Atomic structure of the Fe/Fe3C interface with the Isaichev orientation in pearlite

Abstract

The pronounced mechanical property of pearlitic steels highly correlates with the ferrite (bcc-Fe)/cementite (Fe₃C) boundaries inside. Unraveling the interface structure at an atomic level is essential for interpreting the material’s property. In the present study, using aberration-corrected scanning/transmission electron microscopy combined with density functional theory calculations, we reveal the atomic configuration as well as the electronic structure of the Fe/Fe₃C interfaces with the Isaichev orientation in pearlite. The interface with terminating layer Fe–C–Fe in cementite has the lowest energy due to the formation of interfacial Fe–C bonds. Terrace steps which are frequently observed at the interfaces would not break the lattice match between the two phases.     

The trapping of K and Na atoms by a clean W(110) surface trajectory calculations

The fraction of K and Na atoms initially trapped by the W(110) surface has been measured as a function of the incident energy (0.5–15 eV) and as a function of the incident angle. The trapping probability equals one at low incident energies (E_i ? 0.5 eV) and decreases with increasing energy. The measurements show an increase of trapping with increasing angle of incidence θ_i (measured from the surface normal). Simultaneously the desorption energies Q_i were determined from the temperature dependence of the measured mean residence time on the W(110) surface. We obtained for K: Q_i = 2.05 ± 0.02 eV, and for Na: Q_i = 2.60 ± 0.04 eV.The trapping phenomenon at a solid surface was approximated in a theoretical way by calculating the in-plane trajectory of a projectile scattered from a diatomic surface-molecule. The important feature which showed up was the conversion of tangential to normal momentum of the projectile, and thus the inapplicability of cube models. As a function of the angle of incidence two regimes can be distinguished: at the smaller angles the scattering is governed by simultaneous interaction of the projectile with two neighbouring surface atoms, and at the higher angles of incidence the single particle interaction contributes most to the momentum transfer.     

Fabrication and device characterization of potassium fluoride solution treated CZTSSe solar cell

Post deposition treatment (PDT) for Cu₂ZnSn(S,Se)₄ (CZTSSe) was carried out by simply dipping the absorber into the KF solution at 80 °C. The dipping time of absorber in KF solution was found to be crucial to device parameters of CZTSSe solar cell. The K-doping improved the solar cell efficiency from 4.4% to 7.6% by 1 min dipping whereas the longer than 5 min dipping solar cells showed distorted kink J-V curves. The activation energy of CZTSSe solar cell was increased upto 1 min KF treatment from 0.83 eV to 0.92 eV which indicates interface recombination is reduced significantly. However, the activation energies of 5 min and 10 min dipping solar cells were found to be 0.81 eV and 0.63 eV where dominant recombination was interface recombination. Furthermore, trap energies of 49 meV and 298 meV of pristine CZTSSe solar cell were modified to 33 meV and 117 meV for 1 min treated CZTSSe solar cell. Trap energies of 5 min were calculated to be 112 meV and 147 meV. The proper KF doping passivated the shallow as well as deep defects of CZTSSe solar cell which is reflected in photovoltaic performances directly.     

X-ray diffraction and Mössbauer spectroscopy studies of cementite dissolution in cold-drawn pearlitic steel

Cementite dissolution in cold-drawn pearlitic steel (0.8 wt.% carbon) wires has been studied by quantitative X-ray diffraction (XRD) and Mössbauer spectroscopy up to drawing strain 1.4. Quantification of cementite-phase fraction by Rietveld analysis has confirmed more than 50% dissolution of cementite phase at drawing strain 1.4. It is found that the lattice parameter of the ferrite phase determined by Rietveld refinement procedure remains nearly unchanged even after cementite dissolution. This confirms that the carbon atoms released after cementite dissolution do not dissolve in the ferrite lattice as Fe-C interstitial solid solution. Detailed analysis of broadening of XRD line profiles for the ferrite phase shows high density of dislocations (～10¹⁵/m²) in the ferrite matrix at drawing strain 1.4. The results suggest a dominant role of ?1?1?1? screw dislocations in the cementite dissolution process. Post-deformation heat treatment leads to partial annihilation of dislocations and restoration of cementite phase. Based on these experimental observations, further supplemented by TEM studies, we have suggested an alternative thermodynamic mechanism of the dissolution process.     

