Planar defects and dislocations in transition-metal disilicides

The structures of transition-metal disilicides are constituted of different stacking of identical atomic planes at four different positions A, B, C, D: AB in C11_b structure of e.g. MoSi₂, ABC in C40 structure of e.g. VSi₂ and ABDC in C54 structure of e.g. TiSi₂ disilicides. In comparison with the FCC lattice with the ABC atomic plane stacking along the <111> directions, the occurrence of the fourth position, D, essentially alters the properties of defects and consequently the mechanical properties. The effect of generalized planar defects and their impacts on the dislocation core structures are discussed. In particular, we examine stacking faults and related partial dislocations on the basal planes in different types of disilicides as well as the related twin boundaries and dissociated dislocations. Our analysis of the stacking-fault-like defects is based on the calculations of γ-surfaces using ab initio methods. Predictions of possible metastable defects in all types of disilicides are reported.     

Ab initio studies on the magnetic phase stability of iron

The possible collinear magnetic configurations of the α- and γ-Fe phases have been analyzed using the full-potential linearized augmented plane wave (FLAPW) method and the fixed spin moment (FSM) procedure in order to perform total energy and moment calculations, with which the phase stability could be checked. In the FSM, after the electronic configuration has been self-consistently determined, the resulting energy E is a minimum at the fixed total magnetic moment M per unit cell and volume per atom V. Subsequently, M should be changed through previously chosen steps, in order to get the E(M) curve for a given V. The minima of the curves at a given V represent the true magnetic phases of the system. From our calculations, the so-called AF-II antiferromagnetic phase, which requires a four-atom unit cell to exist, is the collinear ground state of γ-Fe, while for α-Fe the well-known ferromagnetic ground state is found. At some volumes, a crossing or degeneration of two or more magnetic phases appears. This may be regarded as an indication of a true non-collinear magnetic ground state of the system that remains to be studied. Nevertheless, experimental work seems to support that a collinear ferromagnetic arrangement is the ground state of the γ-Fe precipitates present in Fe–Cu mechanically alloyed solid solutions.     

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.     

Ab initio energetic study of oxide ceramics with rare-earth elements

Ab initio energetic calculations based on the density functional theory （DFF） and the projector augmented wave method （PAW） for determining the polymorphisms of lanthanide sesquioxides Ln2O3 （where Ln = rare-earth element, Y, and Sc）, LnMO3 perovskites （where M = AI and Ga）, and Ln2B207 pyrochlores （where B = Ti, Zr, and Hf） were reported. The relative lattice stabilities agreed well with the critically assessed results or the experimental results except the C-type Ln2O3 with a cubic structure, for which the calculated total energies were considerably more negative. With the increase of the Ln^3＋-cation radius, the polymorphic structures showed a degenerative tendency. The tendencies and quantities of the enthalpies of formation of the ternary oxide ceramics synthesized from their constituent binary oxides reasonably agreed with the available experimental results, and valuable thermodynamic properties were afforded to the compound, for which no experimental data is available. The enthalpies of formation of both perovskites and pyrochlores tend to become more negative with the increase of the Ln^3＋-cation radius.     

超声波搅拌-脉冲电沉积法制备纳米镍

在脉冲电沉积过程中,采用超声波搅拌制备了厚度约为90 μm的金属镍镀层.XRD和TEM对不同超声波强度下制备的镀层的分析表明,镍镀层的晶粒尺寸随着超声波强度的变化而变化.在固定脉冲电解参数导通时间(ton)和关断时间(toff)分别为0.2和0.8 ms、平均电流密度为10 A/dm2、镀液pH值为4.0、温度为50 ℃的条件下,当施加的超声波强度由0 W增至50 W时,镀层平均晶粒尺寸从45 nm减至24 nm;但当超声波强度增至70 W时,镀层的平均晶粒尺寸增至38 nm.显微硬度测试结果表明,平均粒径为24 nm的镍镀层的HV高达760.     

