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In this study, we present atomistic mechanisms of 1/2 [111](1̄1 0) edge dislocation interactions with point defects (hydrogen and vacancies) and hydrogen solute atmospheres in body centered cubic (bcc) iron. In metals such as iron, increases in hydrogen concentration can increase dislocation mobility and/or cleavage-type decohesion. Here, we first investigate the dislocation mobility in the presence of various point defects, i.e., change in the frictional stress as the edge dislocation interacts with (a) vacancy, (b) substitutional hydrogen, (c) one substitutional and one interstitial hydrogen, (d) interstitial hydrogen, (e) vacancy and interstitial hydrogen, and (f) two interstitial hydrogen. Second, we examine the role of a hydrogen-solute atmosphere on the rate of local dislocation velocity. The edge dislocation simulation with a vacancy in the compression side of the dislocation and an interstitial hydrogen atom at the tension side exhibit the strongest mechanical response, suggesting a higher potential barrier and hence, the higher frictional stress (i.e., ~83% higher than the pure iron Peierls stress). In the case of a dislocation interacting with a vacancy on the compressive side, the vacancy binds with the edge dislocation, resulting in an increase in the friction stress of about 28% when compared with the Peierls stress of an edge dislocation in pure iron. Furthermore, as the applied strain increases, the vacancy migrates through a dislocation transportation mechanism by attaining a velocity of the same order as the dislocation velocity. For the case of the edge dislocation interacting with interstitial hydrogen on the tension side, the hydrogen atom jumps through one layer perpendicular to the glide plane during the pinning-unpinning process. Finally, our simulation of dislocation interactions with hydrogen show first an increase in the local dislocation velocity followed by a pinning of the dislocation core in the atmosphere, resulting in resistance to dislocation motion as the dislocation moves though the hydrogen-solute atmospheres. With this systematic, atomistic study of the edge dislocation with various point defects, we show significant increase in obstacle strengths in addition to an increase in the local dislocation velocity during interaction with solute atmospheres. The results have implications for constitutive development and modeling of the hydrogen effect on dislocation mobility and deformation in metals. [ABSTRACT FROM AUTHOR]

Nanocomposite α-Fe/Nd2Fe14B alloys with high coercivity were prepared by melt spinning. The influence of Ti addition on the microstructure and magnetic properties of these nanocomposite alloys were investigated. Ti addition has proved to result in relevant improvements in the microstructure and magnetic properties, especially in the coercivity Hc. It is shown that the magnetic properties of α-Fe/Nd2Fe14B nanocomposite are improved by an additional 5 at. % Ti, in particular, Hc from 595 up to 1006 kA/m, and (BH)max from 126.7 up to 135.3 kJ/m3. The coercivity mechanism of the α-Fe/Nd2Fe14B nanocomposite magnets was analyzed by studying the behavior of Hc(T)/Ms(T) versus HNmin(T)/Ms(T) (Kronmüller plot) and the dependence of Mirrev(H)/2Mr on the reverse field H. The results of this analysis show that nucleation is the dominating mechanism for the magnetization reversal in these nanocomposites. The Kronmüller plot gives evidence for a reduction in the exchange coupling resulting in an increase in the coercivity. [ABSTRACT FROM AUTHOR]

The present work reports the formation of an interstitial dislocation loop with a lower primary knock-on atom (PKA) energy in alpha-iron under strain conditions by the use of molecular dynamics simulation. The study was conducted using a PKA energy of 1~10 keV and hydro-static strain from −1.4 to 1.6%. The application of 1.6% hydrostatic strain results in the formation of ½ dislocation loop with a low PKA of 3 keV. This result was associated with a threshold displacement energy decrement when moving from compression to tension strain, which resulted in more Frenkel pairs initiated at peak time. Furthermore, many of the initiated defects were energetically favorable by 2 eV in the form of the interstitial dislocation loop rather than a mono defect.

Radiation damage in reactor materials caused by the collision of the fast neutrons has a great impact on the reliability and safety of nuclear reactors. The element vanadium has attracted interest in many fields due to its advantageous properties in alloys. Thus, molecular dynamics simulation (MD) and first-principles calculation have been executed here to explore the radiation-resistant properties of five materials adding a layer in the bulk (pure iron and four types of Fe-V alloys containing 10%-40% V). The following results were inferred from these simulations. Firstly, the number of Frenkel pairs (FPs) at the stable quenching stage in the bulk decreases when the Fe-V alloy is added as an anti-radiation layer to the bulk. These benefits are evident for the Fe80V20 and alloy layers with more vanadium. The main reason is that the Fe-V binding energy is greater than the Fe-Fe binding energy, which can make the Primary Knock-On atom (PKA) lose more energy at the Fe-V alloy layer. Secondly, the average value of point-defect, cluster and defect clustered fractions in the bulk of Fe-V alloy is smaller than that in the pure iron at the stable quenching stage, especially for the Fe80V20 alloy. [ABSTRACT FROM AUTHOR]

