微观相场法模拟Ni75Al5.3V19.7 中L12和D022结构反位缺陷的演化

运用微观相场法研究Ni₇₅Al_5.3V_19.7合金沉淀过程中L1₂结构和D0₂₂结构反位缺陷发现：在沉淀初期,L1₂结构反位缺陷Al_Ni,V_Ni,Ni_Al,D0₂₂结构反位缺陷V_Ni,Al_Ni关键词： 微观相场 反位缺陷 L1₂结构')" href="#">L1₂结构 D0₂₂结构')" href="#">D0₂₂结构     

Ordering behaviors of the F.C.C. and B.C.C. phase alloys in the InMg,LiMg and AlZn systems

Expressions of the band-structure energy and the electrostatic energy characterized by the usual long-range and short-range order parameters are given to a binary alloy of simple metals with the f.c.c.-type or b.c.c.-type structure.The ordering energy and the local ordering energy are calculated for the InMg, LiMg and AlZn systems. The numerical results explain successfully the facts that the InMg system has the Ll₀-type and Ll₂-type ordered phases, each of which exists over a wide range of concentration and that the LiMg system has a local order with a negative short-range order parameter, while the AlZn system has that with the positive one. A lattice distortion in the Ll₀-type ordered structure of InMg is also discussed.     

Phase transformations and thermal stability of the structure of Ti<Subscript>3</Subscript>Al intermetallic compound at deuteration and plastic deformation

The structural transformations in Ti₃Al intermetallic compound at deuteration with concentrations x = 1.2 and 1.7, heating at 100–400°C, and shear deformation under pressure have been studied. It is established that at a given deuterium concentration deuterides with fcc and orthorhombic lattices are formed; under severe shear deformation, nanocrystalline and amorphous (or close to amorphous) deuterides arise. The reasons for the structure amorphization at deuteration and subsequent plastic deformation are discussed.     

Stability,elastic properties and electronic structures of L12-ZrAl3 and D022-ZrAl3 up to 40 GPa

The effects of high pressures on structure stability, elastic properties and electronic structures of zirconium trialuminide L1₂- and D0₂₂-ZrAl₃ compounds have been investigated by first-principles calculations within the local density approximation. The equilibrium structure and formation energy show that L1₂-ZrAl₃ is more stable than D0₂₂-ZrAl₃ at the applied pressure. The elastic properties and Debye temperatures of (L1₂, D0₂₂)-ZrAl₃ increase with the increasing pressure and the calculated values in the ground state are in good agreement with the available experiment data. The mechanical anisotropic properties were discussed using the universal anisotropic index A^U. The sound velocities along the [100], [110] and [111] directions were also calculated for both phases. The calculated electronic properties under high pressures suggest that the decreased electronic density of states (DOS) at the Fermi level and the changed charges distribution lead to the observed decrease of the structural stability for (L1₂, D0₂₂)-ZrAl₃ under high pressures.     

Strain-split energy bands and piezooptical properties of Si

The energy bands of a crystal of Si under an axial force are calculated using a pseudopotential method including spin-orbit coupling effects. Good numerical agreement with experimentally determined deformation potentials is obtained. The wavelength modulated reflectivity spectra for several uniaxial forces along the [001] and [111] directions are calculated and compared with available experiments. The structure of the spectra is analyzed in terms of the shifts and splittings of the energy bands. It is shown that the optical structure corresponding to the main peak in reflectivity around 3·4 eV is due to transitions associated with several initial points related to the A line and it is well reproduced by an energy band model where the energy value of the L₃—L₁ transition is slightly larger than that corresponding to the Γ_25′–Γ₁₅ one.     

Nucleation of deformation twins in nanocrystalline fcc alloys

An earlier dislocation model for predicting the grain size effect on deformation twinning in nanocrystalline (nc) face-centred-cubic (fcc) metals has been found valid for pure metals but problematic for alloys. The problem arises from the assumption that the stacking-fault energy (γ_SF) is twice the coherent twin-boundary energy (γ_fcc), which is approximately correct for pure fcc metals, but not for alloys. Here we developed a modified dislocation model to explain the deformation twinning nucleation in fcc alloy systems, where γ_SF ≠ 2γ_twin. This model can explain the differences in the formations of deformation twins in pure metals and alloys, which is significant in low stacking-fault energy alloys. We also describe the procedure to calculate the optimum grain size for twinning in alloy systems and present a method to estimate γ_twin.     