Kaonic atoms measurements at the DAΦNE accelerator

Radioactive ⁵⁷Mn⁺(T _1/2?= 1.5 min) ions have been implanted at the ISOLDE facility at CERN with 60 keV energy to fluences <10¹²/cm² into p-type Si_1???x Ge _x (x < 0.1) single crystals held at 300–600 K. The implantation and annealing processes result in the majority of the implanted Mn ions occupying substitutional lattice sites. In the subsequent ⁵⁷Mn nuclear β ^???-decay to the 14.4 keV Mössbauer state of ⁵⁷Fe (T _1/2?= 100 ns), an average recoil energy of 40 eV is imparted to the ⁵⁷Fe daughter atoms which results in a large fraction being expelled into tetrahedral interstitial sites and the creation of a vacancy. The remainder occupies substitutional sites. This technique of recoil production of ^57m Fe_I thus allows for the study of the diffusion characteristics of interstitial Fe. From the temperature dependent line broadening, the activation energies have been determined and decrease with increasing Ge concentration which contributes significantly to the increase of the jump frequency. A similar result has been obtained in n-type SiGe but there the values for the activation energies were much higher.     

Ab initio modelling of the interaction of H interstitials with grain boundaries in bcc Fe

Hydrogen that is accumulated within the grain boundaries can lead to a decrease of the critical strain required to fracture the material. The paper presents results of ab initio modelling of hydrogen–grain boundary interaction in ferromagnetic bcc iron. Modelling was performed using density functional theory with generalised gradient approximation (GGA’96), as implemented in WIEN2k package. Three fully relaxed tilt grain boundaries, Σ5(310), Σ5(210) and Σ3(111), were studied. The supercells contained 40–48 atoms, i.e. 20–24 atoms in each of the two ‘grains’. Calculated formation energies of grain boundaries is 1.44, 1.83 and 1.46 J/m² and the maximum binding (trapping) energies of hydrogen to the boundaries are 0.43, 0.83 and 0.39 eV, respectively. These values are close to other researchers’ data. The higher value of trapping energy of the Σ5(210) boundary is probably due to the asymmetrical atom configurations resulting from mutual rigid shift of the two grains that was necessary to be introduced to provide optimal distances between Fe atoms, unlike the other two boundary types.     

Substitutional-interstitial interactions in niobium-titanium alloys: An internal friction study

The influence of additions of interstitial oxygen and nitrogen on the internal friction spetrum of the niobium-1 at-% titanium alloy was studied. The nature of the various observed relaxation processes introduced by the presence of substitutional titanium is discussed. A thermodynamic analysis was carried out for two pronounced interaction peaks attributed to the stress-induced reorientation of single oxygen or nitrogen atoms around single titanium atoms and the respective binding energies were estimated. It was also found that, over the range of oxygen concentration studied, interstitial oxygen is completely removed from random migration by substitutional titanium atoms acting as trapping centres, whilst nitrogen population is always partitioned between mobile and trapped atoms even at very low relative nitrogen concentrationsC _N/C _Ti. A possible reason for the different behavior of oxygen and nitrogen is suggested.     

Migration and stabilization of hydrogen atoms in KCl

Stabilities of hydrogen atoms at the interstitial (H⁰_i), cation and anion sites (H⁰_sc and H⁰_sa) in KCl have been studied with ESR and thermoluminescence. The close pair between H⁰_i and S^? formed by UV-irradiation of KCl:SH^? recombines first with the activation energy of 0.17±0.02 eV following the emission of light and then H⁰_i becomes mobile with the energy of 0.20±0.02 eV. The energies determined from the decay of ESR signal intensity of H⁰_sc and H⁰_sa are 0.24±0.02 eV and 0.43±0.03 eV, respectively.     

On the nucleation of cementite on bainitic ferrite–austenite interphase boundaries

Among all possible variants of the Isaichev orientation relationship between cementite and ferrite, a single major cementite variant has been observed to appear in bainite. Interphase boundary nucleation of cementite on ferrite–austenite semi-coherent interfaces is considered a plausible reason for this observation. With the aid of known crystallographic relations and habit planes of the ferrite–cementite, ferrite–austenite and austenite–cementite phases, a model for cementite nucleation has been proposed. The interphase-boundary nucleus is assumed to form on a semi-coherent ferrite–austenite interface and to possess ferrite–cementite and austenite–cementite habits as two main facets of the nucleus. It is shown that interphase cementite nucleation will be viable if the energies of all facets of the nucleus are in the semi-coherent range.     

FePc与TiO2(110)及C60界面电子结构研究

基于C₆₀受体和有机分子给体的太阳能电池是目前非常重要的一个研究热点, 利用同步辐射真空紫外光电子能谱(SRUPS) 技术研究了酞菁铁(FePc)与TiO₂(110)及C₆₀的界面电子结构, 以及FePc与C₆₀分子混合薄膜的电子结构. SRUPS价带谱显示, FePc沉积在化学计量比与还原态两种不同的TiO₂(110)表面时, FePc分子的HOMO能级均随FePc厚度的变化发生了移动, 而在化学计量比的TiO₂(110)表面位移较大, 同时发生界面能带弯曲, 说明存在从有机层向衬底的电子转移. 在FePc/C₆₀和C₆₀/FePc界面形成过程中, FePc与C₆₀分子的最高占据分子轨道(HOMO)位移大小基本相同. 由界面能级排列发现, 在FePc与C₆₀的混合薄膜中, FePc分子的HOMO与C₆₀分子的最高占据分子轨道能级差较大, 这有利于提高器件开路电压, 改善器件性能.     

