Defects and Electron Densities in TiAl-based Alloys Containing Mn and Cu Studied by Positron Annihilation

The defects and electron densities in Ti50Al50, Ti50Al48Mn2 and Ti50Al48Cu2 alloys have been studied by positron lifetime measurements. The results show that the free electron density in the bulk of binary TiAl alloy is lower than that of pure Ti or Al metal. The open volume of defects on the grain boundaries of binary TiAl alloy is larger than that of a monovacancy of Al metal. The additions of Mn and Cu into Ti-rich TiAI alloy will increase the free electron densities in the bulk and the grain boundary simultaneously, since one Mn atom or Cu atom which occupies the Al atom site provides more free electrons participating metallic bonds than those provided by an Al atom. It is also found the free electron density in the grain boundary of Ti50Al48Cu2 is higher than that of Ti50Al48Mn2 alloy, while the free electron density in the bulk of Ti50Al48Cu2 is lower than that of Ti50Al48Mn2 alloy. The behaviors of Mn and Cu atoms in TiAI alloy have been discussed.     

基于真空热压扩散法的金刚石/Ti界面生成机制

采用真空热压扩散法在聚晶金刚石表面制备Ti层,探究金刚石表面金属化过程中的界面生成机制。利用扫描电子显微镜和X射线衍射仪,分析了钛层的表面形貌、界面结构和界面间的物相组成,采用能谱仪对界面进行了元素分析,计算了聚晶金刚石与Ti层之间界面的扩散带宽度及生成TiC的化学反应吉布斯自由能变。研究结果表明:在聚晶金刚石表面形成了平整、致密的Ti层,在聚晶金刚石与Ti层界面之间存在C、Ti和Co元素的扩散,在结合界面处产生了一定宽度的元素扩散带,同时在金刚石表面生成了点状TiC。真空热压扩散法实现了金刚石与Ti层的化学结合,可以提高金刚石与Ti层的结合强度。      

铈对Fe-Cr-Ni-Nb-Ti-Al-W合金晶界MC相组成的影响

采用EDS研究了铈对铁基合金Fe Cr Ni Nb Ti Al W晶界析出相MC中M组成的影响。结果表明,随着合金中铈含量的增加,晶界MC相中铌量增加,而钛量相对下降。此结果证明,由于铌与碳的亲和力较钛大,当晶界附近的钛、铌等替代式溶质原子被铈原子驱逐并与碳结合时,有更多的铌参与了晶界MC相的组成。     

掺杂(Fe、Nb)SrTiO3晶界组成的电子能量损失谱(EELS)分析

采用高空间分辨率扫描透射电镜（STEM）对掺杂（Fe^3 、Nb^5 ）SrTiO3晶界进行了观察，并利用采集的电子能量损失谱（EELS）对晶界组成进行了分析，结果表明，掺杂Fe的SrTiO3陶瓷晶界存在1nm左右的非晶膜，同晶体内部相比晶界缺Sr；三叉晶界为钛基玻璃相，Ti、O及Fe含量明显高于晶界，掺Nb的SrTiO3陶瓷晶界同晶粒相比同样缺Sr，三叉晶界主要以SiO2非晶相为主，Fe^3 、Nb^5 对Ti^4 的替位及在晶界的偏析引起SrTiO3晶格畸变是导致晶界组成变化的主要原因。     

Atomistic study of the effect of B addition in the FeAl compound

First principle calculations have been carried out to study energetic of boron atom impurities in bulk and symmetric Σ5(310) tilt grain boundaries of the ordered stoechiometric B2 FeAl intermetallic. A set of configurations was considered for studying the bulk behaviour: B in tetrahedral and octahedral interstitial positions or substituting Al and Fe. For the analysis of the segregation at the grain boundary, calculations were done for B substituting Al and Fe at three different locations and for B filling empty spaces along the interface. In each case, the defect formation energies were calculated to determine the site preference and their relative stability. The results indicate that B doping is metastable in the bulk and tends to segregate along the grain boundary. The overall behaviour of the B atoms at the boundary is essentially driven by the strong Fe–B interactions.     

First-principles study of doping and distribution of Si in TiC

In this work, first principles calculations have been performed to study the doping and distribution of Si atoms in TiC lattice. The results confirm that Si atoms prefer to occupy Ti sites and their segregation on the TiC crystal surface may occur. But in the presence of carbon vacancies on the surface, Si atoms tend to be chemically adsorbed around the vacancies rather than occupy the carbon sites. It is also shown that the diffusion of Si may be very difficult in stoichiometric TiC, in particular the diffusion from bulk to surface. However, the carbon vacancies can considerably decrease the energy barrier and enhance the diffusion of Si atoms.     