Structural phase transition and structural properties in oxygenated La2CuO4+y; y ∼0.02

The crystal structure of the oxygenated La₂CuO_4+y; y ～0.02 has been studied by X-ray diffraction. It is found that the structural phase transition from tetragonal to orthorhombic symmetry occurring at about 450 K (Tc) is of second order and shows diffuse scattering above Tc. The diffuse scattering is related to static and/or dynamic short-range order of cooperative tilts of rigid-like elongated CuO₆ octahedra, which make two-dimensional networks stacked along the [001] axis. The structures at room temperature and 210K are determined using a 4-circle X-ray diffractometer and have the orthorhombic space group ¹⁰D_2h-Pccn. The structure is mainly characterized by anisotropic cooperative tilts of the CuO₄ octahedra around the [100]_p and [010]_p primitive axes in two-dimensional octahedron-networks, where suffix “p” means pseudo K₂NiF₄-type structure. The anisotropy of the tilting-angles results from pushing by excess oxygen atoms to apical oxygen atoms of the CuO₆ octahedra. It is also found by refinement of the structure that the excess oxygen atoms are randomly sited at the atomic coordinates (1/4,1/4,0.244) and/or (1/4, 1/4,0.279).     

Microstructure evolution of a [011] orientation single crystal nickel-base superalloy during tensile creep

Microstructure of [011] oriented single crystal nickel-base superalloy consists of the cubical γ′ phase embedded coherently in the γ matrix, and arranged regularly along 〈100〉 orientations. After tensile creep, the cubical γ′ phase in the alloy is transformed into the strip-like rafted structure along [001] direction under the ordering transformation free-energy and strain energy change. And the directional growing of γ′ phase is attributed to the atoms Al and Ta into (001) plane to form the stable stacking mode.     

HAADF-STEM study of β′-type precipitates in an over-aged Al–Mg–Si–Ag alloy

Coarse, rod-shaped precipitates growing along ?100?Al directions in an Al–1.0?wt% Mg₂Si alloy with 0.5?wt% Ag additions were investigated by high-resolution high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM). All investigated precipitates had complex structures, being composed of domains separated by anti-phase resembling boundaries. The domains consist of a modified hexagonal β′-type structure that contains a considerable amount of Ag. Based on HAADF-STEM images, an average atomic model with space group P-62?m (189) and composition Al₃Mg₃Si₂Ag is proposed, having Al incorporation and Ag replacing certain Si atomic columns. Co-existence with the Ag-free β′-Mg₉Si₅ phase has been observed for some precipitates. The boundaries may be described as full or half units of the orthorhombic U2-AlMgSi precipitate phase. The HAADF-STEM images indicate partial replacements of Al atoms by Ag, in both the β′-type domains and the U2-type boundaries. Ag enrichment of the Al matrix near the precipitate/Al interface was observed for all the investigated precipitates     

Optical studies of phase transitions in oxyfluoride (NH4)2NbOF5

Polarization-optical studies and measurements of the birefringence Δn and the angle of rotation of the optical indicatrix for the (NH₄)₂NbOF₅ crystal have been carried out in the temperature range 100–350 K. Two anomalies of the birefringence have been revealed at the temperatures T ₀₁ = 258 K and T ₀₂ ≈ 219 K. According to the twinning pattern, the crystal undergoes successive changes in symmetry: orthorhombic ↔ monoclinic 1 ↔ monoclinic 2. The twofold axis of the monoclinic phases (or the normal to the plane) is directed along [001]_or. The effect of the uniaxial compression along [011]_or and the electric field E ≈ 25 kV/cm along [100]_or on the twin structure has been studied. The ferroelastic phase transition at T ₀₁ is due to the appearance of the shear deformation x ₄(T) and is accompanied by significant anomalies of the birefringence. Strong pretransition phenomena mask the jumps in the birefringence Δn(T) and in the angle of rotation of the indicatrix φ(T) at T ₀₁.     

