ELECTRONIC STRUCTURE OF THE LaNi5-xGax INTERMETALLIC COMPOUNDS

The equilibrium structures and electronic structure of LaNi5-xGax （x=0, 0.5, 1.0） compounds have been investigated by all-electron calculations. Based on the full geometry optimization, the densities of states and electron densities of LaNi5-xGax are plotted and analyzed. It is clear that the substitution of Ga at the Ni site leads to a progressive filling of the Ni-d bands, the ionic interaction between Ni and Ni, with Ga plays a dominant role in the stability of LaNi5-xGax compounds. The smaller the shift of EF toward higher energy region, the more stable the compounds will be. The increased contribution of the Ni-d-Ga-d interactions near EF and the low energy metal-gallium bonding bands indicate that the compounds become more stable. The results are compared with experimental data and discussed in light of previous studies.     

MOLECULAR DYNAMICS SIMULATION OF STRUCTURE STABILITY OF SILVER NANOCLUSTERS

The structures of Ag clusters with sizes n=13 to 1157 are studied by tight binding molecular dynamics simulation. It is found that the stable structures of Ag clusters follow the sequence amorphous-crystalline-amorphous-crystalline with the cluster size increasing from 13 to 1157. Furthermore, all the shells of Ag clusters are different from the structure of the corresponding bulk Ag     

Adsorption of atomic S and C on Mg（0001） surface

First-principle calculations based on density functional theory were used to study the adsorption of atomic sulfur and carbon on the Mg（0001） surface in a wide range of coverages from 1/4 ML（monolayer） to 1 ML. It is found that the adsorption of atomic S and C on the high coordinate hollow site is more energetically favorable than that on other adsorption sites. S atom is favorable to be adsorbed at on-surface site and C atom is favorable to be adsorbed at subsurface site. The results suggest that when the coverage increases, the binding energy for S and C atoms will decrease and the interaction between adsorbed atoms tends to be stronger. It indicates that as coverage increases, S-Mg and C-Mg interaction weakens.     

Bonding Character and Formation Energy of Point Defects of He and Vacancy on （001） Surface of bcc Iron by First Principle Calculations

The structure and energy of He impurities and vacancy on (001) surface of bcc iron are investigated by an ab initio method. Three cases for stabilities of a He atom at the surface are found: some of He atoms at surface atomic layers (SAL) relax into vacuum gap; some of surface He atoms at octahedral interstitial site relax into more stable tetrahedral interstitial site; some of surface He atoms still stay at tetrahedral interstitial site. The un-stability of the He atom at the surface system can be explained by deformation mechanism of charge densities and electronic densities of states. It is found that formation energy of the point defects from the topmost SAL to bulk-like atomic layer increase gradually, for example, the formation energies of a monovacancy at the first five topmost SALs are equal to 0.33, 1.56, 2.04, 2.02 and 2.11 eV, respectively. The magnetic moments of Fe atoms in the surface atomic layers are also calculated.     

Crystal structure of Mg3Pd from first-principles calculations

Crystal structure of Mg3Pd alloy was studied by first-principles calculations based on the density functional theory. The total energy, formation heat and cohesive energy of the two types of Mg3Pd were calculated to assess the stability and the preferentiality. The results show that Mg3Pd alloy with Cu3P structure is more stable than Na3As structure, and Mg3Pd alloy is preferential to Cu3P structure. The obtained densities of states and charge density distribution for the two types of crystal structure were analyzed and discussed in combination with experimental findings for further discussion of the Mg3Pd structure.     

Molecular simulation of interfacial reaction between TiAl alloy melts and different coatings

The effect of coatings (Y2O3, ZrO2 and Al2O3) on the interfacial reaction of TiAl alloys was studied with molecular dynamics. The binding energy of coatings and the diffusion process of oxygen in the melt were simulated, and then the simulation results were compared with the experimental results. The simulation results indicate that for each of the three simulated coatings, inordinate interfacial reactions have occurred between the coating and the melt. The binding energy results show that Y2O3 has the best stability and is the most difficult to break down. ZrO2 has the greatest decomposition energy and is the easiest to break down in the melt. Besides,the molecular dynamics indicate that the diffusion coefficient of the oxygen atom in Al2O3 is larger than that in the other two coatings, indicating that oxygen diffusion in Al2O3 is the fastest at a given temperature. The experimental results show that the oxygen concentration of the melt with Al2O3 coating is the highest, and the oxygen diffusion is of similar magnitude to the simulation values, from which the conclusion can be obtained that the oxygen concentration is significantly influenced by the coating materials.     

