ON CORE STRUCTURE PROPERTIES AND PEIERLS STRESS OF DISSOCIATED SUPERDISLOCATIONS IN ALUMINIDES: NiAl AND FeAl

<正>The study of dislocation properties in B2 structure intermetallics NiAl and FeAl is crucial to understand their mechanical behaviors.In this paper,the core structure and Peierls stress of collinear dissociated〈111〉{110} edge superdislocations in NiAl and FeAl are investigated with the modified P-N dislocation equation.The generalized stacking fault energy curve along〈111〉direction in {110} slip plane contains two modification factors that can assure the antiphase energy and the unstable stacking fault energy to change independently.The results show that the core width of superpartials decreases with the increasing unstable stacking fault energy,and increases with the increasing antiphase boundary energy.The calculated Peierls stress of〈111〉{110} edge superdislocations in NiAl and FeAl are 475 MPa and 3042 MPa,respectively.The values of Peierls stress in NiAl is in accordance in magnitude with the experimental and the molecular statics simulations results.     

The use of macroscopic modelling of intermetallic phases in aluminium alloys in the study of ferricyanide accelerated chromate conversion coatings

Chromate conversion coatings are used on aluminium alloys, primarily for their renowned corrosion resistant properties. Although these coatings are in common industrial use, neither the protection mechanisms, nor the coating interation with the intermetallic precipitation phases are fully understood. Macroscopic models have been developed in order to represent the galvanic cells present in aluminium alloys due to the presence of such intermetallic particles. Particles modelled include CuAl₂, FeAl₃ and Cu₂FeAl₇, all know to be cathodic to the aluminium matrix. Variations in deposition, both in composition and thickness, are indicative of the mechanisms of deposition over each phase. Characterisation of the coating deposition was carried out using X-ray photoelectron spectroscopy, Rutherford backscattering spectroscopy, Auger electron spectroscopy, scanning electron microscopy with X-ray analysis. Depositional characteristics have been determined for each phase. The coating on the intermetallic phases is primarily Al oxide, and is significantly thinner than the coating on the matrix. This coating on the matrix consists mainly of a mixed Cr/Al oxide. The coating on the intermetallic phases was only one tenth the thickness of the matrix coating, and contained higher levels of Fe, Al and O. Matrix coating chemistry predominated with Cr, O, Fe and N, indicative of a chromate conversion coating. The mechanism for reduced rates of deposition over intermetallic phases was found to be affected by fluorine ion attack leading to intermetallic de-alloying and decomposition of Fe(CN)₆²⁻ accelerator into amide groups on the matrix.     

Enhanced antiferromagnetic coupling in dual-synthetic antiferromagnet with Co2FeAl electrodes

We study dual-synthetic antiferromagnets (DSyAFs) using Co₂FeAl (CFA) Heusler electrodes with a stack structure of Ta/CFA/Ru/CFA/Ru/CFA/Ta. When the thicknesses of the two Ru layers are 0.45 nm, 0.65 nm or 0.45 nm, 1.00 nm, the CFA-based DSyAF has a strong antiferromagnetic coupling between adjacent CFA layers at room temperature with a saturation magnetic field of ∼11,000 Oe, a saturation magnetization of ∼710 emu/cm³ and a coercivity of ∼2.0 Oe. Moreover, the DSyAF has a good thermal stability up to 400 °C, at which CFA films show B2-ordered structure. Therefore, the CFA-based DSyAFs are favorable for applications in future spintronic devices.     

Atom-probe investigations of fine-scale features in intermetallics

Intermetallics have been studied by means of Atom Probe Field Ion Microscopy. Atom-Probe techniques have been used to determine the phase composition and to study the role and the influence of additional elements. The use of the Tomographic Atom Probe makes it possible to map out the distribution of chemical species in a small volume of the material at a near atomic scale. This has been particularly used in order to study segregation of additional elements at interfaces or planar and linear defects in TiAl base and FeAl base alloys.     