THEORETICAL STRENGTH AND PHASE STABILITY OF Cu AND Ni UNDER [100] UNIAXIAL LOADING

Based on Born's criteria we studied phase stability and theoretical strength of fcc crystals of copper and nickel under [100] uniaxial loading. The calculation was carried out using a simple and completely analytical embedded atom method(EAM) potential proposed by the present authors. For Cu, the calculated value of its theoretical strength (0.33×10¹¹ dyn·cm^-2) agrees well with the experimental value (0.30×10¹¹ dyn·cm^-2), while the calculated strain (9.76%) is somewhat larger than the experimental one (2.8%). For Ni, its theoretical strength and strain predicted using the EAM potential are found smaller than those predicted using a pair potential. It is worthy to note that unlike previous calculations, in which pair potentials were used and three unstressed fcc, bcc, and fct structures included (for Ni only fcc state is found stable, while for Cu both fcc and bcc states are predicted stable), in present calculations using EAM potential the [100] primary loading path passes through only two zeroes (a stable unstressed fcc structure and an unstable stress-free bcc structure) either for Cu or for Ni.     

Development of magnetic anisotropies in ultrathin epitaxial films of Fe(001) and Ni(001)

Ultrathin films, bcc Fe(001) on Ag(001), fcc Fe(001) on Cu(001) and Fe/Ni(001) bilayers on Ag, were grown by molecular beam epitaxy. A wide range of surface science tools were employed to establish the quality of epitaxial growth. Ferromagnetic resonance and Brillouin light scattering were used to extract the magnetic properties. Emphasis was placed on the study of magnetic anisotropies. Large uniaxial anisotropies with easy axis perpendicular to the film surface were observed in all ultrathin structures studied. These anisotropies were particularly strong in fcc Fe and bcc Fe films. In sufficiently thin samples the saturation magnetization was oriented perpendicularly to the film surface in the absence of an applied field. It has been demonstrated that in bcc Fe films the uniaxial perpendicular anisotropy originates at the film interfaces. In situ measurements indentified the strength of the uniaxial perpendicular anisotropy constant at the Fe/vacuum, Fe/Ag and Fe/Au interfaces asK _us = 0.96, 0.63, and 0.3 ergs/cm² respectively. The surface anisotropies deduced for [bulk Fe/noble metal] interfaces are in good agreement with the values obtained from ultrathin films. Hence the perpendicular surface ansiotropies originate in the broken symmetry at abrupt interfaces. An observed decrease in the cubic anisotropy in bcc Fe ultrathin films has been explained by the presence of a weak 4th order in-plane surface anisotropy,K _1S=0.012 ergs/cm². Fe/Ni bilayers were also investigated. Ni grew in the pure bcc structure for the first 3–6 ML and then transformed to a new structure which exhibited unique magnetic properties. Transformed ultrathin bilayers possessed large inplane 4th order anisotropies far surpassing those observed in bulk Fe and Ni. The large 4th order anisotropies originate in crystallographic defects formed during the Ni lattice transformation.     

[2.2]Paracyclophane: theoretical study of its lower excited states and of the zero-field splitting parameters D

The lower excited singlet and triplet states and the zero-field splitting parameters D of [2.2]paracyclophane are studied within a semiempirical π theory which takes into account overlap effects between the two benzene rings, transanular and through-bond interaction via the methylene bridges. Whereas the singlet energies depend strongly on the through-bond interaction and the mutual polarization of σ core and π system this is not the case for the energies and zero-field splitting parameters D of the two lowest triplet states. The deformations of the benzene rings in [2.2]paracyclophane lead only to a small decrease of the excitation energies of about 0·2 eV. The D parameter can be written as a sum D = D_A + D_B + D_AB with the intrasubunit contributions D_A and D_B of the conjugated subunits A and B of the phane and an intersubunit term D_AB . We demonstrate that the deformations reduce the intrasubunit terms D_A and D_B and that they are crucial for the decrease of the D values of [2.2]paracyclophane with respect to p-xylene. The difference between the D values of the first and second triplet states is governed by the intersubunit term D_AB which has a different sign in the two states. However, this difference does not depend markedly on the transanular interaction. A further reduction of D_A and D_B in the first triplet state only is caused by transanular interaction by means of symmetrical charge-transfer terms in the wavefunction.     