The role of carbon segregation on nanocrystallisation of pearlitic steels processed by severe plastic deformation

The nanostructure and the carbon distribution in a pearlitic steel processed by torsion under high pressure was investigated by three-dimensional atom probe. In the early stage of deformation (shear strain of 62), off-stoichiometry cementite was analysed close to interphase boundaries and a strong segregation of carbon atoms along dislocation cell boundaries was observed in the ferrite. At a shear strain of 300, only few nanoscaled off-stoichiometry cementite particles remain and a nanoscaled equiaxed grain structure with a grain size of about 20 nm was revealed. 3D-AP data clearly point out a strong segregation of carbon atoms along grain boundaries. The influence of this carbon atom segregation on the nanostructure formation is discussed and a scenario accounting for the nanocrystallisation during severe plastic deformation is proposed.     

Texture competition during thin film deposition – effects of grain boundary migration

In this paper, we describe an implementation of grain boundary migration in the atomistic simulator of thin film deposition (ADEPT), and apply the simulator to study effects of the grain boundary migration on texture evolution. In the implementation, atoms are classified into two categories: those belong to a single grain and those at grain boundaries. An atom is defined as one at a grain boundary if it has more than half of its neighbors occupied and not all of the neighboring atoms are in the same grain. The grain boundary atom is attempted to re-align with neighboring grains to represent the grain boundary migration; the attempt probability is defined by the grain boundary migration coefficient. Our studies show that grain boundary migration does not always assist formation of texture with a top surface of the lowest energy. At the nucleation stage of thin film deposition, high migration coefficient of grain boundaries may enhance the formation of grain nuclei with top surfaces of higher energy, and therefore effectively may suppress formation of textures with a top surface of the lowest energy. This effect may provide an extra dimension to engineer textures of thin films.     

Effect of impurities on grain boundary cohesion in bcc iron

The effect of B, N and O impurities placed in interstitial and substitutional positions at symmetric Σ5(2 1 0) Fe grain boundary is studied by means of first principles calculations. Full relaxation of supercell shape and volume is applied which results in stable asymmetric grain boundaries, depending on the impurity and its position. In all cases the big shifts (0.47–2.33 Å) of grains with respect to each other are observed. The equilibrium distance between the grains is decreased for impurities in substitutional positions, and increased for atoms in interstitial sites, compared to the relaxed clean GB. We found that nitrogen both in interstitial and substitutional positions, and boron in substitutional position enhance cohesion while oxygen in both positions, and interstitial boron, are embrittlers. The magnetic moments of Fe atoms at clean grain boundary are remarkably increased compared to the bulk. They tend to the bulk value in the middle of the grain in an oscillatory way.     

Theoretical investigation of Ti-adsorbed graphene for hydrogen storage using the ab-initio method

Based on first-principles plane wave calculations, it was shown that boron substituted graphene with Ti metal atom adsorption can be used as a high capacity hydrogen storage material. Boron substitution in graphene enhances the Ti metal adsorption energy, which is much larger than that in the case of pure graphene, and than the Ti cohesive energy. The Ti metal atom can be well dispersed on boron-substituted graphene and can form a 2 x 2 pattern because the clustering of the Ti atoms is hindered by the repulsive Coulomb interaction between them. The H2 adsorption behavior on Ti metal atoms was investigated, along with the H2 bonding characteristics and the open-metal states of Ti. It was found that one Ti adatom dispersed on the double sides of graphene can absorb up to eight H2 molecules, corresponding to a 7.9% hydrogen storage capacity. In addition, the adsorption behaviors of non-H2 atoms like C and B were calculated to determine if Ti atoms can remain in an open-metal state in boron-substituted graphene.     

Ion back-scattering analysis of interdiffusion in Cu-Au thin films

Helium ion back-scattering has been used to examine the interdiffusion behavior of the thin film copper-gold system. Techniques for distinguishing bulk diffusion and grain boundary diffusion using back-scattering are presented. At temperatures in the range 200°–500°C, the grain boundary diffusion mechanism is shown to predominate in Cu-Au thin films. The back-scattering results suggest a model in which interdiffusion takes place by very rapid saturation of the grain boundaries in the gold film by copper and a slower filling of the copper grain boundaries by gold. The atoms in the grain boundaries then diffuse into the grains by bulk diffusion. In the parent gold film, we suggest that the amount of Cu Au ₃ formed near the grain boundaries is uniform in depth. In the parent copper film, more Cu₃Au forms near the original Cu-Au interface than further into the copper film. No evidence was found to suggest the formation of a pure layer of CuAu₃ or Cu₃Au. The interdiffusion and compound formation process is found to be characterized by an activation energy of 1.35-1.5 eV.     