Abstract Hydrogen atoms absorbed by metals in hydrogen-containing environments can lead to the premature fracture of the metal components used in load-bearing conditions. Since metals used in practice are mostly polycrystalline, grain boundaries (GBs) can play an important role in hydrogen embrittlement of metals. Here we show that the reaction of GBs with lattice dislocations is a key component in hydrogen embrittlement mechanism for polycrystalline metals. We use atomistic modeling methods to investigate the mechanical response of GBs in alpha-iron with various hydrogen concentrations. Analysis indicates that dislocations impingement and emission on the GB can provoke it to locally transform into an activated state with a more disordered atomistic structure, and introduce a local stress concentration. The activation of the GB segregated with hydrogen atoms can greatly facilitate decohesion of the GB. We propose a hydrogen embrittlement model that can give better explanation of many experimental observations. Graphical abstract Image 1 Highlights • A new hydrogen embrittlement model is proposed for polycrystalline metals. • Dislocation - grain boundary reaction is a key process in hydrogen embrittlement. • Dislocation - grain boundary reaction causes local activation of grain boundary. • Activation of grain boundary segregated with H atoms greatly aids decohesion. [ABSTRACT FROM AUTHOR]

Highlights • Hydrogen permeation through the shot-peened pure Fe was suppressed by a factor of ten at the lower temperature below 453 K. • The effects were disappeared or lost at higher temperatures than 473 K. • Hydrogen was trapped at least two trapping sites induced by the shot-peening treatment. • Amount of hydrogen in the shot-peened surface was three times larger than that in the normal surface of pure Fe. Abstract Effects of the shot-peening on hydrogen permeation and retention behaviors were examined by applying tritium tracer techniques to the gas-driven hydrogen permeation experiments. Hydrogen permeability in shot-peened iron was reduced by a factor of ten in comparison with the as-received iron at lower temperatures of 298 K and 453 K. The effects were disappeared at higher temperatures than 473 K. Hydrogen was trapped at least two trapping sites induced by the shot-peening treatment. The amount of trapped hydrogen in the shot-peened surface was three times larger than that in the normal surface of the as-received iron. [ABSTRACT FROM AUTHOR]

The influence of the microstructure of uniaxial cold-compacted green iron on the sintering shrinkage was investigated. Pores in the green parts are very slightly oriented, while the dimension of the interparticle contact areas is anisotropic. A large and anisotropic anisothermal shrinkage in alpha iron was measured, greater than isothermal shrinkage at the 1120°C. The results were interpreted on the basis of the geometrical and of the structural activity, and the effective diffusivity responsible for neck growth was determined. [ABSTRACT FROM AUTHOR]

Mobile hydrogen, when dissolving in metals, redistributes due to the density gradients and elastic stresses, and enables destruction processes or phase transformations in local volumes of a solvent metal. It is rather important in solid state physics to investigate these interactions. The first-principle calculations performed in terms of the density functional theory, are used for thermodynamic simulation of the elastic stress effect on the energy of hydrogen dissolution in α-Fe crystal lattice. The paper presents investigations of the total energy of Fe-H system depending on the lattice parameter. As a result, the relation is obtained between the hydrogen dissolution energy and stress. A good agreement is shown between the existing data and simulation results. The extended equation is suggested for the chemical potential of hydrogen atom in iron within the local stress field. Two parameters affecting the hydrogen distribution are compared, namely local stress and phase transformations. [ABSTRACT FROM AUTHOR]

We present the calibration of the [ $\alpha$ /Fe] element in terms of ultra-violet excess for 465 dwarf stars with spectral type F0-K2. We used a single calibration, fitted to a third degree polynomial with a square of the correlation coefficient 0.74 and standard deviation 0.05 mag, for all stars due to their small colour range, $0.1<(g-r)_{0} \leq 0.6$ mag, and high frequency in the blueward of the spectrum which minimize the guillotine effect. Our calibration provides [ $\alpha$ /Fe] elements in the range $(-0.05, 0.35]$ dex. We applied the procedure to a high-latitude field, $85^{\circ }\leq b \leq 90^{\circ }$ with size 78 deg and we could estimate the [ $\alpha$ /Fe] elements of 23,414 dwarf stars which occupy a Galactic region up to a vertical distance of $z=9$ kpc. We could detect a small positive gradient, $d[\alpha \text{/Fe}]/dz=+0.032 \pm 0.002~\mbox{dex}\,\mbox{kpc}^{-1}$ , for the range $0< z<5$ kpc, while the distribution of the [ $\alpha$ /Fe] element is flat for further $z$ distances. [ABSTRACT FROM AUTHOR]

High concentrations of vacancies tend to be formed inside the metal materials under irradiation, and then accumulate and cluster together gradually to promote the formation of nanovoids. Generally, these voids act as obstacles for dislocation glide and thereby change/degrade the mechanical behavior of irradiated materials. In this work, the interaction between ellipsoidal nanovoids with edge dislocations in alpha-iron has been studied by atomic simulations. The results illuminate that the ellipsoidal void’s semi-major axis on the slip plane and parallel to the dislocation line is the dominant factor controlling the obstacle strength of ellipsoidal nanovoids. Two other semi-major axes, which are perpendicular to the glide plane and parallel to the Burgers vector, respectively, can also influence the critical resolved shear stress (CRSS) for dislocation shearing the ellipsoidal void. The intrinsic atomic mechanisms controlling above phenomena, such as nanovoid-geometry spatial constraint and nanovoid-surface curvature on dislocation evolution, have been discussed carefully. The classical continuum model has been amended to describe the dislocation-ellipsoidal nanovoid interaction base on current results. In addition, the influence of temperature on the CRSS of ellipsoidal nanovoids has also been investigated. [ABSTRACT FROM AUTHOR]

