Binary Fe-Cr Alloys Research Articles

Fe-Cr二元合金での転位挙動に関する分子動力学解析

Fe-Cr二元合金での転位挙動に関する分子動力学解析

Molecular Dynamics Modelling of the Plastic Deformation Behavior of Fe-Cr Binary Alloys

Molecular Dynamics Modelling of the Plastic Deformation Behavior of Fe-Cr Binary Alloys

Theoretical description of thermodynamic and mechanical properties of multicomponent bcc Fe-Cr-based alloys

Fe-Cr-based steels are broadly considered as important structural materials, e.g., for generation-IV nuclear reactors. In order to address tougher operation conditions in future applications, properties of existing Fe-Cr-based steels must be significantly improved. Multicomponent alloying has been viewed as an efficient tool to achieve this goal. Despite its accepted importance, fundamental knowledge of the effect of the simultaneous alloying of Fe with Cr and other elements, such as Ni, Mo, Al, W, V, and Nb, on the thermodynamic and mechanical properties of the bcc alloys is still quite limited. Here we report results of first-principles simulations of lattice parameters, mixing enthalpies, and magnetic and elastic properties of bcc Fe-Cr-based solid solutions with up to 20% Ni, as well as with small concentrations of other alloying elements, Mo, Al, W, V, and Nb. We predict that alloys with relatively high Ni concentrations containing 2.5%--5% Al demonstrate a simultaneous increase of thermodynamic stability and ductility without significant drop of their elastic moduli with respect to corresponding binary Fe-Cr alloys.

Atomistic modeling of hardening in spinodally-decomposed Fe–Cr binary alloys

Spinodal decomposition in thermally aged Fe–Cr alloys leads to significant hardening, which is the direct cause of the so-called 475∘C-embrittlement. To illustrate how spinodal decomposition induces hardening by atomistic interactions, we conducted a series of numerical simulations as well as reference experiments. The numerical results indicated that the hardness scales linearly with the short-range order (SRO) parameter, while the experimental result reproduced this relationship within statistical error. Both seemingly suggest that neighboring Cr–Cr atomic pairs essentially cause hardening, because SRO is by definition uniquely dependent on the appearance probability of such pairs. A further numerical investigation supported this notion, as it suggests that the dominant cause of hardening is the pinning effect of dislocations passing over such Cr–Cr pairs. Moreover, this finding has a practical merit, that is, the SRO parameter can serve as a good index of hardening of steels, which is critical for evaluating their lifetime.

Small-angle neutron scattering quantification of phase separation and the corresponding embrittlement of a super duplex stainless steel after long-term aging at 300°C

Small-angle neutron scattering (SANS) was applied to quantify the nanostructural evolution during spinodal decomposition in a 25Cr-7Ni (wt.%) super duplex stainless steel isothermally aged at 300 °C, for up-to 48,000 h. Prior to the application on the 25Cr-7Ni alloy, the SANS methodology was validated by comparing results from SANS measurements on binary Fe-Cr alloys with atom probe tomography results. SANS results on the 25Cr-7Ni alloy indicated that decomposition wavelength decreased from 5.1 nm to 4.5 nm, whereas the amplitude increased from 15.0 to 33.4 at.%. This quantitative nanostructural evolution correlated to a hardening of the ferrite phase by 190 HV and a reduction of the sub-size Charpy-V impact toughness from 60 J to 25 J.

First-principles study of bcc Fe-Cr-Si binary and ternary random alloys from special quasi-random structure

