra at 3 angles of incidence, reveals that the atomic arrangement corresponds to truncation of the bulk structure but with about 4% contraction of the first interlayer spacing along 〈001〉 with respect to the bulk.     

Electron states and atomic positions at the {001}, {110} and {111} faces of W and Mo

The results of ab initio calculations on the {001}, {110} and {111} surfaces of W and Mo and on the (√2 × √2)R45° reconstructed W {001} surface are presented. A distribution of surface states in reasonable agreement with experiment is found. A simple parametrisation of the short range repulsive force between transition metal atoms is used to predict, for all these surfaces, relaxations which are comparable with those observed. This same parametrisation indicates that the W and Mo {001} surfaces are stable to proposed reconstructive displacements.     

镍基单晶超合金Ni/Ni3Al晶界的分子动力学模拟

在镍基单晶超合金中，由于单晶Ni的晶格常数比单晶Ni₃Al的稍小，在Ni/Ni3Al晶界面 上必然要出现错配.采用分子动力学模拟了镍基单晶超合金的Ni/Ni₃Al晶界的结 构，考虑 了两个不同的初始模型，并进行了分子动力学弛豫.弛豫的结果均表明：由于晶格的差异形 成的错配能不是通过长程晶格错配的方式来释放，而是通过在局部区域形成位错的方式释放 的.由于Ni₃Al相周围Ni相环境的不同，形成的位错也有所不同. 关键词： 镍基单晶超合金 晶界 分子动力学模拟     

Epitaxial relationships between Al,Ag and GaAs{001} surfaces

Detailed results concerning the orientation of Al and Ag layers grown by MBE onto GaAs{001} surfaces are reported. The observed crystallographic relationships have been found to depend only on the growth temperature in the case of Ag, while for Al the situation is more complex. Possible effects of other parameters such as structure and stoichiometry of the starting GaAs surface and growth rate, are discussed and compared with other results recently reported.     

Simulation of positron annihilation response to mechanical deformation of nanostructured Ni3Al

Simulations of the positron response to the mechanical deformation of Ni₃Al nanowires are performed on modelled samples obtained using molecular dynamics. Particular attention is paid to the evolution of the various open volume defects and their interaction during deformation. Positron simulations are done in conjunction with a general, geometrical analysis of open volumes in the studied samples, which brings complementary information to the positron response results.     

Energy band structure of Ni3P

A band structure calculation is performed for Ni₃P. It is found that the density of states curve closely resembles that of pure Ni.     

First-principles calculations for the elastic properties of Ni-base model superalloys: Ni/Ni3Al multilayers

A model system consisting of Ni[001](100)/Ni3Al[001](100) multi-layers are studied using the density functional theory in order to explore the elastic properties of single crystal Ni-based superalloys. Simulation results are consistent with the experimental observation that rafted Ni-base superalloys virtually possess a cubic symmetry. The convergence of the elastic properties with respect to the thickness of the multilayers are tested by a series of multilayers from 2γ′+2γ to 10γ′+10γ atomic layers. The elastic properties are found to vary little with the increase of the multilayer’s thickness. A Ni/Ni3Al multilayer with 10γ′+10γ atomic layers (3.54 nm) can be used to simulate the mechanical properties of Ni-base model superalloys. Our calculated elastic constants, bulk modulus, orientation-dependent shear modulus and Young’s modulus, as well as the Zener anisotropy factor are all compatible with the measured results of Ni-base model superalloys R1 and the advanced commercial superalloys TMS-26, CMSX-4 at a low temperature. The mechanical properties as a function of the γ′ phase volume fraction are calculated by varying the proportion of the γ and γ′ phase in the multilayers. Besides, the mechanical properties of two-phase Ni/Ni3Al multilayer can be well predicted by the Voigt-Reuss-Hill rule of mixtures.