Copyright of Metallophysics & Advanced Technologies / Metallofizika i Novejsie Tehnologii is the property of G.V. Kurdyumov Institute for Metal Physics, N.A.S.U and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Aluminium alloy free CS1 type steel (0.06 wt% C, 0.45 wt% Mn) and samples of cold roll bonded steel bimetal alloys (MAS15 and MAS16) were fabricated and investigated by X-ray diffraction (XRD), Fe conversion electron Mössbauer spectroscopy (CEMS) at room temperature. XRD has revealed only the existence of the alpha iron solid solution (steel) phase in the steel only sample, while identified steel and metallic Al and Sn constituent phases in the bimetallic alloys. Fe Mössbauer spectroscopy revealed the presence of 4 % secondary iron-bearing phase attributed mainly to iron oxide/ oxyhydroxides (ferrihydrite) besides the steel matrix on the surface of the steel sample. A significant difference between the occurrences of the secondary phase of differently prepared bimetal alloys found in their Fe CEM spectra allowed to identify the main phase of debris as different iron oxide/ oxyhydroxides. [ABSTRACT FROM AUTHOR]

The Autonomous Basin Climbing method and Rate Theory are applied to investigate the interstitial emission mechanism in alpha-Fe. The atomic trajectories and potential energy landscape of the interstitial emission process induced by the Σ3<110>{111} symmetrical tilt grain boundary are presented. By comparing with vacancy hopping mechanism, the grain boundary influence range at finite temperature is revealed. We uncover the energetic and geometric essentials of the interstitial emission mechanism, and find connections between two previously reported defect-grain boundary interactions at long time scale. Surprisingly in nanolayered structures, higher grain boundary density raises the activation energy barriers of interstitial emission. This phenomenon is strongly correlated with the energy and local stress distribution of the grain boundary. [ABSTRACT FROM AUTHOR]

The accumulation and clustering of He atoms at Σ 3 〈1 1 0〉 {1 1 2} and Σ 73 b 〈1 1 0〉 {6 6 1} grain boundaries (GBs) in body-centred cubic Fe, as well as their effects on GB reconstruction, have been investigated using atomic-level computer simulations. The accumulation of He atoms and the evolution of the GB structure all depend on local He concentration, temperature and the original GB structure. At a local He concentration of 1%, small He clusters are formed in the Σ 3 GB, accompanied by the emission of single self-interstitial Fe atoms (SIAs). At a He concentration of 5%, a large number of SIAs are emitted from He clusters in the Σ 3 GB and collect at the periphery of these clusters. The SIAs eventually form 〈1 0 0〉 dislocation loops between two He clusters. It is likely that impurities may promote the formation of 〈1 0 0〉 loops and enhance their stabilities in α-Fe. At a He concentration of 10%, the large number of emitted SIAs are able to rearrange themselves, forming a new GB plane within the Σ 3 GB, which results in self-healing of the GB and leads to GB migration. In contrast to the Σ 3 GB, He clusters are mainly formed along the GB dislocation lines in the Σ 73 b , and the emitted SIAs accumulate at the cores of the GB dislocations, leading to the climb of the dislocations within the GB plane. As compared to bulk Fe, a higher number density of clusters form at GBs, but the average cluster size is smaller. The product of cluster density and average cluster size is roughly constant at a given He level, and is about the same in bulk and GB regions, and varies linearly with the He concentration. [ABSTRACT FROM AUTHOR]

As part of a series of experiments based on the application of ultrasound to structural materials and metal alloys, a sample of alpha iron (Ferrite) was subjected to ultrasounds in order to investigate the possible emission of nuclear particles and the possible formation of new isotopes. The sample was subsequently subjected to Instrumental Neutron Activation Analysis (INAA) to highlight the possible transformations that occurred, or the formation of new natural isotopes or the variation of the natural isotopic composition of the material. [ABSTRACT FROM AUTHOR]

The development of vacancy clusters in α-Fe is essential for understanding the formation of irradiation damage. In this work, the structures and the development of vacancy clusters in α-Fe have been studied by atomistic computer simulation. It was found out that larger clusters can form by combinations of smaller cluster units including triangular trimers and square tetramers. Furthermore, the formation of stacking faults, cracks or voids can be explained by the development of the clusters. [ABSTRACT FROM AUTHOR]

Abstract: Formation and migrations energies were calculated for multi-vacancies V n (n =1–15) in bcc Fe bulk and its surfaces using density functional theory. In this work, we analyze the evolution of the formation energy, the migration processes and the stable configurations of V n . For small clusters (mono- and divacancies), our results are in agreement with those from the literature. We also focused on the study of the interactions between surfaces (Fe(100) and Fe(110)) and a vacancy, which enabled an assessment of the interactions the between vacancy and free surfaces. For V 3, we found that the more compact structure (labeled (112)) is a more stable configuration than that of V 4 (111122). The migration mechanisms of V 2 and V 3 are decomposed to identify the position of the transition states, and to thus explain the low migration energy of V 3. An analysis of the structure of different clusters shows that for n ⩽10, the more stable structures are built from 2NN and 1NN divacancies. We suggest some stable compact configurations for large clusters. [Copyright &y& Elsevier]