Using a combination of first-principles calculations with special quasi-random structure and quasi-harmonic approximation methods, we have calculated formation energy, magnetic moments, mechanical properties and thermodynamic properties of binary FeCr and ternary FeCrSi random alloys. The results agree well with available theoretical and experimental data. The body centered cubic structures and ferromagnetic phase are employed to calculate these properties in this work. The phase stability studies show that FeCr alloys are stable systems in Cr concentrations below 6% region. However, binary FeCr alloys are the systems with a miscibility gap for Cr concentrations above 6%. The ternary FeCrSi systems show much less formation energy than FeCr systems. For ternary FeCrSi alloys, Fe9Cr22Si1 shows maximum value of formation energy. Studies on magnetism have shown that Si is a kind of nonmagnetic material, it can be used to reduce magnetic moments of FeCr alloys. Doping a little Si into FeCr alloys can form more stable systems and improve bulk modulus, shear modulus and young's modulus in both low-Cr and rich-Cr concentration regions. Si element can generate effects on Debye temperature and volumetric thermal expansion coefficient of the alloy. But Si element can not generate obvious effects on constant volume heat capacity of FeCr alloys.

Positron annihilation study on the phase transition of thermally aged Fe-Cr binary alloys at 748K

ABSTRACTThe phase transition of Fe1-xCrx alloys after thermal aging at 748 K has been investigated by means of positron annihilation spectroscopy. It was observed that the implanted positrons preferentially annihilate with Fe electrons. The Cr concentration dependence of the W-parameter before thermal aging is quantitatively described by a preferential positron annihilation model within the first two coordination shells. After thermal aging, the W-parameter increased for all alloys, probably on account of an enhanced preferential positron annihilation of Fe atoms according to the growth of Cr-rich phases by thermal aging. Simultaneously, Fe-rich phases were formed and the interface condition between the Fe- and Cr-rich phases changed, providing another mechanism by which positrons are preferentially annihilated with Fe electrons in the Fe-rich phases.

Evaluation of Cr Concentration Effect on Displacement Cascades in Fe–Cr Alloys with Piecewise Potential

A piecewise potential was constructed with combination of the concentration-dependent EAM (CD-EAM) potential and Ziegler–Biersack–Littmark (ZBL) potential, and the displacement cascades in random Fe–Cr binary alloys with a wide range of Cr concentration from 0 to 20% were simulated to evaluate the effect of Cr concentration on defect formation and evolution during the cascades. The Cr concentration has little or no effect on the fraction of vacancy clusters, the number of surviving Frenkel pairs, or the self-interstitial and vacancy cluster sizes and a slight effect on the peak time and peak defect number, but a considerable effect on the Cr enrichment in the peak and surviving defects in the cascaded Fe–Cr alloys. With the increase in Cr concentration, the Cr enrichment in the peak and surviving defects exhibits a downtrend. The higher enrichment degree in the surviving defects than in the peak defects shows more difficult recombination of Cr SIAs with vacancies.

Damping capacity, magnetic and mechanical properties of Fe-18Cr alloy

Iron-chromium based alloys are known as potentially high damping corrosion-resistance alloys with good mechanical properties and workability. The main structural mechanism of enhanced damping in the Fe-Cr alloys is magneto-mechanical coupling due to reversible and irreversible motion of magnetic domain walls, which is also linked with magnetostriction of the alloys. In order to create new complex alloyed materials with improved functional properties, it is important to study experimentally the basic relation between structure, mechanical and functional properties of binary Fe-Cr alloys. In this paper, we used cold-rolled sheets of a high-purity Fe-18Cr alloy to study the correlation between heat treatment, grain size, damping capacity and magnetostriction. Damping capacity of the samples was measured at bending using forced vibrations by means of dynamical mechanical analyzer Q800 TA Instruments and using free-decay of bending vibrations in different structural states after various annealing treatments. The results show that the optimal properties for Fe-18Cr binary alloy were achieved after annealing of cold-rolled sheets at 840 °C. Homogenizing by annealing at 1200 °C for 3 h before the final heat treatment shifts the annealing temperature for maximal damping capacity to 900 °C with approximately the same value of damping capacity and decreases mechanical properties due to the significant increase in the grain size. Slow cooling of the samples after high-temperature annealing causes a marked decrease of the impact toughness, reduction of damping capacity and an increase in the coercive force of the Fe-18Cr alloy. This effect can be explained by spinodal decomposition of the α-solid solution and formation of local zones enriched with Cr or Fe.