Ni segregation and thermal stability of reversed austenite in a Fe–Ni alloy processed by QLT heat treatment

High-resolution transmission electron microscopy(HRTEM) and X-ray diffraction(XRD) were used to investigate Ni segregation and thermal stability of reversed austenite(RA) in a Fe–Ni alloy processed by quench–lamellarize–temper(QLT) heat treatment. The results show that the 77 K impact energy of the alloy increases with RA content increasing. As an austenite-stabilizing element, Ni is found to segregate in RA, though Ni is not evenly distributed within RA. The amount of segregations increases near the boundary(twice as high as the balanced content)and decreases to some extent in the center of the RA regions. Ni concentration in matrix near the boundary is lower than that in matrix far from the boundary because of Ni atom transportation from a to c near the boundary. RA in this alloy has high heat and mechanical stability but is likely to lose its stability and transform to martensite when a mechanical load is applied at ultralow temperatures(77 K), which induces plasticity.     

Effects of electric field treatment on microstructures of GH4199 superalloy after long-term aging

The effects of electric field intensity and treatment temperature on the microstructures of GH4199 superalloy after long-term aging were investigated. The results show that the number and size of carbides and TCP（σ phase and μ phase） phase in the alloy increase with increasing electric field intensity at the same heat treatment temperature and holding time. While the number and size of carbides and TCP phase are weekly influenced by treatment temperature with lower electric field intensity of 2 kV/cm. When the treat temperature is up to 1 093 K, annealing twins appear in the alloy, and the number of twins increases with increasing holding time. Since the electric field can provide the enough energy for the movement of vacancies and atom, it is considered that the nucleus of the twins formed with formation stack faults due to the mismatch of local atom in crystal caused by the vacancies, and the twins will grow with the increase of holding time. Meanwhile, such promoting effects on atom movement of the electric field increase with the increase of the electric field intensity, meantime the carbides and TCP phase grow fast with the increase of electric field intensity.     

Liquid structure and viscosity of In80Cu20 alloy

The structure and dynamic viscosity of In80Cu20 alloy melt in the temperature range from 600 ℃ to 1 000℃ were investigated by using a high-temperature X-ray diffractometer and a torsional oscillation viscometer. The experiments show that there exist medium range order (MRO) structures in In80Cu20 alloy melt in a low temperature range above liquidus. The MRO structures are weakened with increasing temperature and disappear when the temperature surpasses 800 ℃. The nearest interatomic distance r1 and the coordination number Ns of In80Cu20 alloy melt decrease as temperature increases from 650 ℃ to 1 000 ℃. Thermal contraction of atom clusters can be found in the heating process. The viscosity of In80Cu20 alloy melt drops as temperature increases and meets with the exponential relation. No sudden change in structure occurs in the measured temperature range. DSC curve of In80Cu20 alloy during cooling process was measured. It is found that there is no noticeable variation of heat during cooling from 1000 ℃ to 600 ℃ , which testifies further that there is no sudden change in structure of In80Cu20 alloy melt.     

Electrodeposition of Ni–Co alloy films onto titanium substrate

Binary bright Ni–Co alloy films were electrodeposited on titanium in the chloride–sulfate electrolytes.The influences of Co2?concentration, current density, and temperature on the Ni–Co alloy films electrodeposition were investigated. The films were analyzed by scanning electron microscope(SEM), energy dispersive spectroscopy(EDS), and X-ray diffraction(XRD). Cathodic polarization for Ni–Co codeposition was performed on Ti working electrodes. With the increase of Co2?concentration, the Ni content in the films decreases and the current efficiency increases slightly. The Ni content increases with the increase of temperature, while it decreases with the increase of current density to a minimum and then increases. The cathodic reduction peak potential is measured to be-1.34 V. Anomalous deposition is found to occur in the Ni–Co codeposition. The SEM of Ni–Co alloy films shows that hydroxide particles are not present on the surface and fine grain, compact, smooth, and bright Ni–Co alloy films are obtained. The XRD result indicates that the deposited Ni–Co alloy film is Ni-solid solution with a facecentered cubic in structure.     