Dislocation pipe diffusion seems to be a well-established phenomenon. Here we demonstrate an unexpected effect, that the migration of interstitials such as carbon in iron may be accelerated not in the dislocation line direction , but in a conjugate diffusion direction. This accelerated random walk arises from a simple crystallographic channeling effect. is a function of the Burgers vector b, but not , thus a dislocation loop possesses the same everywhere. Using molecular dynamics and accelerated dynamics simulations, we further show that such dislocation-core-coupled carbon diffusion in iron has temperature-dependent activation enthalpy like a fragile glass. The 71° mixed dislocation is the only case in which we see straightforward pipe diffusion that does not depend on dislocation mobility. [ABSTRACT FROM AUTHOR]

Boron is a common alloying element in modern steels with a significant influence on the mechanical properties already at concentrations of only a few parts per million. The effect of boron depends on its distribution in the microstructure. Here, we characterize the elemental factors that determine the boron distribution in a-iron by density functional theory calculations. Boron as point defect has been considered in substitutional and interstitial sites. The calculated migration barriers for the substitutional and interstitial mechanisms show the first nearest-neighbor hops being preferred over second nearest-neighbor hops. A dissociative mechanism shows boron migrating via an interstitial mechanism to be likely trapped by vacancies. In order to characterize the interaction with other point defects, we determined the distance-dependent interaction energy of a boron defect with a vacancy, a second boron, and with hydrogen, carbon, nitrogen, oxygen, aluminum, silicon, phosphorus, and sulfur atoms. We find that substitutional boron binds strongly to interstitial point defects with dumbbell formation and weaker to substitutional point defects. Interstitial boron tends to repel substitutional and interstitial point defects. We find a similarity of substitutional boron and vacancies regarding their influence on elastic properties and their interaction with point defects in α-iron. [ABSTRACT FROM AUTHOR]

An extended variant of Smirnov statistic theory [12] is presented that describes the equilibrium concentrations of vacancy-hydrogen atom complexes as a function of temperature. Calculations based on the first-principle values of interaction energy show that, at low temperatures, the majority of vacancies are bound with several hydrogen atoms, and the concentration of vacancies sharply increases. [ABSTRACT FROM AUTHOR]

Abstract: Fifty healthy participants took part in a double-blind placebo-controlled study in which they were either given auditory alpha activity (8–12Hz) training (N =18), random beta training (N =12), or no training at all (N =20). A novel wireless electrode system was used for training without instructions, involving water-based electrodes mounted in an audio headset. Training was applied approximately at central electrodes. Post-training measurement using a conventional full-cap EEG system revealed a 10% increase in alpha activity at posterior sites compared to pre-training levels, when using the conventional index of alpha activity and a non-linear regression fit intended to model individual alpha frequency. This statistically significant increase was present only in the group that received the alpha training, and remained evident at a 3month follow-up session, especially under eyes open conditions where an additional 10% increase was found. In an exit interview, approximately twice as many participants in the alpha training group (53%) mentioned that the training was relaxing, compared to those in either the beta (20%) or no training (21%) control groups. Behavioural measures of stress and relaxation were indicative of effects of alpha activity training but failed to reach statistical significance. These results are discussed in terms of a lack of statistical power. Overall, results suggest that self-guided alpha activity training using this novel system is feasible and represents a step forward in the ease of instrumental conditioning of brain rhythms. [Copyright &y& Elsevier]

The mechanical behaviour of steels is strongly related to their underlying atomistic structures which evolve during thermal treatment. Cu-alloyed α-Fe undergoes a change in material behaviour during the ageing process, especially at temperatures of above 300°C, where precipitates form on a large time-scale within the α-Fe matrix, yielding first a precipitation strengthening of the material. As the precipitates grow further in time, the material strength decreases again. This complex process is modelled with a multiscale approach, combining Kinetic Monte Carlo (KMC) with Molecular Dynamics (MD) simulations in a sequential way and exploiting the advantages of both methods while simultaneously circumventing their particular disadvantages. The formation of precipitates is modelled on a single-crystal lattice with a diffusion based KMC approach. Transferring selected precipitation states at different ageing times to MD simulations allows the performance of nano tensile tests and the analysis of failure initiation. The anisotropic tensile behaviour is investigated in the [100], [110] and [111] directions, showing monotonically decreasing tensile strengths and deformation strains. Hence precipitation strengthening is mainly due to dislocation–precipitate interactions which are non-existent at small tensile loadings in this scenario. At the point of ductile failure, dislocations are generated at the interfaces between precipitates and the Fe matrix. Straining in the [100] direction, they lie on {110} and {112} glide planes, as expected. With the method presented here, the changes of the anisotropic tensile moduli are related to different states of thermal ageing, i.e., to nucleation, growth and Ostwald ripening of Cu precipitates. [ABSTRACT FROM PUBLISHER]