Composition dependence of solid-liquid interfacial energy of Fe-Cr binary alloy from molecular dynamics simulations

Composition dependence of the solid-liquid interfacial energy for Fe-Cr binary alloy is closely investigated by the molecular dynamics simulation in conjunction with the capillary fluctuation method. The solid-liquid interfacial energy of Fe-Cr alloy doesn’t increase monotonically with increasing Cr composition but takes the minimum at approximately Fe-19.1at%Cr. Regarding the anisotropy effect, the interfacial energy of solid-liquid interface with (1 0 0) orientation is larger than those of (1 1 0) and (1 1 1) orientations at all compositions. Moreover, the effect of solute partition on the solid-liquid interfacial energy is discussed on the basis of a conceptual calculation using the solid-liquid biphasic system with various hypothetical compositions. The fluctuation of the interface becomes smooth at the solid-liquid interface with the small partition coefficient, which results in a large solid-liquid interfacial energy.

Ab initio based examination of the kinetics and thermodynamics of oxygen in Fe-Cr alloys

The behavior of oxygen in Fe-Cr binary alloys is important to understand for a variety of applications including fuel cell interconnects and nuclear energy systems. In this work, we performed an ab initio investigation of the binding of oxygen in the entire compositional range of Fe-Cr at minima and saddles. This database was subsequently used to parametrize concentration-dependent local cluster expansions which were utilized to examine the kinetics of oxygen transport in the alloy and the thermodynamics of the system. The behavior of oxygen in the alloy system was investigated using our cluster expansion model and a convex hull diagram was recorded. The most favorable composition for the incorporation of oxygen was found to occur at 70% Cr. The dependence of the oxygen solution energy on the Cr occupation of the nearest neighbor shells was carefully characterized. It was determined that the third nearest neighbor Cr occupation was unfavorable and that, generally speaking, the first and second nearest neighbor shell Cr occupation was favored with a few exceptions. Kinetic Monte Carlo models at dilute oxygen levels were performed using two models [a kinetically resolved activation (kRA) model and a cluster expansion model for the saddle points]. The models qualitatively agreed with the observed trends in the literature which reported a decrease in diffusivity at dilute Cr levels, but the models predicted different behaviors beyond the dilute Cr limit. Finally, using grand canonical Monte Carlo, we further examined the thermodynamics of oxygen incorporation into the alloy. Together, our results offer new insight into the behavior of oxygen in Fe-Cr alloys that have ramifications on the early stages of the corrosive behavior of the material.

First-principles localized cluster expansion study of the kinetics of hydrogen diffusion in homogeneous and heterogeneous Fe-Cr alloys

Metal alloys have a wide range of technological applications, from structural materials to catalysts. In many situations, the transport of hydrogen, whether intentionally for hydrogen storage and fuel cell applications or unintentionally in the case of tritium uptake in nuclear materials, is an important concern. Fe-Cr binary alloys, in particular, may be viewed as a simple model system to represent ferritic steels used in nuclear energy systems and, more generally, as a model binary alloy for examining the role of alloying elements on transport. In this work, we used density functional theory, cluster expansion, and kinetic Monte Carlo to study hydrogen kinetics in Fe-Cr alloys. In random homogeneous alloys, we observed that hydrogen diffusivity first decreased with increasing Cr content and then increased to its levels in pure Cr. Furthermore, the effects of heterogeneity, as might be induced by irradiation, were explored and it was concluded that local structural heterogeneities for the same overall Cr concentration may significantly affect the hydrogen diffusivity. The effect was attributed to the relative binding energy of hydrogen in different metals and this understanding was then utilized to predict hydrogen transport behavior in different element-segregated grain/grain boundary combinations and thus identify solute/solvent alloys where hydrogen transport might be either hindered or enhanced. Finally, we comment on the potential impact of radiation-induced segregation on hydrogen behavior in nuclear energy systems.