On the possibility of rhenium clustering in nickel-based superalloys

In order to elucidate the role of this element in superalloy metallurgy, the binding energy of Re–Re pairs and the stability of small Re clusters in the nickel face-centred cubic (fcc) lattice is investigated using ab initio density functional theory. It is shown that the formation of Re–Re nearest neighbour pairs is energetically unfavourable, and that this repulsive energy is dramatically reduced as soon as the solute atoms move further apart from one another. Furthermore, small nearest neighbour and second neighbour Re clusters are found to be unstable. The calculations are repeated for W and Ta, which lie beside Re in the periodic table; the results are essentially the same, except that some Ta–Ta higher order pairs have a positive binding energy, consistent with the Ni–Ta binary phase diagram exhibiting several ordered intermetallics. The predictions show that Re clusters are unstable in fcc Ni and it is unlikely that clustering has a role in improving creep and fatigue properties (the rhenium-effect) in Ni-based superalloys.     

Al-Pb互不溶体系机械合金化过程中固溶度的计算

计算了 8种fcc金属 (Ag ,Al,Au ,Cu ,Ni,Pb ,Pd和Pt)和Al Pb互不溶体系的嵌入原子势 (EAM) ,并计算了用EAM模型计算的结构稳定性。计算结果和实验结果吻合较好 ,而且拟合得到的fcc模型在结构上是稳定的。运用拟合得到的数据计算了Pb在Al中的溶解热 ,结果与abinitio计算结果相近。根据机械合金化扩展固溶度的理论 ,计算了Al Pb互不溶体系机械合金化后的固溶度 ,约为 0 .19% (摩尔分数 )。     

First-principles study of vacancy formation and migration in clean and Re-doped γ′-Ni3Al

Using density functional theory calculations in conjunction with the climbing images nudged elastic band method, we studied the vacancy formation and migration in clean and Re-doped Ni₃Al. Both the chemical potential of the species and the magnetic effect are considered to determine the vacancy formation energy. We also simulated the vacancy migration in a complete set of migration paths. The evaluated vacancy formation energy and activation energy for the motion of vacancy compared well with the experimental results. Also, the obtained migration ways for the diffusion of Ni and Al atoms are consistent with previous theoretical predictions and experimental observations. Magnetism is found to influence both the vacancy formation and migration. Our results reveal that Re doping can inhibit the formation of Ni vacancies but facilitate the formation of Al vacancies, and can also inhibit the migration of neighboring vacancies. While the doped Re atom on the Al site is stable, the Re atom on the Ni site can diffuse within the Ni-sublattice mediated by Ni vacancies.     

金属氟化物改善MgH2体系解氢性能的机制

采用基于密度泛函理论的Dmol4.1程序包,通过计算替换Mg、形成Mg空位、体系移走H原子所需能量及电子结构的改变,对金属氟化物改善MgH2体系解氢性能的机制进行探讨.结果发现:形成Mg空位所需能量明显高于Fe、Ti、Zr、V、Ni、Nb、Cr、Cu替代Mg所需能量,与形成Mg空位相比,低温下替代Mg更利于MgH2体系解氢:NiF2、NbF5、ZrF4作为催化剂利于MgH2体系解氢,理论计算的强弱顺序与实验结果一致:NiF2、NbF5、ZrF4金属氟化物改善MgH2体系解氢性能主要在于NiF2中的Ni、NbH中的Nb、ZrH2中的Zr分别替代MgH2中的Mg,加速了化学反应:NiF2+3MgH2=MgF2+Mg2NiH4、2NbF5+5MgH2=5MgF2+2NbH+4H2、ZrF4+2MgH2=2MgF2+ZrH2向右进行,使结构稳定的MgH2发生转变生成了Mg2NiH4、NbH和ZrH2等氢化物.电子态密度的进一步分析结果发现:Ni、Nb、Zr替代MgH2体系中的Mg,使超胞中心原子与其周围第1、2近邻H原子组成的八面体区域,成键的总电子数在费米能级以下按Ni、Ti、Zr的替代顺序增多,表明对应MgH2体系的结构稳定性按Ni、Ti、Zr的替代顺序增强,而解氢按Ni、Ti、Zr的替代顺序在下降,较好解释了NiF2、NbF5、ZrF4利于MgH2体系解氢,理论计算的强弱顺序与实验结果的一致性.     

