Pure Bcc Iron Research Articles

First-principles study of the interaction of solutes with ∑3(11-1) symmetric tilt grain boundaries in α-Fe

Abstract The structural, cohesive and magnetic properties of a symmetric Σ3(70.53)[011](11-1) tilt grain boundary in pure bcc iron and with commonly used alloying elements (Si, Co, Mn, Ti, Cu, Mo, Nb, V, Cr and Ni) by means of density functional theory calculations are studied. Solubility and segregation energies were calculated for different positions of dissolved atoms. Calculations show a tendency for impurities to segregate near the boundary. It was found that the substituting Co, Cu and Ni in the layer adjacent to the boundary have an embrittling effect, while other atoms enhance the cohesion of the grains. Magnetic moments on GB atoms are significantly higher than those on bulk atoms.

A phase field model for dislocation climb under irradiation: Formalism and applications to pure bcc iron and ferritic alloys

A phase-field model allowing to describe dislocation climb under irradiation is presented. Whereas previous phase-field models of the literature devoted to this phenomenon only take into account vacancies, our approach includes the effect of self interstitial atoms (SIAs) as required in the context of irradiated metals. Beyond the fact that it rigorously ensures the balance between the quantity of point defects, vacancies and SIAs, absorbed or emitted by the loop and the loop evolution, the present model has several originalities. First, it is capable to quantify the climb rate for systems far from equilibrium, which is commonly the case under irradiation. This required supplemental methodological developments since we clearly show that a mere generalization of existing phase-field models is not satisfactory to tackle this specificity. Secondly, it alleviates the often adopted assumption of perfect sink through the introduction of a kinetic parameter related to the dislocation jog density. A preliminary generic study of dislocations considered as nonperfect sinks leads to nonintuitive results, since the climb rate decreases when the dislocation jog density increases. Thirdly, the possibility to consider different types of interacting microstructural defects in the model allows to show the significant role of the point defect thermal equilibrium fractions on the climb rate in pure bcc iron under irradiation conditions. Finally, the climb model is coupled to the chemical diffusion equations in the same phase-field formalism. For this purpose, a multi-time step algorithm is proposed in order to couple phenomena with different characteristic time scales by several orders of magnitude, namely climb, point defect and chemical diffusion. It allows to study the interaction between the motion of the dislocation and the well-known phenomenon of radiation induced segregation in a Fe–Cr alloy. It is shown that the shape and size of the solute atmosphere can strongly depend on the dislocation motion under irradiation.

Hydrogen-assisted fatigue crack propagation in a pure BCC iron. Part II: Accelerated regime manifested by quasi-cleavage fracture at relatively high stress intensity range values

Hydrogen effect on fatigue performance at relatively high values of stress intensity factor range, ΔK, of pure BCC iron has been studied with a combination of various electron microscopy techniques. Hydrogen-assisted fatigue crack growth rate is manifested by a change of fracture features at the fracture surface from ductile transgranular in air to quasi-cleavage in hydrogen gas. Grain reference orientation deviation (GROD) analysis has shown a dramatic suppression of plastic deformation around the crack wake in samples fatigued in hydrogen. These results were verified by preparing site-specific specimens from different fracture features by using Focused Ion Beam (FIB) technique and observing them with Transmission Electron Microscope (TEM). The FIB lamella taken from the sample fatigued in air was decorated with dislocation cell structure indicating high amount of plasticity, while the lamella taken from the quasi-cleavage surface of the sample fatigued in hydrogen revealed a distribution of dislocation tangles which corresponds to smaller plastic strain amplitude involved at the point of fracture. These results show that a combination of critical hydrogen concentration and critical stress during fatigue crack growth at high ΔK values triggers cleavage-like fracture due to reduction of cohesive force between matrix atoms.