In situ compression transmission electron microscopy was used to investigate the deformation behavior of alpha-Fe nanoblades. The results showed an inhomogeneous deformation process, composed of a series of short displacement bursts and single large displacement bursts. The short displacement bursts were associated with the collective motion of dislocations within the specimen, whilst the large displacement bursts occurred through dislocation-assisted slip band formation nucleated from the surface of the nanoblades. [ABSTRACT FROM AUTHOR]

Diffusion of W in the 723-1153 K temperature range both in paramagnetic and ferromagnetic α-Fe was studied, diffusion couples were manufactured by W evaporation onto high-purity Fe samples. Measurements were made using the Heavy Ion Rutherford Backscattering (HIRBS) technique as the analysis tool. A straight Arrhenius plot was obtained in the paramagnetic region with a break at the Curie temperature (1043 K) followed by a curved plot at lower temperatures as a product of the effect of ferromagnetism on diffusion. A straight Arrhenius plot was obtained in the paramagnetic region with a break at the Curie temperature (1043 K) followed by a curved plot at lower temperatures resulting from the effect of ferromagnetism on diffusion. A previous developed model for the diffusion of non-magnetic impurities in ferromagnetic Fe fits the data perfectly well, giving a temperature dependent diffusivity according to [ABSTRACT FROM AUTHOR]

We propose a mathematical description of the refined shape of absorption lines in Mössbauer spectra obtained using a resonant detector, taking into account distortions in the lineshape as a result of the influence of factors introduced by the experimental equipment and associated with the properties of the source and detector. We present the results of treatment of the absorption spectra of alpha iron and sodium nitroprusside within the proposed mathematical model. [ABSTRACT FROM AUTHOR]

Abstract: This paper addresses the problem of solid solution strengthening of α-iron. The strengthening effect by alloying of substitutionally or interstitially dissolved atoms is obtained by simulating the interaction of a dislocation in an α-iron crystal with random as well as special arrangements of dissolved atoms. For the detection of the position of a dislocation during the simulation an algorithm is implemented which allows to analyze dislocation movements in detail. Solid solution strengthening is determined for different concentrations of Cu and Ni atoms. It is found that the movement of edge dislocations in the case of Ni atoms is nearly unhindered in contrast to when Cu atoms are present. The critical shear stress for dislocation movement is found to be significantly higher for Cu-alloyed α-iron as compared to Ni-alloyed α-iron. Interstitial C atoms in α-iron are significantly stronger obstacles for the dislocation movement than substitutional Cu atoms. The strengthening level thereby attains values similar to those found in experiments. In order to analyze the mechanisms of solid solution strengthening, a model system is used by the construction of a special interaction potential in which by the choice of the interaction parameters the lattice constants and the shear modulus of the alloyed element can be modified independently. Especially, dissolved elements can be simulated which are different from α-iron only by their lattice constants or only by their shear modulus, respectively. In this way it is found that dissolved elements with a smaller shear modulus than α-iron lead to a larger strengthening value than elements with a larger shear modulus than α-iron. Moreover, elements with a larger lattice constant than α-iron are found to be stronger obstacles than elements with a smaller lattice constant than α-iron. [ABSTRACT FROM AUTHOR]

Abstract: Kinetics of the apparent isothermal and the non-isothermal crystallization of α-Fe phase within the amorphous Fe81B13Si4C2 alloy were investigated by an X-ray diffraction (XRD) and by a differential scanning calorimetry (DSC). It was established that the apparent isothermal crystallization of α-Fe phase within amorphous Fe81B13Si4C2 alloy could be described by the Johnson–Mehl–Avrami (JMA) kinetic model (with parameter n iso =4.0). The apparent isothermal crystallization process includes a constant rate of nucleation and three-dimensional growth of nuclei. The results of X-ray diffraction (XRD) data of the isothermally crystallized samples confirmed the above established kinetic model. From the kinetic analysis of the non-isothermal crystallization of the α-Fe phase within this amorphous alloy, it was concluded that the autocatalytic two-parameter Šesták–Berggren (SB) reaction model (with kinetic exponents M=0.72 and N=1.02) describes well the studied process under the given conditions. The non-isothermal crystallization process involves the constant nucleation rate of stable nuclei with additional secondary two-dimensional (surface) nucleation and overlapping of the growing nuclei on account of the non-isothermal activation. [Copyright &y& Elsevier]

Abstract: The effects of He-vacancy (He-V) clusters on the mobility of an a/2〈111〉{110} edge dislocation in α-Fe are investigated by atomic simulation with empirical potentials at 0K. A number of small He n V m (n/m 0–4) clusters initially placed at the same position on the slip plane are comparatively studied. The results show that the interaction of He-V clusters with edge dislocations depends on not only the helium-to-vacancy (He/V) ratio, but also the cluster size. The small He-V clusters with low He/V ratios have a small effect on the dislocation mobility, but the larger clusters with higher He/V ratios significantly increase the critical resolved shear stress for dislocation glide. One of interesting results is that the He-V clusters almost stay at their original positions, and do not move along with the dislocation. [Copyright &y& Elsevier]