Detection of phase separation of neutron-irradiated Fe–Cr binary alloys using positron annihilation spectroscopy

Phase separation in Fe–Cr binary alloys irradiated with neutrons at 473 K and 573 K was investigated using positron annihilation spectroscopy. Using positron annihilation coincidence Doppler broadening (CDB) measurements, the phase separation progress was observed in neutron-irradiated samples at 473 K and 573 K. Vacancy clusters were detected in Fe–xCr (x = 0, 9, 15, 30, 45, 50, and 100) during 473 K irradiation using positron annihilation lifetime measurements, but were not detected in Fe–xCr (x = 70, 85, and 91) irradiated at 473 K or in any samples irradiated at 573 K. Additionally, in Fe–xCr (x = 70, 85, and 91) irradiated at 473 K, all positrons were annihilated with core Fe electrons as determined from CDB ratio curves. Thus, vacancy clusters were not detected in the Fe-rich phase. There was a possibility that vacancy clusters are formed in the Cr-rich phase, but they were not detected by the PAS. Therefore, another method is necessary to investigate this further. Vickers hardness tests indicated that neutron-irradiated samples were harder than unirradiated samples. The contribution of phase separation and neutron-irradiation defects to increased hardness was dependent on the irradiation conditions including temperature and dose.

Electron-irradiation-induced Cr segregation in Fe-Cr model alloy pre-implanted with hydrogen ions

Reduced activation ferritic/martensitic (RAFM) steels are widely accepted as the candidate structural materials for fusion reactors. To gain knowledge about Cr behaviour of these steels in such environments, Fe-Cr binary alloys have to be investigated. In this study, Fe-10 at.% Cr ferritic model alloy pre-implanted with hydrogen ions was electron-irradiated in a high voltage electron microscope (HVEM) at the temperature ranging from 300 °C to 600 °C. Streaks were observed in the diffraction patterns in the model alloy after electron irradiation at 450–550 °C and explained reasonably by shape effect. The analysis of transmission electron microscopy (TEM) reveals that the observed streaks are induced by a kind of needle-like precipitates with the directions along 〈100〉. The dependence of the length and density of the precipitates on the temperature were studied. In addition, contrast experiments were conducted with pure Fe and Fe-10 at.% Cr alloy with and without hydrogen ion implantation, respectively. After electron irradiation, no streak was observed in the hydrogen-implanted pure Fe and Fe-10 at.% Cr alloy without the hydrogen implantation. Therefore, it is assumed that the needle-like precipitates could be the complex of Cr and H, which segregate and precipitate along 〈100〉 direction.

Enhanced Corrosion Resistance of Ultrafine-Grained Fe-Cr Alloys with Subcritical Cr Contents for Passivity

This work presents an experimental validation that an ultrafine grained (UFG) structure of binary Fe-Cr alloys fabricated by severe plastic deformation (SPD) can bring about dramatic improvements in the corrosion resistance. More specifically, UFG Fe-Cr alloys with subcritical chromium content for passivation (8% and 10% Cr) was found to exhibit passivity, and was resistant to corrosion in an aqueous 0.6 mol/L NaCl solution whereas coarse-grained Fe-12% Cr, which are known as a stainless steel and passive in most dilute aerated solution, was degraded by corrosion. The findings indicated that the critical threshold of Cr content required for establishing a protective layer by self-passivation in binary Fe-Cr alloy is microstructure dependent, and can be reduced by grain size reduction to a sub-micron scale by SPD.

MODELING RADIATION-INDUCED SEGREGATION IN BINARY ALLOYS

A computer code to calculate the time and space dependencies of the defect and component concentrations of a binary alloy is developed. This code is based on the theoretical model for radiation-induced segregation governed by first and second Fick’s laws with taking into account inverse Kirkendall effect. The system of three coupled partial differential equations for concentrations of one of components and point defects is converted to the system of ordinary differential equations in time by a discretization procedure. Numerical solutions of this system are obtained under appropriate initial and boundary conditions by means of the MATLAB. The modeling of radiation-induced segregation in binary Fe-Cr alloys is carried out for various initial concentrations of components, temperatures, dose rates and doses. The process of achievement of steady state at dose rising is demonstrated. The calculated concentration profiles are compared with experimental profiles published in literature. The sensitivity of model to input parameters is done and the capabilities of proposed model are estimated. This computer code for multicomponent alloys is developed.