Hydrogen-assisted fatigue crack propagation in a pure BCC iron. Part I: Intergranular crack propagation at relatively low stress intensities

The role of hydrogen on intergranular (IG) fracture in hydrogen-assisted fatigue crack growth (HAFCG) of a pure iron at low stress intensity was discussed in terms of the microscopic deformation structures near crack propagation paths. The main cause of IG fracture was assumed to be the hydrogen-enhanced dislocation structure evolution and subsequent microvoids formation along the grain boundaries. Additionally, the impact of such IG cracking on the macroscopic FCG rate was evaluated according to the dependency of IG fracture propensity on the hydrogen gas pressure. It was first demonstrated that the increased hydrogen pressure results in the larger area fraction of IG and corresponding faster FCG rate. Moreover, gaseous hydrogen environment also had a positive influence on the FCG rate due to the absence of oxygen and water vapor. The macroscopic crack propagation rate was controlled by the competition process of said positive and negative effects.

Positron studies of interaction between yttrium atoms and vacancies in bcc iron with relevance for ODS nanoparticles formation

The very high calculated binding energy of vacancy (V)–Ysub (1.45eV) in Fe makes it be one possible earliest formation stage of (Y, Ti, O) nanoclusters in ODS alloy. Our direct slow positrons annihilation observations are used to valid the interaction between V and Y. The pure bcc iron samples have been implanted by 1.2MeV Y ions at three fluences from 1×1014 to 3×1015cm−2. Vacancy clusters are observed for all these three fluences. Their size and concentration decrease with Y concentration measured by using SIMS. Two hypotheses are proposed to explain the results, including the formation of complexes Vm–Yn and/or of precipitates Ym–Xn (X=Y, O, etc.). In addition, vacancy clusters are detected deeper than predicted by SRIM calculation due to, at least for a part, channelling which is confirmed by Marlowe calculation and SIMS measurements.

Growth of a brittle crack (0 0 1) in 3D bcc iron crystal with a Cu nano-particle

We present new results of molecular dynamics (MD) in 3D bcc Fe–Cu crystal with embedded central through crack (001)[110] of Griffith type loaded in mode I. Such sample geometry and border conditions in MD were chosen to invoke a cleavage crack extension through a bcc Cu nano-particle placed in front of the crack. The stress field and bond breakage caused by the crack and Cu inhomogeneity are analysed on the atomistic level. The character of crack extension and corresponding wave patterns are studied in the Fe–Cu system. The study is motivated by a question how a small Cu bcc precipitate changes the brittle behavior of the crack in comparison with pure bcc iron loaded in mode I.

Thermo-physical properties of body-centered cubic iron–magnesium alloys under extreme conditions

Using density functional theory formulated within the framework of the exact muffin-tin orbitals method, we investigate the thermo-physical properties of body-centered cubic (bcc) iron–magnesium alloys, containing 5 and 10 atomic % Mg, under extreme conditions, at high pressure and high temperature. The temperature effect is taken into account via the Fermi–Dirac distribution of the electrons. We find that at high pressures pure bcc iron is dynamically unstable at any temperature, having a negative tetragonal shear modulus ( C ′ ). Magnesium alloying significantly increases C ′ of Fe, and bcc Fe–Mg alloys become dynamically stable at high temperature. The electronic structure origin of the stabilization effect of Mg is discussed in detail. We show that the thermo-physical properties of a bcc Fe–Mg alloy with 5% Mg agree well with those of the Earth’s inner core as provided by seismic observations.

3D atomistic simulation of the interaction between a ductile crack and a Cu nanoprecipitate

We present the results of molecular dynamic simulations of ductile–brittle behavior at an edge [1 1 0] crack neighboring with a Cu nanoprecipitate in bcc iron. This crack in pure bcc iron emits dislocations in the ⟨1 1 1⟩{1 1 2} slip systems and it is stable up to very high loads, in qualitative agreement with experiments and continuum predictions. Our question is how a copper precipitate influences the dislocation emission from the crack front and how the dislocations interact with the precipitate.