Abstract: Experimental and analytical studies have shown that the presence of hydrogen enhances dislocation emission. However, the effect of hydrogen on dislocation emission is not well understood. In this study, we investigated the effect of hydrogen on dislocation emission from a mode II crack in alpha iron using an atomistic model. We found that hydrogen decreases the stacking fault energy of alpha iron, resulting in an enhancement of dislocation emission. It is clarified that hydrogen atoms existing within a few angstroms of the crack tip only enhance the dislocation emission. We also confirmed that dislocation emission enhancement can occur at realistic hydrogen concentrations of hydrogen gaseous condition under thermal equilibrium conditions. [Copyright &y& Elsevier]

At approximately 13GPa, body-centered cubic alpha-iron undergoes a fully reversible, pressure-induced phase transition into hexagonal close-packed epsilon-iron. Microstructural evidence of this phase transition has been identified in the fully reverted alpha-iron as a large number of {332} twins found primarily as secondary twins within {112} primary twins. The {332} twins were produced during high-pressure shock-loading of 1018 steel at a peak pressure above the alpha-epsilon phase transition pressure. The twins were identified using electron backscattered diffraction and transmission electron microscopy. [Copyright &y& Elsevier]

Abstract: The dimer method for searching transition states has been used to systematically study possible migration paths of vacancies, He interstitials and He-vacancy (He/V) clusters at Σ11<110> {323} and Σ3<110> {111} grain boundaries (GBs) in α-Fe. Vacancies trapped at the GBs diffuse along the GBs with migration energies much less than that within the perfect crystal. Long-time dynamics simulations of diffusion pathways reveal that vacancies migrate one-dimensionally along specific directions in both GBs: directly along close-packed rows in the Σ3 GB, and in zigzag paths within the Σ11 GB. Also, dimer saddle point searches show that He interstitials can diffuse along the GBs with migration energies of 0.4–0.5eV, similar to those of individual vacancies at the GBs, and the corresponding mechanisms are determined. The rate-controlling activation energy for migration of a He-divacancy cluster in the GBs determined using the dimer method is about 0.9eV. This is comparable to the migration energy for a He-divacancy cluster in bulk α-Fe. [Copyright &y& Elsevier]

Abstract: Activation with three different gases (H2, CO and synthesis gas) over an Fe100/K1.4/Si4.6/Cu2.0 catalyst was conducted to investigate the effects of pretreatment gas on Fischer-Tropsch Synthesis (FTS) activity and selectivity. Catalyst slurry was withdrawn from the reactor at increasing time intervals of FTS for Mössbauer spectroscopic analysis. Activation with CO produced the highest syngas conversion while H2 generated the lowest; syngas activation produced a slightly lower conversion than CO activation. CO activation transformed the majority of the iron into χ-Fe5C2 and Magnetite with only 12% ɛ-Fe2.2C being detected. Unlike the CO activated catalyst, the syngas activated iron catalyst resulted in a lower amount of χ-Fe5C2 than ɛ-Fe2.2C. The initial high (64%) content of ɛ-Fe2.2C decreased gradually to below 30% while CO conversion decreased from 83% to 55%. During this period, χ-Fe5C2 increased from initial 10% to 33%. Magnetite changed little during the process while the form of carbides interchanged. Hydrogen activation yielded a low CO conversion of 50% and only 8% χ-Fe5C2 and 16% ɛ-Fe2.2C was formed while Magnetite was as high as 75% after the FTS reaction rate became constant. Although activation gas type had a significant effect on syngas conversion, hydrogen, syngas and CO activations produced similar H2 to CO usage ratio, hydrocarbon product distribution, olefin fraction, alpha value and CO2 selectivity. [Copyright &y& Elsevier]

Abstract: Despite extensive investigations, the distribution of hydrogen around a stress singularity field is still not well understood. In this study, we conducted molecular statics (MS) analyses of the hydrogen-trap energy around a {112}<111> edge dislocation in alpha iron. The distribution of hydrogen in crystals is generally assumed to be dominated by hydrostatic stress. However, the MS results indicate that the hydrogen-trap energy is sensitive to shear stress as well as hydrostatic stress, thus indicating that strong trap sites are distributed across a wide range on the slip plane around the dislocation core. We also performed molecular dynamics simulation of hydrogen diffusion, and revealed the anisotropic diffusion behaviour of hydrogen around the dislocation core. [Copyright &y& Elsevier]

A letter to the editor is presented discussing a study about the atomistic simulation of the effects of hydrogen on the mobility of edge dislocation in alpha iron.

Abstract: White cast irons perform much less favourably in industrial service environments such as ball mills than would be predicted by standard laboratory abrasion tests. Pin abrasive tests are widely thought to simulate high stress abrasion, but in reality they grossly over-estimate service lives of white iron mill liners compared to pearlitic steel liners. The ‘impact-abrasion’ hypothesis has suggested that the key difference between laboratory and service conditions is impact. However, we have found no evidence of such a phenomenon in ball mills, and instead the observed behaviour is attributed to high stress abrasion in the service environment. The defining characteristics of high stress abrasion are re-stated in a way that emphasises the load able to be transmitted to the wearing surface through the abrasive particle. Data are presented which indicate that abrasive wear mode can be identified from quantitative performance comparisons between key classes of wear-resistant alloys. Three useful diagnostics are proposed. Using these diagnostics it is shown that pin abrasive tests do not represent high stress abrasion in any industrially-relevant sense. Appropriately modest performance of white irons can be reproduced in various laboratory apparatus having zero or negligible impact energy: RWAT-analogues using solid metal wheels (xWAT), the Abex block-on-track apparatus, and a simple laboratory ball mill abrasion test (BMAT). Of these, the BMAT most closely reproduces the contact and particle mechanics of service conditions, and can give quantitative alloy performance comparisons closely matching those in service. This paper discusses some features of the BMAT and presents preliminary data for white cast irons. [Copyright &y& Elsevier]