Defect analysis with positron annihilation — Applications to Fe aluminides

Measurements of positron annihilation at various temperatures at constant pressure give information about the formation of thermal defects and their enthalpy in metals and alloys. In the FeAl system the formation enthalpy decreases from 1.6–2.0 eV in pure -Fe to 1.37 eV (Fe7at%Al) and 0.91 eV in the D0₃ structure. Additionally, the formation volume of the defects can be determined from measurements varying the pressure at constant temperature. The formation volume increases from 0.88 in Fe7at%Al to 1.42 in Fe40at%Al (=mean atomic volume). The change of the formation volume indicates different types of defects. With the further knowledge of the positron lifetimes in these materials we suggest triple defects as the main defects in this system. In quenching experiments the migration enthalpy is measured using the fast Doppler broadening technique. In Fe₃Al we observe a migration enthalpy of 0.5 eV, which is influenced by an apparing change of order in a temperature range above the D0₃ B₂ transition as determined from the phase diagram by Massalski.Thickness reduction of Fe6.3at%Si single crystals changes positron lifetimes with increasing deformation, particularly by rolling less than 5%.Dedicated to Professor Dr. rer. nat. Dr. h.c. Hubertus Nickel on the occasion of his 65th birthday     

Atomic Arrangement in B2 FeAl Prepared by Self-Propagated High-Temperature Synthesis at Varying Al Content and Annealing

Atomic arrangement in B2 FeAl prepared by self-propagated high temperature synthesis (SHS) as a function of Al concentration and annealing temperature has been studied by ⁵⁷Fe Mössbauer spectroscopy (MS) and X-ray diffraction (XRD). The increase of B2 FeAl isomer shift (IS) and lattice parameter (a) with Al concentration in the whole concentration range has been detected. This may originate from the formation of Al antisite atoms. Calculation of s-electron populations against number of Al antisites using a cluster approach and MO LCAO method supported this assumption. Annealing resulted in atomic rearrangements both in near-stoichiometric and Al-rich B2 FeAl.     

Comparison of the background corrected valence band XPS spectra of Fe and Co aluminides and silicides with their electronic structures

The background corrected valence band XPS spectra and the electronic structures of FeAl, FeSi, CoAl and CoSi were studied. Clean surfaces of the polycrystalline samples were obtained by in situ fracturing of the samples in an XPS spectrometer. The energy loss parts of the Fe 2p, Co 2p and valence band spectra were removed by the deconvolution method using Al 2s or Si 2s spectra as response functions. CoAl exhibited a satellite peak in the Co 2p region, but the other compounds had no clear satellite peaks in the Co 2p and Fe 2p regions. The experimentally background corrected valence band spectra were compared with the calculated spectra using the first-principle band calculation. There were large discrepancies between the spectra above the binding energy of 5 eV. These indicated that the experimental spectra could not be explained by the electronic structures of the ground states alone.     

Self-consistent APW band structure calculations for the intermetallic compounds FeAl,CoAl, and NiAl

Self-consistent augmented plane wave (APW) band structure calculations have been performed for the compounds FeAl, CoAl, and NiAl. For CoAl as a representative example the following results are presented: energy eigenvalues, band structure, radial charge densities and charge transfer. The charge distribution for all three compounds is displayed in tabular form and sections through theFermi surface are shown. The results are compared with previous band structure calculations on these substances. Reference is made to papers which have used the present results to calculate several physical properties of the investigated compounds.

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Selbstkonsistente Bandstrukturrechnungen für die intermetallischen VerbindungenFeAl, CoAl undNiAl

Zusammenfassung Für die Verbindungen FeAl, CoAl und NiAl wurden selbstkonsistente APW (augmented plane wave) Bandstrukturrechnungen durchgeführt. Energieeigenwerte, Bandstruktur, radiale Ladungsdichten und Ladungsüberführung werden für CoAl als repräsentatives Beispiel angegeben. Für alle drei Verbindungen wird die Ladungsverteilung in Tabellenform dargestellt, und es werden Schnitte durch dieFermioberfläche gezeigt. Die Ergebnisse dieser Arbeit werden mit den Resultaten früherer Berechnungen verglichen. Ferner wird auf Arbeiten hingewiesen, in welchen auf Grundlage der vorliegenden Bandstrukturrechnungen verschiedene physikalische Eigenschaften der genannten Verbindungen ermittelt wurden.