Fe-Cr Alloy System Research Articles

Macroscopic magnetic hardening due to nanoscale spinodal decomposition in Fe–Cr

The Fe–Cr alloy system is the basis of ferritic steels, which are important structural materials for many applications, including their use in future fusion reactors. However, when exposed to elevated temperatures and radiation, the Fe–Cr system can undergo phase separation, resulting in Fe-rich (α) and Cr-rich (α’) nanoscale regions. This in turn generates the so-called “475 °C embrittlement” and modifies the magnetic properties. The correlation between the microstructural and magnetic changes is however poorly understood, which currently prevents the possibility of assessing the material in a non-destructive way by magnetometry. Here, we study the microstructural decomposition of an Fe–40Cr alloy induced by annealing at 500 °C for extensive time scales and its impact on the magnetic properties using magnetometry and advanced experimental methods, such as atom probe tomography, transmission electron microscopy (TEM), and micromagnetic simulations. Upon annealing, the alloy rapidly exhibits a spinodal decomposition morphology with a typical length scale of about 10 nm. With increasing annealing time, the hardness assessed by Vickers testing, the magnetic saturation, and the coercivity increase, which correlates with an increase in α-volume fraction and the system's heterogeneity. The magnetic domain patterns imaged by TEM and interpreted with the help of micromagnetic simulations reveal at the nanometer scale the impact of decomposition on the magnetic response of Fe–Cr.

Effect of Cr addition on room temperature hydrogenation of TiFe alloys

This paper discusses the effect of AB2 (Ti(Cr, Fe)2) phase on the hydrogenation properties of a Ti–Fe–Cr alloy system. Five Ti–Fe–Cr based alloys were fabricated by varying the Cr content. The microstructural analysis results revealed that the fraction of the Ti(Cr, Fe)2 phase increased with the increasing Cr content. The first hydrogenation test results indicated that all the alloys could absorb a significant amount of hydrogen at room temperature (30 °C) without a separate activation process. This behavior improved when the Ti(Cr, Fe)2 phase existed in the AB phase; the kinetics of the first hydrogenation tended to increase with the fraction of Ti(Cr, Fe)2 phase. The enhancement in the first hydrogenation kinetics of the Ti–Fe–Cr based alloys was attributed to the synergetic effect of the interface between the AB and Ti(Cr, Fe)2 phases and the inherent fast hydrogenation of the Ti(Cr, Fe)2 phase. However, the total hydrogen storage capacity decreased when the fraction of Ti(Cr, Fe)2 phase increased.

A study on microstructural evolution and corrosion behavior of spark plasma sintered Fe–Cr alloy system

Iron (Fe) and chromium (Cr) powders at various concentrations of Cr (6, 9, 12 and 14 wt%) were mechanically alloyed via high-energy ball milling for 10 h. The milled Fe–Cr alloy powders were analyzed using scanning electron microscopy (SEM) and X-ray diffraction (XRD) for understanding the impact of varying chromium content on the characteristics of the milled powder. The powder samples were then consolidated via spark plasma sintering (SPS) at 1000 °C for 45 min at 80 MPa pressure. The density of the consolidated samples was measured and found to reach a maximum of 98%. Microstructural characterization of the spark plasma sintered samples was performed using SEM and XRD. Electrochemical corrosion tests were performed on spark plasma sintered samples in 0.5 M H2SO4, 0.05 M NaH2PO4 and 1 M NaOH solutions using potentiodynamic polarization, electrochemical impedance spectroscopy and Mott–Schottky analysis. The corrosion behavior of the spark plasma sintered materials was compared with that of a wrought HT-9 steel (12Cr based ferritic–martensitic steel). The work presented here is a comparative study of microstructural evolution and corrosion behavior between the different spark plasma sintered Fe–Cr model alloys and wrought HT-9 steel.