Focused electron beam induced chemical vapor deposition was performed in a scanning electron microscope with a field emission gun using a precursor of iron carbonyl. Due to the longer deposition time and higher gas pressure than those of our previous electron beam-induced deposition method, a new type of deposition occurred. A large amount of nanosized crystals were produced around the focused beam irradiation point on a carbon substrate at room temperature. The nanocrystals were systematically characterized using transmission electron microscopy with electron energy loss spectroscopy (EELS), and were identified to be single crystals of α-Fe. [ABSTRACT FROM AUTHOR]

The interfacial energies between α and γ iron were calculated for Kurdjumov-Sachs (K-S), Nishiyama-Wassermann (N-W), and one orientation relationship (OR), which may represent irrational OR, with varying interface orientation. The relaxation of atomic structure in the vicinity of the interface was performed by the Monte Carlo method using an embedded atom method (EAM) potential. Whereas the polar plot of calculated interfacial energies exhibited small and large cusps for the K-S and N-W ORs, an almost equiaxed energy surface with shallow cusps was obtained for the irrational ORs. The equilibrium shape of an a particle, N-W oriented with the γ matrix, was a thick rectangular plate with broad facets containing monatomic ledges. In contrast, the equilibrium shape of a particle, K-S oriented with the γ matrix, was elongated nearly in the closed-packed directions with {112}γ type and another broad facet that achieves relatively good matching. The volume of these shapes in the Wulff space tends to be smaller for K-S than that of the N-W OR. The equilibrium shape of the grain boundary α particle at the γ grain boundary was calculated under the assumption that the α particle has K-S OR with one of the two γ grains using the modified Wulff construction proposed earlier. The variation of the shape of the α particle with γ grain boundaries at an early stage of precipitation may primarily be ascribed to the change in the variant of the smallest nucleation activation energy and less prominently to the change in the γ grain boundary energy. [ABSTRACT FROM AUTHOR]

This review provides a brief survey of the computational methods which have been adopted to model fracture in materials, particularly polycrystalline metals and alloys, and considers in greater detail the use of geometrical modelling. The different modes of fracture, transgranular and intergranular brittle and ductile, are discussed, together with the modelling tools adopted for macroscale, microscale and nanoscale applications. The procedures which have been used for creating computer models of two– and three–dimensional polycrystalline materials and for propagating cracks through them are then outlined. Applications of these methods to investigate the propagation of cleavage cracks across grain boundaries, to study the influence of texture, grain shape, impurity segregation and prior creep cavitation, to examine the ductile–to–brittle transition region in ferritic steels and to consider the influence of work hardening on ductile fracture are then presented. The predictions are compared with experimental results and proposals for future studies are discussed. [ABSTRACT FROM AUTHOR]

We present results of a large-scale atomic-level study of dislocation–precipitate interaction. We have considered a 1/2 〈1 1 1〉 edge dislocation gliding in α-iron containing coherent copper precipitates of size from 0.7 to 6 nm over a temperature range from 0 to 450 K. The results demonstrate that some features are qualitatively consistent with earlier theoretical conclusions, e.g. the critical resolved shear stress (CRSS) is proportional to L-1 and ln(D), where L and D are precipitate spacing and diameter. Other features, which are intrinsic to the atomic-level nature of the dislocation–precipitate interaction, include strong dependence of the CRSS on temperature, dislocation climb and precipitate phase transformation. [Copyright &y& Elsevier]

Despite the existence of a number of published results, the data on the solubility of carbon in alpha iron are still inaccurate. An analysis of published experimental results shows that available values vary greatly (between 50 and 100 ppm by wt, for example, at 600°C). These discrepancies make it difficult to optimize the metallurgical processes of low-carbon or ultralow-carbon alloys. An experimental methodology, using the measurement of the thermo-electric power (TEP) of the alloy, was set up. This enabled us to deduce the quantity of free interstitials in the matrix by measuring the amount of interstitials which segregate on dislocations after a deformation of the sample. This technique was used in the case of an Al-killed steel containing 0.2 pct Mn. The limit of solubility of carbon was determined with a precision of ± 2 ppm between 550°C and 730°C. This limit of solubility can be analytically described by the relation C(wt pct) = 6.63 exp (-11.8kcal⋅mol-1/RT), which is shown to be valid only for temperatures above 400°C. We show experimentally that the residual concentration of carbon at low temperature is much greater than the value predicted by the extrapolation of this relation. Complementary studies on steels with various C and Mn contents allow us to verify the validity of the proposed methodology. [ABSTRACT FROM AUTHOR]

