An ab initio study of Ti-Y-O nanocluster energetics in nanostructured ferritic alloys

So-called oxide dispersion strengthened steels or nanostructured ferritic alloys (NFAs) contain nanoprecipitates which give them exceptional mechanical properties and resistance to radiation effects. However, the structure and composition of these nanoprecipitates are still uncertain. To help clarify the nature of the smallest nanoprecipitates, density functional theory calculations are used to investigate the most stable Ti, Y, and O nanocluster computational units in Fe. Two distinct methods for searching for stable nanoclusters are proposed: one in which nanoclusters are restricted to the body-centered cubic Fe lattice and one in which the nanocluster structures are strained variants of bulk Ti and Y oxides. We discovered that nanoclusters that are structurally similar to bulk Ti and Y oxides are significantly more stable than nanoclusters that are restricted to the Fe lattice. Consequently, the most stable nanoprecipitates in Ti-Y-O NFAs are more likely to be small oxide phases than coherent solute-enriched clusters.     

Investigation of the structural stability of Co2NiGa shape memory alloys via ab initio methods

CoNiGa alloys have received considerable interest due to their high-temperature shape memory properties. Although there have been many investigations on the mechanical and magnetic behavior of these materials, little is known about the microscopic basis for the observed macroscopic behavior. In this work, we discuss the stability of Co₂NiGa-based structures. The austenite phase is modeled as fully ordered L2₁ and as partially ordered B2. The stability of the austenite phase with respect to tetragonal distortions is examined. Lattice dynamics calculations suggest that the mechanism for the transformation is different from that of the much more studied and chemically similar Ni₂MnGa shape memory alloy (SMA). The electronic basis for the observed metastability of the cubic Co₂NiGa austenite is found to be qualitatively different from that observed in other ferromagnetic SMAs, especially Ni₂MnGa.     

Structure at abrupt copper–alumina interfaces: An ab initio study

The atomistic structure and the energetics of Cu(1 1 1)/-Al₂O₃(0 0 0 1) interfaces for two experimentally observed interface orientation relationships were investigated from first-principles by means of the mixed-basis pseudopotential method. In the first orientation relationship, the direction of Cu is parallel to the direction of -Al₂O₃, and in the second the Cu is rotated with respect to the -Al₂O₃ by 90° around the interface normal [0 0 0 1]. Numerous candidate systems were considered for each case, covering all high-symmetry interface configurations, and with either Al or O termination of the -Al₂O₃(0 0 0 1) surfaces. For each of the most stable interfaces, full relaxations of the positions of all atoms were performed. It is found that despite the different atomistic structures of the two interface types, their interface stabilities, in terms of the work of separation, and their local atomistic structures are similar.     

Point defects and diffusion in ordered alloys: An ab initio study of the effect of vibrations

We present a detailed investigation of the influence of atomic vibrations on the point defect and diffusion properties of ordered metallic alloys, by means of ab initio calculations with density-functional theory. Considering the case of Ni₂Al₃ which provides a rich panel of defect-related properties, our study reveals that the behaviour of this compound is largely monitored by self-interstitials, whereas such defects are usually ignored in metallic compounds. The vibration free energies are obtained for the full set of point defects of Ni₂Al₃, showing that these quantities are strongly defect-dependent, and significantly modify the free energy of the compound in an intricate composition-dependent manner. The second key-issue is the first ab initio full analysis of attempt frequencies, via the coupling of vibration analysis and saddle-point search for significant atomic jumps. This analysis indicates that attempt frequencies range over several orders of magnitude and exponentially increase with migration energies. We show the importance of these factors in reaching realistic composition-dependent diffusion coefficients.     

Elastic constants of Al and TiN calculated by ab initio method

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Theoretical study on the second hyperpolarizabilities of tetrathiafulvalene (TTF) and tetrathiapentalene (TTP) using highly correlated ab initio MO and the density functional theory methods

We investigate the static second hyperpolarizabilities (γ) of tetrathiafulvalene (TTF) and tetrathiapentalene (TTP) using the ab initio molecular orbital (MO) method. The cationic radical states of these systems (TTF⁺ and TTP⁺) are expected to have large negative γ values, which are rare in organic systems, based on our classification rule of γ. It turns out that at the higher-order electron correlation level TTP⁺ gives a large negative γ value though TTF⁺ gives a positive γ. We also investigate the applicability of the density functional theory (DFT) methods to the calculation of the γ values for these systems. By tuning the mixing parameter of DFT/HF exchange term, a DFT method turns out to semiquantitatively reproduce the γ values of TTP and TTP⁺ at the higher-order electron correlation method, i.e., CCSD(T), while fail in reproducing the γ value of TTF⁺. This suggests the necessity of further improvement in correlation functional for obtaining reliable γ values of charged radical states.     