First principles study of site substitution of ternary elements in NiAl

Site substitution of ternary elements in ordered compounds influences the electronic structure and hence the properties of compounds at the continuous level. The electronic structure and binding energy of a number of NiAl-X alloy systems (X=Ti, V, Cr, Mn, Fe, Co, Zr, Nb, Mo, Hf, Ta, W, Si, Ga, or Ge) were calculated using the discrete variational cluster method based on the local density approximation of the density functional theory. The site preference of the ternary additions to NiAl was investigated by employing the Bragg–Williams model to analyse the calculated binding energy. The results show that all the considered ternary elements possess stronger preference to the Al sublattice sites than a Ni atom does. A new method of identifying sublattice substitution of ternary additions in NiAl was proposed by comparison of the binding energies per atom of the ternary and the binary clusters involving the fourth nearest neighbours. The analysis suggests that Fe and Co atoms occupy the Ni sublattice sites, whereas Si, Ga and Ti atoms occupy the Al sublattice sites. The remaining elements may substitute for both sublattices: Mn is most likely to go for the Ni sublattice; V, Cr, Zr, Nb, Mo, Hf, Ta, W and Ge have a larger preference for the Al sublattice, but Cr and W do not show significant preference to any sublattice. The densities of states involving alloying additions of Co, Si and Cr were further investigated to clarify the site preference of the alloying additions.     

NiCrAlFe精密电阻合金中的L12有序结构研究

有序结构的形成是决定NiCrAlFe精密电阻合金电学性能的关键因素。本文利用第一性原理赝势平面波方法，计算了合金处于无序固溶体结构和形成L12型有序结构时的结合能、态密度、晶格常数等参数，并利用高分辨透射电子显微镜（HRTEM）对合金进行了结构表征，还测试了固溶态和时效态的电阻率。从结合能来看，该合金形成L12有序结构时比无序固溶体更加稳定；态密度以及部分态密度计算结果则表明，在L12有序结构中，Ni、Cr、Al、Fe会强烈成键而使得整个合金体系变得稳定；HRTEM分析结果证明固溶态合金经过和时效处理后出现了L12有序结构，而且该有序结构的晶格常数与计算值基本一致。对比无序固溶体与L12型有序结构费米能级处的态密度值发现，当形成L12有序结构时合金的导电能力较无序固溶体下降，电阻率升高，与实际测试结果吻合。     

Effects of TM on stability of structure corresponding to prepeak of amorphous Al_(90)TM_5Ce_5 Alloys

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Nin(n=2-6)原子簇的电子结构和磁性研究

采用MS—Xα方法研究了Nin(n=2—6)原子簇的电子结构和原子磁矩,发现团簇的几何对称性对原子磁矩和电子态密度的分布有重要影响．具有Oh点群对称的八面体原子簇Ni6的^3Eg轨道上存在很强的负交换耦合,呈现反铁磁耦合趋势;具有C3υ点群对称的三边金字塔结构的原子簇Ni5位于塔顶点的Ni原子与基面上的Ni原子磁矩方向相反,但大小不等,呈现出亚铁磁交换耦合特征．与金属Ni相比,有些Ni原子团簇磁性增强,有些团簇磁性减弱．这一结果能够较好地解释铁磁超微颗粒呈现出的表面磁性异常现象．