Intermediate temperature creep mechanisms in Ni 3 Al

The intermediate-temperature creep response of single-crystal Ni 3 Al(Ta) has been investigated along both [ ] and [001] axial orientations. The effect of the existing deformation structure (i.e. pre-straining) on the [ ] creep response was reported. The creep responses of virgin specimens and specimens prestrained at room temperature (RT) and 520°C are compared. In order to compare the dislocation structures prior to creep, the microstructure of specimens which had been deformed at a constant strain rate at RT and 520°C, but not subjected to creep, was also examined. Creep curves show that the temperature of pre-strain influences the subsequent creep properties. The primary creep response, like the yielding response, appears to be controlled by the kink size distribution, while the secondary creep response is thought to be controlled by the kink separation (or the length of the Kear-Wilsdorf locks). Specimens crept along [ ] display steady state creep properties and rectangularly oriented [ ](010) dislocations, while a virgin specimen crept along [001] displays an increasing secondary creep rate (inverse creep) and d110 ¢{100}-type dislocations. Inverse creep along [001] is thought to be the result of an increasing density of edge kink octahedral sources where there is little resolved shear stress on the cube planes.     

Hydrothermal synthesis and electrochemical properties of α-MoO3 nanobelts used as cathode materials for Li-ion batteries

Uniform α-MoO₃ nanobelts were successfully synthesized by the hydrothermal process at 180°C for 20 h of the acidic solutions with different pH values of 0–0.75, adjusted using HCl (conc.). XRD and SEM results revealed that the pH of the precursor solutions played an important role in the phase, impurities, and morphology of the products. At the pH=0, the perfect α-MoO₃ nanobelts with a few tens of microns long were synthesized. By the TEM characterization, orthorhombic MoO₃ has a distinctive layered structure along the [010] direction, consisting of distorted MoO₆ octahedrons connected by common corners along the [100] direction and common edges along the [001] direction. The electrochemical measurement showed that the α-MoO₃ nanobelts have high specific charge capacity.     

Structural state of β-solid solution in rapidly quenched alloys of Al50Cu50− x Fex (x = 17, 13, 6) and ordered η1-AlCu phase formation

Conventional transmission and scanning electron microscopy were employed to investigate the structural state and phase transformations of the β(CsCl)-solid solution in rapidly quenched alloys of Al₅₀Cu_{50? x} Fe _x with respect to the Cu/Fe ratio. We studied the alloys from the central (x?=?17 and 13) and border areas (x?=?6) of the β-solid solution homogeneity region. The structural state of the β-solid solution was characterized by premartensitic structural instability of the bcc lattice for x?=?17 and 13, and the presence of combined short-range order accompanied by the intense diffuse scattering for x?=?6. This short-range order can be described by the ordering of atoms and vacancies in the planes (111)β and ω-like atomic displacements (longitudinally polarized waves of displacement in the direction of the [111]β). The structural state of the β-solid solution with the combined short-range order was regarded as a pretransition state for the revealed transformation with homogeneous precipitation of the nanodispersed phase. The precipitation phase was attributed to an orthorhombic Al(Cu, Fe) η₁-phase, belonging to the family of ordered β-based phases with orientation relationships of [100]_η1 ||[110]_β, [010]_η1 ||[110]_β, [001]_η1 ||[001]_β. We concluded that the atomic structure of the η1-phase is characterized by ordering, accompanied by ω-like atomic displacements of the adjacent layers in 3d metals (Cu, Fe) and aluminum.     