Temperature dependence of ellipsometric spectra of Fe and CrNiAl steel

AbstractWe report on the temperature dependence of optical response of pure bcc iron and CrNiAl steel. Ellipsometric spectra, covering the 1.4‐3.0 eV spectral range, have been obtained during temperature cycles reaching 650 °C in UHV chamber. Since the penetration depth of light is rather small and the data are sensitive to the quality of sample surfaces, we have examined polished, etched and annealed samples. We identify characteristic features of the temperature dependence, consisting in spectral shifts of interband transitions and a redistribution of spectral weight. We have also detected reproducible irreversible changes in the initial stages of annealing cycles when the samples are heated to at least 300 °C. With increasing temperature, we observe a pronounced reduction of the minority‐spin interband contribution. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Magnetism of U/Fe multilayers

Magnetic multilayers are known to have behaviour shaped by the intrinsic magnetic properties of their constituents and of their interactions. Multilayers composed of Uranium (5f electrons) and transition metal (3d electrons) provide the unique combination of a potentially large orbital moment with strong electronic hybridisation effects between the extended 5f states and the strongly magnetic 3d states. In this study U/Fe multilayers with layer thicknesses 20 A ̊ < U<100 Å and 30 A ̊ < Fe<140 Å were fabricated by dc sputtering in a UHV chamber. X-ray reflectivity (XRR) spectra show good layer structure with little interdiffusion at interfaces. We have investigated the magnetic moment profile normal to the layers by polarised neutron reflectivity (PNR) and specifically whether magnetic moments are induced on the uranium layers by the proximity of the iron layers. The PNR results show a magnetic moment on the iron layers reduced from that of pure bcc iron and they set an upper limit on any moment on the U layers. A more sensitive search for the U moment has involved resonant X-ray magnetic reflectivity measurements at energies around the UM IV edge using different geometries of circular polarisation helicity and applied magnetic field. Some evidence of U moments has been observed.

Effects of Ti doping on the structures and magnetic properties of films

(Fe, Ti) - N films with the titanium concentration were prepared on Si(100) and NaCl substrates held at by facing-target sputtering. The effects of Ti doping on the structures and magnetic properties of Fe - N films were investigated. The crystal structures of the (Fe, Ti) - N films were examined with an x-ray diffractometer and a transmission electron microscope. The formation of magnetic iron nitride phases is sensitive to the Ti doping concentration. Films with contain and phases, and films with contain and TiN phases, but there is no magnetic iron nitride phase in the films with . The saturation magnetization of (Fe, Ti) - N films with was larger than that of pure bcc iron. was 2.6 T for samples, which is higher than the value for pure iron by 17%.

Epitaxial Fe16N2 films grown by sputtering

We have been able to obtain α″-Fe16N2 films using an underlayer template to induce the epitaxial growth of this metastable phase. They are epitaxial in the (001) direction and show single crystallinity in plane. Furthermore, they are deposited by simple reactive nitrogen sputtering. They have an average magnetic moment of 250 emu/g, considerably larger than the moment (217 emu/g) for pure bcc iron. Conversion electron Mössbauer spectroscopy gives three hyperfine fields corresponding to three different iron sites, as expected for this structure.

Epitaxy of cubic iron silicides on Si(111)

Recently, new pseudomorphic iron silicide phases with the cubic structure have been synthesized on Si(111) by molecular beam epitaxy (MBE) at room temperature (RT). The interface between the pseudomorphic phases and Si(111) is energetically very favourable compared to stable ϵ-FeSi/Si(111) or stable β-FeSi 2/Si(111) interfaces. It enables the epitaxy of various phases which are structurally similar, like Fe 3Si and even pure bcc iron at room temperature. The orientation was found to be always pinned to the type B orientation of the interface. On Fe 3Si a new √3 × √3 R30° surface reconstruction was observed.

The calculated electronic and magnetic structure of martensite iron nitrides

The electronic structure and local magnetic moment of BCT-FeN compounds were calculated by the first-principles self-consistent cluster method. For the alpha '-martensite, the author found that the magnetic moment of the first-neighbour Fe atoms of N was reduced while that of the second-neighbour Fe atoms was increased compared with those of pure BCC-iron; this was attributed to the concomitant effects of p-d interactions and tetragonal elongation of BCT-FeN, and was in good agreement with experiment. For the Fe16N2 structure, the calculated average magnetic moment is smaller than that of experiment.