Bcc Fe Crystal Research Articles

Development of Fe-based diluted nanocrystalline alloy by substituting C for P in FeSiBCCu system

Here, an Fe85.7Si1B10C1P2Cu0.3 diluted nanocrystalline alloy exhibiting relatively high Bs of 1.69 T and low Hc of 2.58 A/m is successfully developed. In addition, the effects of substituting C with P on the thermal behavior, magnetic properties, microstructure, and magnetic domains of Fe85.7Si1B10C3−xPxCu0.3 (x = 0, 1, 2, and 3 at%) amorphous alloys are investigated. It is found that the addition of P not only improves glass forming ability, but also enhances thermal stability of the amorphous alloy. Furthermore, the suitable P addition can also optimize the soft magnetic properties under heat treatment. For the Fe85.7Si1B10C1P2Cu0.3 alloy, the improved Bs is due to the formation of bcc-Fe crystals and the lowest Hc is caused by the low magneto-crystalline anisotropy, which are both induced by the construction diluted nanocrystalline structure. The formation of this unique microstructure is due to the competition between stabilization effect on amorphous phase of higher crystallization activation energy and the facilitation effect on the formation of bcc-Fe crystal by atomic-scale preexisting crystal-like clusters.

Atomistic simulation of shear deformation at bcc-Fe grain boundary and precipitation strengthening by Cr23C6

We investigate the temperature and the tilt angle dependence of the shear deformation behavior at a body-centered cubic (bcc)-Fe grain boundary (GB) by utilizing classical molecular dynamics simulations. We found that the shear deformation at a [001] axial bcc-Fe GB accommodates GB migration. The critical shear stress needed to induce GB migration was found to decrease with an increase in the temperature or molecular dynamics simulation time, because GB migration is a thermally activated process. The tilt angle dependence of the critical shear stress is successfully explained by the presence of a specific structure unit composed of under-coordinated atoms. It was suggested that Cr 23 C 6 precipitation within the bcc-Fe crystal decreases the critical stress required to generate dislocations, because of the interface between the precipitation and parent phases. On the other hand, Cr 23 C 6 precipitation on a [001] axial bcc-Fe GB was found to increase the critical shear stress, while the precipitation didn’t change the shear deformation mechanism. The degree of strengthening was verified by theoretical equation to evaluate the shear strengthening by the ordered precipitates.

Molecular Dynamics Study on the Impact of Cu Clusters at the BCC-Fe Grain Boundary on the Tensile Properties of Crystal

The molecular dynamics (MD) method was used to simulate and calculate the segregation energy and cohesive energy of Cu atoms at the Σ3{111}(110) and Σ3{112}(110) grain boundaries, and the tensile properties of the BCC-Fe crystal, with the grain boundaries containing coherent Cu clusters of different sizes (a diameter of 10 Å, 15 Å and 20 Å). The results showed that Cu atoms will spontaneously segregate towards the grain boundaries and tend to exist in the form of large-sized, low-density Cu clusters at the grain boundaries. When Cu cluster exists at the Σ3{111}(110) grain boundary, the increase in the size of the Cu cluster leads to an increase in the probability of vacancy formation inside the Cu cluster during the tensile process, weakening the breaking strength of the crystal. When the Cu cluster exists at the Σ3{112}(110) grain boundary, the Cu cluster with a diameter of 10 Å will reduce the strain hardening strength of the crystal, but the plastic deformation ability of the crystal will not be affected, and the existence of Cu clusters with a diameter of 15 Å and 20 Å will suppress the structural phase transformation of the crystal, and significantly decrease the plastic deformation ability of the crystal, thereby resulting in embrittlement of the crystal.

An investigation of hydrogen embrittlement of 12Cr2Mo1R(H) steel by slow strain rate tests and first-principles calculation

Purpose With excellent mechanic properties and hydrogen embrittlement (HE) resistance, 12Cr2Mo1R(H) steel is suitable to make hot-wall hydrogenation reactors. However, longtime exposure to a harsh environment of high-pressure hydrogen at medium temperature in practical application would still induce severe hydrogen uptake and eventually damage the mechanical properties of the steel. The study aims to evaluate the HE resistance of the steel under different tensile strain rates after hydrogen charging and analyze the hydrogen effect from atomic level. Design/methodology/approach This research studied the HE properties of 12Cr2Mo1R(H) steel by slow strain rate tests. Meanwhile, the effect of hydrogen on the structures and the mechanical properties of the simplified models of the steel was also investigated by first-principle calculations. Findings Experimental results showed that after hydrogen pre-charging in this work, hydrogen had little effect on the microstructure of the steel. The elongations and reduction of cross-sectional area of the samples reduced a lot, by contrast, the yield and tensile strengths changed slightly. The 12Cr2Mo1R(H) steel was not very susceptible to HE with a maximum embrittlement index of about 20.00%. First principles calculation results showed that after H dissolution, lattice distortion occurred and interstitial H atoms would preferentially occupy the tetrahedral interstitial site in bcc-Fe crystal and increase the stability of the supercells. With the increase of H atoms added into the simplified model, the steel still possessed a good ductility and toughness at a low hydrogen concentration, while the material would become brittle as the concentration of hydrogen continued to increase. Originality/value These finds can provide valuable information for subsequent HE studies on this steel.