On the miscibility gap at 800 K in the Fe-Cr alloy system

Issues pertinent to the miscibility gap at 800 K in a Fe73.7Cr26.3 alloy viz. the kinetics of the phase decomposition and borders of the latter were investigated by means of the Mössbauer-effect spectroscopy. The kinetics was revealed to well follow the Johnson-Mehl-Avrami-Kolgomarov equation testifying to a nucleation and growth mechanism underlying the decomposition. A solubility limit of Cr in the Fe matrix was determined to be 17.3(6) at% and that of Fe in the Cr matrix was found as equal to 18.5(9) at%. The results obtained have been compared with various theoretical predictions.

Role of nitrogen in the active–passive transition behavior of binary Fe–Cr alloy system

N effect on the active–passive transition behavior of Fe21Cr(0, 4)N (at%) alloys was investigated in sulfuric acid and borate–phosphate buffer solutions over a wide range of pH. The active–passive transition was observed only in a strong acid solution with a lower pH level than 2 regardless of the solution types. Alloyed N elevated a passivation potential of the FeCr matrix due to the strong CrN bonding formation in the matrix, thus, critical dissolution current density value was observed to increase. In the solution condition above pH 3, more protective passive film with higher Cr content was already formed on N-bearing alloy below the corrosion potential, thus the Fe oxidation current density appeared to be reduced.

Study of phase stability in the σ-FeCr system

Formation energy of the σ-phase in the Fe-Cr alloy system, ΔE, was computed versus the occupancy changes on each of the five possible lattice sites. Its dependence on a number of Fe-atoms per unit cell, NFe, was either monotonically increasing or decreasing function of NFe, depending on the site on which Fe-occupancy was changed. Based on the calculated ΔE values, the average formation energy, 〈ΔE〉, was determined as a weighted over probabilities of different atomic configurations. The latter has a minimum in the concentration range where the σ-phase exists. The minimum in that range of composition was also found for the free energy calculated for 2000 K and taking only the configurational entropy into account.

Waste-recycling Monte Carlo with optimal estimates: Application to free energy calculations in alloys

The estimator proposed recently by Delmas and Jourdain for waste-recycling Monte Carlo achieves variance reduction optimally with respect to a control variate that is evaluated directly using the simulation data. Here, the performance of this estimator is assessed numerically for free energy calculations in generic binary alloys and is compared to those of other estimators taken from the literature. A systematic investigation with varying simulation parameters of a simplified system, the anti-ferromagnetic Ising model, is first carried out in the transmutation ensemble using path-sampling. We observe numerically that (i) the variance of the Delmas-Jourdain estimator is indeed reduced compared to that of other estimators; and that (ii) the resulting reduction is close to the maximal possible one, despite the inaccuracy in the estimated control variate. More extensive path-sampling simulations involving an FeCr alloy system described by a many-body potential additionally show that (iii) gradual transmutations accommodate the atomic frustrations; thus, alleviating the numerical ergodicity issue present in numerous alloy systems and eventually enabling the determination of phase coexistence conditions.

Formation of Columnar-Shaped Structure of Fe in Fe–Cr–Sn Thin Films and Its Shape-Magnetic Anisotropy

In this work studied has been the fabrication method of columnar-structured Fe thin film for manifestation of a shape anisotropy in Fe–Cr alloy system, in which the phase separation occurs naturally by the thermal treatment. In order to obtain such a columnar structure, we have utilized the phase separation on a Sn-nanoparticle-dispersed Si substrate, by exploiting an excellent metallurgical affinity of Fe and Sn, and moreover we have employed applying an external magnetic field. Resultingly, we have found that the columnar-structured Fe thin films are likely to be formed after annealing. Thus obtained Fe–Cr–Sn thin film was found to exhibit a high shape-magnetic anisotropy, where the out-of-plane coercivity Hc⊥ was about 750 Oe, being five times larger than the in-plane coercivity Hc|| of about 150 Oe.