Towards the ab initio based theory of phase transformations in iron and steel

Despite of the appearance of numerous new materials, the iron based alloys and steels continue to play an essential role in modern technology. The properties of a steel are determined by its structural state (ferrite, cementite, pearlite, bainite, martensite, and their combination) that is formed under thermal treatment as a result of the shear lattice reconstruction γ (fcc) → α (bcc) and carbon diffusion redistribution. We present a review on a recent progress in the development of a quantitative theory of the phase transformations and microstructure formation in steel that is based on an ab initio parameterization of the Ginzburg–Landau free energy functional. The results of computer modeling describe the regular change of transformation scenario under cooling from ferritic (nucleation and diffusion-controlled growth of the α phase) to martensitic (the shear lattice instability γ → α). It has been shown that the increase in short-range magnetic order with decreasing the temperature plays a key role in the change of transformation scenarios. Phase-field modeling in the framework of a discussed approach demonstrates the typical transformation patterns.     

Phase stability,electronic structure and mechanical properties of ternary-layered carbide Nb4AlC3: An ab initio study

In this paper we calculated the phase stability, electronic structure and mechanical properties of Nb₄AlC₃ by means of a first-principles pseudopotential total energy method. Based on thermodynamical calculations of the two possible crystal structures of Nb₄AlC₃, α-type Nb₄AlC₃ is confirmed to be the preferred equilibrium phase at ambient conditions. The chemical bonding displays layered characteristics that have commonly been reported for MAX ceramics. The equation of state and compressibility of α-Nb₄AlC₃ were investigated. The material exhibits anisotropic elasticity under hydrostatic pressure: it is more compressible along the c direction than along the a and b directions. The second-order elastic coefficients, bulk modulus, shear modulus and Young’s moduli were reported and compared with those of Nb₂AlC. Since the salt-rock-type Nb–C slab is thicker in Nb₄AlC₃ than that in Nb₂AlC, the former material shows higher elastic stiffness than the latter one; at the same time, Nb₄AlC₃ may display quasi-ductility, which has been well documented for MAX ceramics.     

Structure and chemical analysis of aluminum wear debris: Experiments and ab initio simulations

Sliding can produce severe plastic deformation and drive material far from equilibrium. Commonly, it also involves material transfer, mechanical mixing and formation of tribomaterial that influences both friction and wear. This paper describes aluminum wear debris generated by sliding in different environments. One portion of the debris is highly strained and another portion contains a large amount of oxygen, but not in the form of a simple oxide such as α-alumina. Annealing the debris allows strain relaxation, weight loss and changed electrical conduction characteristics. Characterization of debris before and after annealing suggests that the tribomaterial and the debris consist partly of an aluminum–oxygen solid solution and partly of hydroxylated material. These constituents are not normally formed in the absence of external forces. Quantum mechanical ab initio simulations were performed to complement the experiments. The simulations indicate that tetrahedral sites in aluminum are favored for oxygen in solid solution.     

Electronic,optical and thermoelectric properties of Ce3PdIn11 and Ce5Pd2In19: An ab initio study

Density functional calculations with Engel and Vosko generalized gradient approximation are applied to investigate the electronic, optical and thermoelectric properties of Ce₃PdIn₁₁ and Ce₅Pd₂In₁₉ compounds. Analysis of the calculated band structure of Ce₃PdIn₁₁ and Ce₅Pd₂In₁₉ demonstrates their metallic character. The calculated densities of states N(E_F) of Ce₃PdIn₁₁ and Ce₅Pd₂In₁₉ at the Fermi level are 19.60 states/eV and 33.50 states/eV, respectively. The bonding nature in these compounds is discussed via the calculated contour map of the charge density in (1 1 0) crystallographic plane. Imaginary parts of the complex dielectric function show considerable isotropy between 3 and 14 eV Ce₃PdIn₁₁ have large dielectric constant. Thermoelectric properties results reveal that both compounds possess high Seebeck coefficient and electrical conductivity at high temperature. This is the first quantitative theoretical prediction of the theremoelectric properties for these investigated compounds and still awaits experimental confirmations.     