Influence of vanadium oxide on BSCCO (2 2 2 3) phase formation and related properties

The preparation of BSCCO 2 2 2 3 superconducting powder was studied with addition of V2O5 ranging from 0.3 to 0.5 molar index. Various compositions were prepared both containing and without Pb and subsequently treated with different firing cycles, to promote the incorporation in lattice sites of vanadium atoms. Different densification procedures were attempted and final bulk samples characterized to evaluate microstructural, mechanical and electrical properties. The addition of vanadium was found to increase the formation rate of high Tc phase and about 90% of 2 2 2 3 phase has been obtained in 80 h. However, this element was found to present an outstanding segregation trend at grain boundaries (especially with Sr) and to be responsible of a marked delay in 2 2 2 3 phase formation in respect to lead-doped samples. Transport properties seem positively affected even if considerable contamination at grain boundary and lowering in offset critical temperature are unavoidable. © 1998 Chapman & Hall     

Effect of crystal defects on solute atoms segregation during cooling

Different quantity and configurations of crystal defects were obtained in an austenite of Fe-30%Ni alloy and an ultra low carbon bainitic steel by different deformations and annealing times at high temperature. The boron segregation at grain boundaries and subgrain boundaries during air cooling were revealed by means of particle tracking autoradiography technique. It was found that the non-equilibrium segregation was resisted in the deformed grains after recovery and polygonization, the boron depletion was more in the recrystallized grains than in the deformed original grains during the cooling. The subgrain boundaries and polygonized dislocation cells had a significant effect on the boron non-equilibrium segregation during the air cooling, but the quantity of dispersed dislocations had not. The result indicated that during segregation process the interaction of boron atoms with dislocations was sensitive to the dislocations configuration rather than the total number of defects in grain.     

Atom probe tomographic observation of hydrogen trapping at carbides/ferrite interfaces for a high strength steel

A three-dimensional atom probe (3DAP) technique has been used to characterize the hydrogen distribution on carbides for a high strength AISI 4140 steel. Direct evidence of H atoms trapped at the carbide/ferrite interfaces has been revealed by 3DAP mapping. Hydrogen is mainly trapped on carbide/ferrite interfaces along the grain boundaries. Slow strain rate tensile (SSRT) testing shows that the AISI 4140 steel is highly sensitive to hydrogen embrittlement. The corresponding fractographic morphologies of hydrogen charged specimen exhibit brittle fracture feature. Combined with these results, it is proposed that the hydrogen trapping sites present in the grain boundaries are responsible for the hydrogen-induced intergranular fracture of AISI 4140. The direct observation of hydrogen distribution contributes to a better understanding of the mechanism of hydrogen embrittlement.     

Diffusion distribution of 57Co atoms in nanocrystalline Co79Nb14B7 alloy

Distribution of ⁵⁷Co atoms in a nanocrystalline Co₇₉Nb₁₄B₇ alloy after annealing at different temperatures was studied by means of ⁵⁷Co emission Mössbauer spectroscopy. After annealing below the recrystallization temperature, the diffusing ⁵⁷Co atoms were found in all phases expected to be present in the nanocrystalline alloy. They do not prefer the intergranular phase. Paths of diffusing atoms are probably close to interfaces between crystallites (grain boundaries) and/or intergranular phase. With increasing annealing temperature, when the nanocrystalline state transforms into a coarse polycrystal, the ⁵⁷Co atoms occupy regular sites in the bulk Co and Co₃B phases.     