A comparison of the magnetic anisotropy of [001] and [111] oriented Co/Pd Multilayers

Superlattices of [001]_fcc Co/Pd with varying Co thicknesses from one to eight atomic layers per modulation period were epitaxially grown on NaCl by vapour deposition in UHV. Transmission electron diffraction indicates lattice coherence between the Co and the Pd layers for Co thicknesses up to six atomic layers. If deposited at a substrate temperatureT _s=50°C, only the superlattices containing Ci-monolayers show perpendicular magnetization. By raisingT _s to 200°C, the perpendicular anisotropy for Co monolayers is increased, and is also observed for Co bilayers. We suggest that this is due tolayer smoothening, which increases Néel's interface anisotropy. For more than 6 atomic layers of Co a loss of coherence is observed atT _s=50°C, accompanied by a structure transformation to hcp Co with a (0001)_Co(111)_Pd orientation.Non-epitaxial polycrystalline [111]-multilayers have a different anisotropy versus thickness behaviour. For such multilayers the range of Co thicknesses giving perpendicular magnetization is extended from 8 Å up to 12 Å atT _s=200°C. The different behaviour of the single crystal [001] films is caused by a strong volume contribution to the anisotropy, which favours in-plane magnetization, opposing the perpendicular interface anisotropy. This easy-plane term is attributed to magneto-elastic anisotropy due to stretching of the Co layers, via a positive magnetostriction.     

Magnetization and Specific Heat of UCo0.95T0.05Al (T = Fe, Ni) Single Crystals

We present the first single-crystal study of Fe and Ni doped U(Co_1-x T _x )Al compounds in comparison to the parent compound, the itinerant 5f-electron metamagnet UCoAl. Magnetization and specific-heat data measured in the temperature range of (1.8-300) K and in fields up to 10 T are analyzed and discussed. Both compounds exhibit a strong uniaxial anisotropy with the dominating magnetic response along the c-axis of the hexagonal ZrNiAl-type structure. Ni substitution yields an increase of the critical field for metamagnetism; UCo_0.95Fe_0.05Al is ferromagnetic below 30 K. Temperature evolution of the entropy change between 0 and 10 T points to involvement of magnetic correlations or spin fluctuations in the paramagnetic compounds UCoAl and UCo_0.95Ni_0.05Al.     

Magnetic circular dichroism of [Co/Pd] and [CoB/Pd] multilayered films

Magnetic circular dichroism (MCD) and X-ray absorption spectra (XAS) at the Co L_2,3-edge of [Co/Pd]₂₀ and [CoB/Pd]₂₀ multilayered films, which were fabricated at 260 °C with different magnetic layer thicknesses (δ), have been measured. The lineshapes of XAS–MCD show that the electronic state of Co 3d of the films hardly changes even when sputtered at higher temperatures. The expectation values of orbital and spin angular momentum (〈L_z〉 and 〈S_z〉) are estimated using the sum rule, and it is found that 〈L_z〉/〈S_z〉 in δ<0.5 nm is larger than that in δ>0.5 nm.     

Interatomic bonds and the tensile anisotropy of trialuminides in the elastic limit: a density functional study for Al3(Sc,Ti, V,Cr)

The tensile anisotropy in the elastic limit of Al₃(Sc,?Ti,?V,?Cr) intermetallic compounds in both the L1₂ and D0₂₂ crystal structure has been investigated using first-principles density-functional calculations. In both crystal structures the main bonding character comes from the saturation of dominant d ³ (L1₂) and d ⁴ (D0₂₂) hybrid orbitals located on the TM atoms. The series Al₃Sc?→ Al₃V corresponds to gradual d-band filling and leads to a gradual increase of bond-strength and covalent bond formation. The magnetism of Cr breaks this trend in the Al₃Cr compound (for both ferromagnetic and antiferromagnetic configurations). In this series, a trend towards an increased anisotropy of the elastic constants, Young modulus Y and Poisson ratio ν is observed. The easy and hard directions of tension can be simply identified by the variation of Y, which corresponds to the presence or absence of covalently bonded–Al–TM–chains. A high anisotropy of the Poisson ratio arises also from an alternation of atoms in the lateral directions and can be understood in the same terms.