Theoretical characterisation of irreversible and reversible hydrogen storage reactions on Ni-doped C60 fullerene

An attempt has been made to characterise the irreversible and reversible hydrogen storage reactions on Ni-doped C₆₀ fullerene by using the state of the art density functional theory calculations. The single Ni atom prefers to bind at the bridge site between two hexagonal rings of C₆₀ fullerene, and can bind up to four hydrogen molecules with average adsorption energies of ?0.85, ?0.83, ?0.58, and ?0.31 eV per hydrogen molecule. No evidence for metal clustering in the ideal circumstances and the hydrogen storage capacity is expected to be as large as 8.9 wt%. While the desorption activation barriers of the complexes nH₂NiC₆₀ (n = 1, 2) are outside the desirable energy window recommended by the department of energy for practical applications (–0.2 to –0.6 eV), the desorption activation barriers of the complexes nH₂NiC₆₀ (n = 3, 4) are inside this window. The irreversible 2H₂ + NiC₆₀ and reversible 3H₂ + NiC₆₀ interactions are characterised in terms of several theoretical parameters such as: (1) densities of states and projected densities of states, (2) pairwise and non-pairwise additivity, (3) infrared, Raman, and proton magnetic resonance spectra, (4) electrophilicity, and (5) statistical thermodynamic stability.     

Effects of the Be_(22)W phase formation on hydrogen retention and blistering in mixed Be/W systems

We have performed first-principles density functional theory calculations to investigate the retention and migration of hydrogen in Be_(22) W, a stable low-W intermetallic compound. The solution energy of interstitial H in Be_(22) W is found to be 0.49 eV lower, while the diffusion barrier, on the other hand, is higher by 0.13 eV compared to those in pure hcp-Be. The higher solubility and lower diffusivity for H atoms make Be_(22) W a potential beneficial secondary phase in hcp-Be to impede the accumulation of H atoms, and hence better resist H blistering. We also find that in Be_(22) W, the attraction between an interstitial H and a beryllium vacancy ranges from 0.34 eV to 1.08 eV, which indicates a weaker trapping for hydrogen than in pure Be. Our calculated results suggest that small size Be_(22) W particles in hcp-Be might serve as the hydrogen trapping centers, hinder hydrogen bubble growth, and improve the resistance to irradiation void swelling, just as dispersed oxide particles in steel do.     

First principle calculation of accurate native defect levels in CaF<Subscript>2</Subscript>

We report on the first principle density functional calculation of the charge transition levels of native defects (vacancies and interstitials) in CaF₂ structure. The transition level was defined as the Fermi level where two charge states of given defect have the same formation energy. The common error in the band gap inherited to semiclocal density functional has been accounted for by incorporating the hybrid density functional method, leading to correct placement of the transition levels within the band gap. The band gap size from hybrid calculation has been validated using the full potential, Linearized Augmented Planewave method with the Modified-Becke-Johnson exchange potential. Prior to level calculations, we ensured that an agreement between the formation energies from small (95–97 atoms) and large (323–325 atoms) supercells was achieved after applying the Makov-Payne correction method. Our calculated transition level for the anion vacancy was 2.97 eV below the conduction band, agreeing with the experimental optical absorption band at 3.3 eV associated with the electron transition from the ground state F-center to the conduction band in CaF₂.     

Athermal migration of vacancies in iron and copper induced by electron irradiation

Abstract

Irradiation with high-energy particles induces athermal migration of point defects, which affects defect reactions at low temperatures where thermal migration is negligible. We conducted molecular dynamics simulations of vacancy migration in iron and copper driven by recoil energies under electron irradiation in a high-voltage electron microscope. Minimum kinetic energy required for migration was about 0.8 and 1.0 eV in iron and copper at 20 K, which was slightly higher than the activation energy for vacancy migration. Around the minimum energy, the migration succeeded only when a first nearest neighbour (1NN) atom received the kinetic energy towards the vacancy. The migration was induced by higher kinetic energies even with larger deflection angles. Above several electron-volts and a few 10s of electron-volts, vacancies migrated directly to 2NN and 3NN sites, respectively. Vacancy migration had complicated directional dependence at higher kinetic energies through multiple collisions and replacement of atoms. The probability of vacancy migration increased with the kinetic energy and remained around 0.3–0.5 jumps per recoil event for 20–100 eV. At higher temperatures, thermal energies slightly increased the probability for kinetic energies less than 1.5 eV. The cross section of vacancy migration was 3040 and 2940 barns for 1NN atoms in iron and copper under irradiation with 1.25 MV electrons at 20 K: the previous result was overestimated by about five times.     