Formation and migration of helium pair in bcc Fe from first principle calculations

We investigated the energetics and configurations of two interstitial He atoms in bcc Fe crystal using first-principle method. Two interstitial He atoms will bind together as helium pair if the initial He-He distance is less than 2.82 Å. From a formation energy decomposing analysis we found that, pairing of two He atoms would reduce the interface between He and Fe matrix which mitigates the perturbation of He-1s and Fe-3d orbitals. As a dominant part of the fomation energy, the electron variation energy caused by two close He atoms is less than that of two far He atoms, which accounts for the reason of binding between two close He atoms. Besides, the migration of helium pair along direction [1 0 0] was investigated by a He-He formation energy hypersurface. Through the comparison with Nudged Elastic Band calculation, it’s found that this special energy hypersurface was effective in revealing the migration of helium pair in metals.

Diffusion coefficient of hydrogen interstitial atom in α-Fe, γ-Fe and ε-Fe crystals by first-principle calculations

The hydrogen diffusion process is a crucial issue related to hydrogen embrittlement. In this work, first-principle calculations were performed to investigate the diffusion behavior of hydrogen atoms in different Fe crystalline lattices. The hydrogen atom diffused in bcc Fe crystal through migrating from tetrahedral interstice to its nearest tetrahedral site; hydrogen atom diffused in fcc Fe along octahedral site-tetrahedral site-octahedral site; the diffusion path of the hydrogen atom in hcp Fe was from an octahedral site to its nearest octahedral site. The order of the diffusion coefficients of the hydrogen in the three crystal structures was Dbcc = 1.379 × 10−4 cm2/s exp (−1120/T) > Dfcc = 3.22 × 10−4 cm2/s exp (−8425/T) > Dhcp = 6.161 × 10−4 cm2/s exp (−8830/T). In addition, the diffusion behavior of a hydrogen atom near a vacancy was also investigated to determine the ability to trap a hydrogen atom in a vacancy defect in bcc-Fe and fcc-Fe, and the results indicated that hydrogen had higher mobility in the presence of a vacancy in the bcc Fe crystal.

Molecular dynamics study on diffusion behavior of Li in α-Fe

Liquid lithium has been considered as a candidate material for several components of future fusion devices. Since the containment materials are usually ferrous alloys, molecular dynamics simulations were performed to study the diffusion behavior of lithium atoms in bcc iron lattices. The stable interstitial sites for lithium atoms and the diffusion mechanism, as well as the diffusion barrier, were computed by using nudged elastic band method. The influence of vacancies was analyzed. The results suggest a strong binding effect. Through abundant simulations, the interaction between lithium atoms inside iron lattices was investigated, which interprets the forming process of lithium clusters. The temperature effect on the diffusion behavior of lithium atoms was discussed for all the cases including in the system with pure iron atoms, vacancies and interstitial lithium atoms. In addition, the diffusion coefficient of Li atoms in bcc Fe crystal was evaluated and analyzed.

Structure and magnetic properties of Fe epitaxial thin films prepared by UHV rf magnetron sputtering on GaAs single-crystal substrates

Fe thin films were prepared on GaAs single-crystal substrates of (100) B3 , (110) B3 , and (111) B3 orientations by ultra high vacuum rf magnetron sputtering. The effects of substrate orientation and substrate temperature on the film growth, the structure, and the magnetic properties were investigated. On GaAs(100) B3 substrates, Fe(100) bcc single-crystal films are obtained at 300 °C, whereas Fe films consisting of bcc(100) and bcc(221) crystals epitaxially grow at room temperature (RT). Fe(110) bcc and Fe(111) bcc single-crystal films are respectively obtained on GaAs(110) B3 and GaAs(111) B3 substrates at RT–300 °C. The in-plane lattice spacings of these Fe epitaxial films are 0–9% larger than the out-of-plane lattice spacings due to accommodation of lattice mismatch between the films and the substrates. The film strain is decreased by employing an elevated substrate temperature of 300 °C. The in-plane magnetization properties are reflecting the magnetocrystalline anisotropy of bulk bcc–Fe crystal.