Ab initio study of the interaction of chlorides with defect-free hydroxylated NiO surfaces

DFT periodic calculations have been performed to model the adsorption and sub-surface insertion of Cl atoms at increasing coverage on undefective hydroxylated NiO(1 1 1) terraces characteristic of the surface of the passive film on nickel. The results evidence that structural relaxation, splitting the mixed topmost OH/Cl anionic plane, releases the repulsive interactions induced by Cl adsorption, but the effect decreases with increasing Cl coverage with a loss of ∼1.1 eV in stability at surface saturation. Cl sub-surface insertion in the first inner anionic plane also releases the repulsive interactions in the topmost plane but the effect becomes favourable only at high Cl coverage because of inter-layer mixing after surface reconstruction. At low Cl coverage, an unrelaxed diatomic Ni-OH layer weakly bonded to the oxide and likely to dissolve is formed after sub-surface insertion, suggesting a hybrid mechanism of local oxide thinning. Adsorption of Cl by place exchange is energetically favourable but does not seem to promote dissolution. Insertion into the lattice may promote dissolution but it is energetically favourable only at high Cl surface coverage, which is not a likely situation due to Cl repulsive interaction on the surface. For undefective terraces of the passive film, the reported calculations do not confirm the existing hypotheses of adsorption-induced oxide thinning or chloride sub-surface penetration.     

Ab initio study of electronic densities of states at copper–alumina interfaces

The electronic structure of Cu(1 1 1)/α-Al₂O₃(0 0 0 1) interfaces with two experimentally observed orientation relationships were studied in terms of local site and angular-momentum-projected electronic densities of states. The electronic band structures were calculated using density functional theory in the local density approximation. The local densities of unoccupied states are compared with experimentally determined electron energy loss near-edge structures. A very good coincidence was found for the interfacial oxygen and aluminum atoms. Interpretation guidelines for the electron energy-loss near-edge structures (ELNES) are suggested for further experimental measurements. The local bonding across the interface is analyzed in terms of local densities of occupied states.     

Ab initio based study of finite-temperature structural,elastic and thermodynamic properties of FeTi

We employ density functional theory (DFT) to calculate pressure dependences of selected thermodynamic, structural and elastic properties as well as electronic structure characteristics of equiatomic B2 FeTi. We predict ground-state single-crystalline Young's modulus and its two-dimensional counterpart, the area modulus, together with homogenized polycrystalline elastic parameters. Regarding the electronic structure of FeTi, we analyze the band structure and electronic density of states. Employing (i) an analytical dynamical matrix parametrized in terms of elastic constants and lattice parameters in combination with (ii) the quasiharmonic approximation we then obtained free energies, the thermal expansion coefficient, heat capacities at constant pressure and volume, as well as isothermal bulk moduli at finite temperatures. Experimental measurements of thermal expansion coefficient complement our theoretical investigation and confirm our theoretical predictions. It is worth mentioning that, as often detected in other intermetallics, some materials properties of FeTi strongly differ from the average of the corresponding values found in elemental Fe and Ti. These findings can have important implications for future materials design of new intermetallic materials.     

X-ray study of phase transformations during heating of cast iron

Conclusions In the liquid and solid-liquid states there are microregions of the cementite type in cast iron.Institute of Casting Problems, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 9, pp. 45–47, September, 1982.     

Ab initio development of a high-strength corrosion-resistant austenitic steel

A new austenitic steel alloyed with carbon + nitrogen is developed based on ab initio calculations of the electronic structure, which were carried out using the local density functional theory, and on experimental studies by means of conduction electron spin resonance. It is shown that alloying of CrMn austenite with carbon + nitrogen increases the concentration of conduction electrons and assists their more homogeneous spatial distribution, which results in short-range atomic ordering and, consequently, in a higher thermodynamic stability of iron-based face-centered cubic solid solutions. The developed CrMnCN steel is characterized by a yield and ultimate strength of 600 and 1000 MPa, respectively, in combination with an extremely high fracture energy. The notch-impact toughness exceeds 400 J. The resistance to impact wear is comparable with that of Hadfield steel. Corrosive properties are in some corrosive media similar to those of austenitic Cr18Ni10 steel. Possible applications of the developed steel are discussed.     

Ab initio polarizabilities calculations of singly charged polyacetylene oligomers

We present results for the static longitudinal linear polarizability and second-order hyperpolarizability of small polyacetylene chains bearing positively and negatively charged solitons, obtained through the second-order Møller–Plesset perturbation theory (MP2) method. Hartree–Fock (HF) calculations for these properties was performed only for negatively charged chains. The standard 6-31G basis set was used in all calculations. Our ab initio calculations showed that, regarding singly charged structures, only the second hyperpolarizability is affected by the ionization state. For both, positive and negative structures, it is shown that the electron correlation effect enhances the linear polarizability, and even more markedly the second hyperpolarizabilities.