Effects of chromium content on the nitrided layer of binary Fe-Cr alloys

Binary alloys were gas-nitrided at a temperature of 520 °C for 70 h and with a N2/NH3 ratio of 15 to 20%. The thickness of the nitrided layers was evaluated by Vickers hardness testing. The morphology and composition of the nitrided layers were investigated by light and scanning electron microscopy (SEM). Phase depth distributions were calculated by comparison of peak intensities method, and the elemental analyses were made by EDS. Increasing chromium content increases the solubility of nitrogen in both the compound and the diffusion layers, and then their hardness. It also increases CrN amount and the surface porosities, it reduces the nitrided depth but it does not affect the thickness of the compound layer. Continuous precipitation of CrN is noted until 5%Cr. For higher chromium content, discontinuous precipitation occurs hampering the diffusion of nitrogen and coarsening the precipitates. The compromise between nitrogen diffusion and CrN growth is carried out for 3%Cr.

The Effect of Short-Range Order on Passivation of Fe-Cr Alloys

While it is well known that 12–13 at.% chromium is required for stainless-like passivation behavior of binary Fe-Cr alloys, there remain outstanding questions regarding the compositional dependence of this behavior. In order to explore these issues, we examined the passivation behavior of short-range ordered (SRO) Fe-Cr alloys and compared this to that of their random solid solution counterpart. Our results reveal that for alloys containing between 12–18 at.% Cr, passivation in the SRO alloys is delayed and we attribute this to ordering effects on percolation behavior. Finally, we discuss some major outstanding questions regarding passivation in the Fe-Cr system and suggest that first-principles based calculations could make important contributions to our understanding of passivation in this system.

Effect of cooling rate after solution treatment on subsequent phase separation during aging of Fe-Cr alloys: A small-angle neutron scattering study

The effect of cooling rate after solution treatment on the initial structure of concentrated binary Fe-Cr alloys and the effect of the initial structure on phase separation during subsequent aging has been investigated. The nano-scale compositional fluctuations in the bulk of the alloys are studied using small-angle neutron scattering and the results are compared with simulations using the Cahn-Hilliard-Cook (CHC) model. The alloys investigated represent different mechanisms of phase separation and at higher Cr content, when spinodal decomposition (SD) is favored, the initial Cr compositional fluctuations due to slow cooling after solution treatment reduce the kinetics of phase decomposition, whereas, at lower Cr composition when nucleation and growth is favored, the kinetics of phase decomposition is more rapid. Regardless of the nominal Cr composition of the alloy, the phase decomposition after extended aging up to 300 h at 748 K is always larger for the more non-random initial structure. The CHC modeling of the cooling process and subsequent initial aging (below 10 h) is in reasonable qualitative agreement with the experimental results for the Fe-40 wt.% Cr alloy decomposing via SD. However, the modeling approach must be refined for accurate quantitative modeling of the full SD process, including coarsening.

Primary radiation damage of an FeCr alloy under pressure: Atomistic simulation

The primary radiation damage of a binary FeCr alloy deformed by applied mechanical loading is studied by an atomistic molecular dynamics simulation. Loading is simulated by specifying an applied pressure of 0.25, 1.0, and 2.5 GPa of both signs. Hydrostatic and uniaxial loading is considered along the [001], [111], [112], and [210] directions. The influence of loading on the energy of point defect formation and the threshold atomic displacement energy in single-component bcc iron is investigated. The 10-keV atomic displacement cascades in a “random” binary Fe–9 at % Cr alloy are simulated at an initial temperature of 300 K. The number of the point defects generated in a cascade is estimated, and the clustering of point defects and the spatial orientation of interstitial configurations are analyzed. Our results agree with the results of other researchers and supplement them.