Stress-induced movement of hydrogen to the high tensile region in AISI 4340 steel was measured at room temperature and 202 °C. A neodymium hydrogen detection method was used to determine the location and concentration of hydrogen. The AISI 4340 stress rings with local hydrogen content up to 0.69 ppm and loaded to 16.87, × 103 kg cm-2 did not fail, whereas a stress ring with 0.73 ppm did fail for the test condition cited. The predicted hydrogen concentration versus fracture time curves at constant tensile stress indicated a good agreement with the stress ring test data. The partial molal volume of hydrogen, υ¯H, was calculated to be 2.47 cm3 per g-atom for AISI 4340 steel, which was greater than the reported value for alpha iron at 2.0 cm3 per g-atom. Knowing the value υ¯H and the hydrogen content for equation RT In Cσ/Co = υ¯Hσ/3 the increase of hydrogen concentration by a given applied stress can be calculated. [ABSTRACT FROM AUTHOR]

The present work reports the formation of an interstitial dislocation loop with a lower primary knock-on atom (PKA) energy in alpha-iron under strain conditions by the use of molecular dynamics simulation. The study was conducted using a PKA energy of 1~10 keV and hydro-static strain from −1.4 to 1.6%. The application of 1.6% hydrostatic strain results in the formation of ½<111> dislocation loop with a low PKA of 3 keV. This result was associated with a threshold displacement energy decrement when moving from compression to tension strain, which resulted in more Frenkel pairs initiated at peak time. Furthermore, many of the initiated defects were energetically favorable by 2 eV in the form of the interstitial dislocation loop rather than a mono defect. [ABSTRACT FROM AUTHOR]

Abstract: In order to understand the evolution of materials under irradiation conditions similar to those in future fusion reactors, we have irradiated high purity iron in situ in a transmission electron microscope with 1MeV Fe+ ions while simultaneously implanting 15keV He+ ions, at 500°C. Once ∼1dpa/500appm He were reached, helium bubbles and large dislocation loops were observed. The study reveals that helium bubbles nucleated heterogeneously: a majority of them were observed inside the large dislocation loops. [Copyright &y& Elsevier]

Abstract: The diffusion of tungsten in α-iron is important for the application of ferritic-iron alloys to thermal power plants. These data, over a wide temperature range across the Curie temperature, have been recently reported. We show that these diffusion coefficients can be satisfactory reproduced in terms of the bulk elastic and expansivity data by means of a thermodynamical model that interconnects point defects parameters with bulk qualities. [Copyright &y& Elsevier]

Abstract: In the context of the density functional theory (DFT), we use the ab-initio electronic structure code, SIESTA, coupled to an economic technique to find activated states (or migration barriers), in order to study atomic defects diffusion through the crystal lattice: The monomer method [V.P. Ramunni, M.A. Alurralde, R.C. Pasianot, Phys. Rev. B 74 (2006) 054113]. We test our technique calculating the activated transition states of interstitial hydrogen (H) between two sites of high symmetry, tetrahedral and octahedral, in the α and β phases of Fe. The results are discussed in the context of experimental data and other available models. [Copyright &y& Elsevier]

Abstract: Metals possessing a bcc lattice have been considered as materials with high resistance to irradiation swelling. The aggregation of vacancies is the main reason of the swelling and the interaction of carbon atoms with vacancies affects the mechanical properties of metals. The complex of vacancy-carbon atom in α-Fe is modeled by MD method using a pair inter-atomic potential. The defect formation energy of vacancy-carbon atom complex was obtained by MD method. The stable site of carbon near a vacancy in three dimensions was discussed. [Copyright &y& Elsevier]

Abstract: Molecular dynamics simulations were performed to estimate the dissociation energies of helium interstitials, vacancies and self-interstitial atoms from small helium–vacancy clusters. Several sets of empirical potentials have been tested and compared with available ab initio calculations in order to provide the best combination of potentials to study the stability of small helium bubbles. The behavior of the cluster seems to be better described using Ackland potential for the Fe–Fe interactions and Juslin potential for the Fe–He interactions. From the calculations, it appears that the dissociation energies mainly depend on the helium-to-vacancy ratio rather than the cluster size. The helium/vacancy crossover slightly varies with increasing number of vacancies, but the crossover defining the loop-punching regime decreases strongly with increasing cluster sizes. [Copyright &y& Elsevier]

Two symmetric tilt grain boundaries (GBs) (Σ3〈110〉{112} and Σ11〈110〉{332}) in alpha iron were performed to investigate the dynamics of defect-loaded GBs under shear deformation. The results show that the loaded self-interstitial atoms (SIAs) reduce the critical stress of the coupled GB motion in the Σ3 GB, but increase the critical stress in the Σ11 GB. The loaded SIAs in the Σ3 GB easily form 〈111〉 clusters and remain in the bulk when the GB moves away. However, the SIAs move along with the Σ11 GB and combine with the vacancies in the bulk, leading to the defect self-healing. The helium (He) atoms loaded into the GBs significantly affect the coupled GB motion. Once He clusters emit interstitials, the Σ11 GB carries those interstitials away but the Σ3 does not. The loaded He atoms reduce the critical stress of the Σ3 GB, but increase the critical stress of the Σ11 GB. [ABSTRACT FROM AUTHOR]