Stability of valence alternation pairs across the substoichiometric region at Ge/GeO2 interfaces

Through hybrid density functional calculations, we compare the Ge–Ge bond energy with the formation energy of a valence alternation pair as the O concentration varies across the Ge/GeO₂ interface. First, hole trapping energies are calculated for three atomistic models with different O concentrations: bulk Ge with isolated O atoms, amorphous GeO, and amorphous GeO₂ with an O vacancy. The reaction is then broken down in three steps involving the breaking of a Ge–Ge bond, charge transfer processes involving dangling bonds, and the formation of a threefold coordinated O atom. The energy of each elemental reaction is estimated through suitable model calculations. The charge transition levels resulting from this analysis agree with those obtained for the atomistic models. Our estimates indicate that hole trapping at low O concentrations occurs at no energy cost for p-type germanium owing to the formation of threefold-coordinated O atoms. Applied to n-type Ge, our analysis indicates that electron trapping in dangling bonds obtained from the breaking of Ge–Ge bonds is unfavorable. The formation energy of a valence alternation pair is evaluated and discussed in relation to previous results.     

Study of the hydrogen diffusion and segregation into Fe-C-Mo martensitic HSLA steel using electrochemical permeation test

Diffusion and trapping mechanisms are studied in order to improve Hydrogen embrittlement (HE) resistance of high yield strength steels. Investigations were carried on Fe-C-Mo model steel with a quenched and tempered martensitic microstructure. Hydrogen diffusion was studied by using the electrochemical permeation technique. The influence of the charging current densities in 1 M H₂SO₄ at ambient temperature shows a relation between the apparent diffusion coefficient D_app and the apparent subsurface concentration of hydrogen C_0app. Two domains can be separated and are mainly associated with a competition between two distinct processes: hydrogen trapping and hydrogen diffusion. These results are correlated to the quantities of reversible and irreversible traps into the membrane. Moreover, the experimental results revealed that the apparent diffusion coefficient increases and the total amount of trapped hydrogen decreases with temperature. The activation energy of the diffusion process (0.26 eV) and the trapping process (0.58 eV) are supposed to be, respectively, affiliated with lattice diffusion and with trapping on incidental dislocations and/or on martensitic lath interfaces due to misorientations (geometric necessary dislocations).     

Hydrogen induced interfacial polarization at PdSiO2 interfaces

Internal photoemission experiments were performed to study the polarization layer induced by hydrogen at PdSiO₂ interfaces. It was found that this polarization layer behaves like a true dipole layer where one monolayer of hydrogen atoms at the interface corresponds to a barrier lowering of 0.5 eV in accordance with earlier measurements on threshold voltage shifts of Pd-SiO₂ -Si field effect transistors. Furthermore experiments on AlSiO₂ structures indicate that the catalytic dissociation of H₂ on metals like Pd and Pt is necessary for the introduction of a hydrogen polarization layer at a metal-SiO₂ interface.     

Hydrogen-vacancy interaction in W and Mo

The hydrogen-vacancy interaction in W and Mo was investigated by applying the perturbed angular correlation technique, using the isotope¹¹¹In as a probe. Hydrogen trapping at InV₂ manifests itself as a change of the vacancy-induced quadrupole frequency. We have observed trapping of up to two H atoms. The binding energies for the first H atom are 1.07(2) eV in Mo and 1.16(2) eV in W. These results are close to the values calculated with the effective-medium theory. The frequency shifts are both negative and amount to a few Mrad/s per trapped H atom. Contrary to the theoretical predictions, the binding energies for the second H atom differ strongly: 0.44(3) eV in Mo and 0.99(2) eV in W. The corresponding frequency shifts have opposite signs. Therefore, the lattice positions of the first H atom in Mo and W are similar, while those of the second H atom must be completely different.     

Subthreshold sputtering at high temperatures

The sputtering of tungsten from a target at a temperature of 1470 K during irradiation by 5-eV deuterium ions in a steady-state dense plasma is discovered. The literature values of the threshold for the sputtering of tungsten by deuterium ions are 160–200 eV. The tungsten sputtering coefficient measured by the loss of weight is found to be 1.5×10^?4 atom/ion at a deuterium ion energy of 5 eV. Previously, such a sputtering coefficient was usually observed at energies of 250 eV. The sputtering is accompanied by a change in the target surface relief, i.e., by the etching of the grain boundaries and the formation of a wavy structure on the tungsten surface. The subthreshold sputtering at a high temperature is explained by the possible sputtering of adsorbed tungsten atoms that are released from the traps around the interstitial atoms and come to the target surface from the space between the grains. The wavy structure on the surface results from the merging of adsorbed atoms into ordered clusters.