Effect of nucleated Cu phase on magnetic properties and electronic structures in bcc Fe: Ab initio study

Using the first-principles calculations, the change in magnetism and electronic structures during Cu nucleation in bcc Fe system was investigated. For modeling the Fe-rich bcc FeCu alloy, FexCu1−x (x≥0.75) were employed in the supercell system. Nonmagnetic Cu atoms were precipitated on the (100) plane of bcc Fe crystal, whereas the magnetized Cu atoms preferred to be precipitated on the (110) plane. Magnetization energies showed a linear decrease as the Cu concentration increased in bcc Fe. The magnetic moments of Fe atoms increased for larger concentration of Cu atoms. Electron density of states showed that the enhanced magnetic moment of Fe atoms resulted from the shift of minority spin states toward higher levels, which was associated with the bond formation between Cu and Fe atoms.

Structural, electronic, and magnetic properties of bcc iron surfaces

Trends in atomic multilayer relaxations, surface energy, electronic work function, and magnetic structure of several low-Miller-index surfaces of iron are investigated employing density functional theory total energy calculations. The calculated topmost layer relaxations reproduce well the experimental contractions and their variation with the surface crystallographic orientation, and surface roughness. The multilayer relaxation sequences correlate with the reduced coordination in surface layers and can be explained in terms of a simple electrostatic picture. The surface energies scale almost linearly with the surface roughness. They agree well with the experimental surface tensions and show a small anisotropy in agreement with predictions based on measurements for other metals. The equilibrium shape of a bcc Fe crystal is determined and discussed. The work function anisotropy is calculated and rationalized in terms of changes in the valence charge distribution. Significantly increased local magnetic moments of atoms in the surface region are determined. The correlation between the anisotropy of the surface magnetic moments and atomic coordination in the outermost layers is demonstrated to follow a simple rule.

Interaction Analysis between Edge Dislocation and Self Interstitial Type Dislocation Loop in BCC Iron Using Molecular Dynamics

Self-interstitial atom (SIA) type dislocation loop is one of the possible candidates of the so-called matrix damage that causes hardening and embrittlement of blanket structural materials of fusion reactors and/or pressure vessel materials of light water reactors. We present in this paper molecular dynamics computer simulation results on the interactions between an edge dislocation and a SIA loop with Burgers vectors of b = a 0 /2 [111] and b = a 0 /2 [111], respectively, which are introduced in bcc-Fe crystal. Then shear stresses of several different magnitudes are applied so that the dislocation moves to meet the SIA loop. General observation is that the SIA loops with diameter of ∼2 nm can be obstacles to dislocation motion, and the strength as obstacles to dislocation motion depends on applied stress. The origin of the stress dependent strength can be explained athermally using the elastic theory of dislocation interaction. In most cases, the SIA loops are absorbed by the edge dislocations to form a large super-jog after the interactions. This suggests a possibility of localized deformation of irradiated bcc-Fe due to the formation of dislocation channeling.

Simulation of crack healing in BCC Fe

A molecular dynamics model is developed to investigate the microcrack healing process in BCC Fe. Results show that the temperature has a significant effect on the process of microcrack healing. During the time of 6 ps in this study, crack healing proceeds rapidly at the temperature of 1173 K. When the temperature is reduced, the reduction rate of the crack size decreases dramatically. The critical temperature of microcrack healing in BCC Fe crystal without pre-existing dislocations is about 673 K. Some defects such as dislocations and voids appear during healing process, and their positions change continuously. The distribution of defects in the cell is not homogenous.

Phonon spectrum and related thermodynamic properties of bcc-Fe with an edge dislocation

The phonon spectrum and related thermodynamic properties of perfect bcc-Fe crystal and bcc-Fe with a [1 0 0] edge dislocation are calculated with the recursion method by using F-S N-body potential. For perfect bcc-Fe crystal, our calculated results of phonon spectrum, vibrational entropy, heat capacity and vibrational energy are in excellent agreement with the experiment and related calculation results. The configuration of [1 0 0] edge dislocation obtained by molecular dynamics relaxation shows that a string of interstices is formed along the dislocation line, thus the local vibrational densities of states (LDOS) of atoms in the dislocation core are quite different from the bulk phonon spectrum, and there exist some sharp peaks in certain frequencies. However, for the atoms away from the dislocation core, their LDOS's gradually approach the bulk spectrum. Meanwhile, the local vibrational energies of the dislocation core atoms are also evidently changed except for the atoms on the slip plane. These results indicate that the bondings between the atoms in dislocation core are greatly changed. Furthermore, it is also proved that the average vibrational entropy of bcc-Fe with a [1 0 0] edge dislocation is higher than that of perfect crystal.