The oxygenated phenomena of the undoped large-grain and small-grain polycrystalline diamond films

The polycrystalline diamond films in this research were deposited using a methane/hydrogen gas mixture in a microwave plasma assisted chemical vapor deposition system. Large-grain, several μm size crystallite, diamond films and small-grain, sub-micron size crystallite, diamond films were prepared by diamond paste and diamond powder nucleation method, respectively. It is found that there is no oxygen incorporated into the diamond films during the microwave plasma chemical vapor deposition process at the synthesis temperature between 900°C and 1000°C. However, the oxygenated phenomena did appear for both of the large-grain and the small-grain polycrystalline diamond films after the films were exposed to air for a period of time. It was shown that the large-grain diamond films are oxygenated more than the small-grain diamond films as the samples were exposed to air for a period of time and also after the chemical cleaning treatment. It is indicated that the oxygenated phenomena of the diamond films come from two contributors, the diamond crystallite surfaces and the diamond grain boundaries. The reaction between the diamond grain boundaries and the air is fast and the oxidized dangling bonds are hard to remove. However, the oxidized dangling bonds on the diamond crystallite surfaces are gradually formed and are easily etched away by the hydrogen plasma.     

Ni3Al合金硼原子占位及晶界偏聚

依据ＬＩ２型金属间化合物八面体，四面体间隙的特点，并利用硬球模型计算了上述各类间隙的空球半径大小，发现Ｎｉ３Ａｌ合金中６Ｎｉ八面体间空球半径比其他间隙大，使硼原子在更易进入６Ｎｉ八间隙，由于富Ｎｉ－Ｎｉ３Ａｌ合金晶界比Ａｌ－Ｎｉ３Ａｌ合金有更我的６Ｎｉ八面体间隙，所以硼在富Ｎｉ－Ｎｉ３Ａｌ晶界偏聚较多，另外富Ａｌ－Ｎｉ３Ａｌ合金晶界原子间作用力比富Ｎｉ－Ｎｉ３Ａｌ合金晶界原子间作用力大，这也阻碍了     

Chemisorption and diffusion of hydrogen atoms on single-walled carbon nanotubes

Density functional theory has been performed to investigate the chemisorption and diffusion of H atoms on the surface of single-walled carbon nanotubes (SWNTs). The results show that the binding energy of a single hydrogen atom on the SWNTs surface decreases as the diameter of the tube increases and is not affected by the chirality of the tube much. Two hydrogen atoms favor binding at adjacent and opposite positions rather than at alternate carbon site. As for the diffusion of H atoms on the tube, it is found that an isolated H atom can diffuse rather than desorb on the small SWNT upon heating. As the tube diameter increases, the diffusion barrier for H atom on the surface decreases. Further study shows that when the H atom diffuses around another H atom, the diffusion barriers vary with the relative sites of the two H atoms.     

Ti doped hematite thin film photoanode with enhanced photoelectrochemical properties

Ti-doped Fe₂O₃ thin films were prepared on fluorine-doped SnO₂ substrate as visible light active photoelectrochemical anodes. The fabricated films were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), atomic force microscope (AFM), X-ray energy dispersive spectroscopy and X-ray photoelectron spectroscopy (XPS). XRD data showed all films exhibited rhombohedral hematite phase, and the cell parameters showed that Titanium atoms substituted Fe atoms in the hematite lattice. AFM demonstrated that Ti doping could decrease the particle size on the surface compared with pure hematite. XPS results presented that Ti atom concentration was about 2.23 % in the doped film surface. The incident photon to electron conversion efficiency of Ti doped α-Fe₂O₃ film reached 23 % at 400 nm under 0.30 V bias versus AgCl in 1 M NaOH, which was nearly four times than that of undoped film. Titanium atoms in α-Fe₂O₃ lattice could increase the conductivity of hematite film. And excited electrons and holes in the bulk film could be separated more efficiently, rather than recombining with each other rapidly as that in pure hematite, which ultimately prolonged the life of electrons and holes and obtained the high efficiency Fe₂O₃ photo anode.     

Pt和Au修饰锐钛矿型TiO2(101)面的第一性原理研究

采用平面波超软赝势方法研究了Pt和Au修饰锐钛矿型TiO₂(101)面的结构稳定性及电子结构。结果显示贵金属原子在TiO₂(101)符合化学计量比的条件下, 在其表面的吸附作用不强, 对电子结构的影响也较小。但是发现在富O条件下, Pt和Au原子容易吸附在表面Ti空位的位置, 与Au原子不同, Pt原子有从TiO₂表面扩散进入体相晶格中的趋势。而在富Ti条件下, Pt和Au原子容易吸附在O1空位的位置。对可能存在的几种空位缺陷吸附模型进行了电子结构的计算。结果表明: 空位缺陷的产生不仅有利于Pt和Au原子“湿化”TiO₂(101)表面, 也有利于带隙中产生贵金属原子的5d杂质能级。