Reassessment of the Ce–Ni binary system supported by key experiments and ab initio calculations

The Ce–Ni system is reassessed by means of CALPHAD (CALculation of PHAse Diagram) approach, supplemented with decisive experiments and enthalpies of formation computed via VASP (Vienna ab initio simulation package) code. Fourteen alloys are prepared in order to check the general feature of the established Ce–Ni phase diagram and to provide new phase diagram data for the refinement of previous modeling. The phase identification for both as-cast and annealed states is performed by means of X-ray diffraction technique, and the phase transition temperatures are measured by differential thermal analysis. Electron probe microanalysis is employed to determine two tie lines between CeNi₅ and (Ni). The enthalpies of formation for CeNi₂ and CeNi₅ are calculated using VASP code with a desire to clarify the discrepancies among the literature data. A set of self-consistent thermodynamic parameters for the Ce–Ni system are finally obtained. Significant improvements have been made, compared with the previous assessments.     

缓蚀剂哌啶在不锈钢钝化膜表面吸附行为的量子化学从头算研究

采用多种量子化学方法对缓蚀剂哌啶进行计算,由构象转变能垒确定了分子的吸附构象。以Ｏ－Ｆｅ－Ｏ－Ｆｅ－Ｏ和Ｏ－Ｃｒ－Ｏ－Ｃｒ－Ｏ模拟氧化膜的一维链,施行量化学从头算研究,得到了分子的稳定构型及电子结构。根据化学反应普遍扰理论分析能动蚀剂与氧化膜的作用方式。     

Prediction of high-temperature point defect formation in TiO2 from combined ab initio and thermodynamic calculations

A computational approach that integrates ab initio electronic structure and thermodynamic calculations is used to determine point defect stability in rutile TiO₂ over a range of temperatures, oxygen partial pressures and stoichiometries. Both donors (titanium interstitials and oxygen vacancies) and acceptors (titanium vacancies) are predicted to have shallow defect transition levels in the electronic-structure calculations. The resulting defect formation energies for all possible charge states are then used in thermodynamic calculations to predict the influence of temperature and oxygen partial pressure on the relative stabilities of the point defects. Their ordering is found to be the same as temperature increases and oxygen partial pressure decreases: titanium vacancy → oxygen vacancy → titanium interstitial. The charges on these defects, however, are quite sensitive to the Fermi level. Finally, the combined formation energies of point defect complexes, including Schottky, Frenkel and anti-Frenkel defects, are predicted to limit the further formation of point defects.     

Thermodynamic analysis of the filling fraction limits for impurities in CoSb3 based on ab initio calculations

The thermodynamic stabilities of alkaline earth (Ca, Sr and Ba), and rare earth (La, Ce and Yb), filled CoSb₃ skutterudites have been studied using a plane-wave density functional method. By combining the formation energy of inserting an impurity into the intrinsic void of CoSb₃ and that of secondary phases ISb₂ and CoSb₂, it is found that the filling fraction limit (FFL) or the maximum filling fraction of an impurity I corresponds to the minimum formation energy for a mixed chemical reaction route that results in the formation of filled skutterudite I_yCo₄Sb₁₂ at the maximum filling as well as the formation of secondary phases. Theoretically estimated FFLs of various impurities in the voids of CoSb₃ are in good agreement with the reported experimental data. A schematic phase diagram for filled CoSb₃ is given. Discussion on the effect of the ionic radius of a filler and the content of Sb on FFL is presented.     

第一性原理研究Pt-Zr系统中化合物的生成焓/体模量与原子体积的线性相关性

通过第一性原理的计算方法,研究118种不同结构的Pt-Zr中间化合物,并选取相关的物理性能,如结构稳定性、晶格常数、生成焓、弹性常数以及体模量等进行计算。根据计算得出的生成焓信息,绘制Pt-Zr系统的基态能量曲线。计算得到的物理相关信息为未来的热力学计算和原子尺度模拟提供基础数据。在选取的 Pt-Zr化合物中,存在两组线性相关关系：生成焓与原子体积成正线性相关,而体模量与原子体积成负线性相关关系。