Formation of Columnar-Shaped Structure of Fe in Fe–Cr–Sn Thin Films and Its Shape-Magnetic Anisotropy

In this work studied has been the fabrication method of columnar-structured Fe thin film for manifestation of a shape anisotropy in Fe–Cr alloy system, in which the phase separation occurs naturally by the thermal treatment. In order to obtain such a columnar structure, we have utilized the phase separation on a Sn-nanoparticle-dispersed Si substrate, by exploiting an excellent metallurgical affinity of Fe and Sn, and moreover we have employed applying an external magnetic field. Resultingly, we have found that the columnar-structured Fe thin films are likely to be formed after annealing. Thus obtained Fe–Cr–Sn thin film was found to exhibit a high shape-magnetic anisotropy, where the out-of-plane coercivity Hc⊥ was about 750 Oe, being five times larger than the in-plane coercivity Hc|| of about 150 Oe.

Mössbauer-effect study of the phase separation in theFe-Cr system

In situ and conventional room temperature 57Fe-siteMössbauer spectroscopy (MS) were used to study the phase separationprocess in the Fe-Cr alloy system. It was shown that MS iswell able to distinguish between (i) nucleation and growth and(ii) spinodal decomposition, the two mechanisms held responsible forthe separation. The Fe-rich branch of the spinodal line was locatedat 80.8⩽x⩽84 at.% Fe at T = 415 °C. Itwas also shown that the saturation of the average hyperfine fieldcannot be taken as an indicator of the termination of the separationprocess. The kinetics of the process dramatically depends on theaging temperature, being faster by a factor of 1.4 atT = 440 °C than at T = 415 °C.

Determination of N-body potential for Fe–Cr alloy system and its application to defect study

Attempts have been made to construct the N-body potential for Fe–Cr alloy system by mixing Finnis-Sinclair potential for Fe and that for Cr under the rule for the electron density given by Ackland and Vitek and the rule for the pair term given by Johnson through the adjustment of the heat of the solution of Fe–Cr alloy and the size factor of the Cr atom in the Fe matrix to the experimental values. The obtained potential was used for the determination of the defect structures, e.g., a vacancy-Cr complex ( V-Cr) and a self-interstitial atom (SIA-Cr) complex ( I-Cr). The binding energy of this complex is very small, i.e., 26 meV, which is not contradictory to the experimental result, namely, less than 105 meV obtained in the muon experiment and the vacancy migration temperature obtained by the positron annihilation lifetime experiment, i.e., about 200 K for both pure Fe and Fe–Cr alloy. Calculations were made for the various types of I-Cr complexes, but mixed dumbbell type complex is not the most stable structure. The most stable configuration is the structure where Cr atom sits beside the 〈1 1 1〉 Fe crowdion and the binding energy for this structure is 0.12 eV.

Crossover Phenomenon in Dynamical Scaling of Phase Separation in Fe-Cr Alloy

A neutron-scattering study of phase separation in the Fe-Cr alloy system clearly indicates dynamical scaling. The results are compared with the predictions of recent scaling theories. Both the time evolution of the characteristic wave vector and the scaling function of the structure factor show crossover phenomenon. The crossover is attributed to a transition from a coagulation to a condensation process at a late stage of phase separation in alloy systems.

TSEE from Fe-Cr Alloy System and its Application to the Estimation of Gasoline Deterioration

The exoelectron glow curves for oxide surface on Fe-Cr alloy were measured as a function of Cr content. It was seen that with increasing Cr content the total counts of thermally stimulated exoelectron emission (TSEE), threshold temperature for measuring the glow curve (starting temperature of the glow curve) and the activation energy clearly indicate a good coincidence with the oxidation process of the surface. The interaction of Fe-Cr alloy with gasoline was examined by applying the test for the oxidation induced period (ASTM D525). The induction period became minimum at about 4% Cr, and the deposit of gasoline gum was maximum at its composition. The results were compared with that of TSEE, and it was found that the catalytic effect of Fe-Cr alloy on the deterioration of gasoline is explained from the starting temperature of the glow curves.

TSEE from Fe-Cr Alloy System and its Application to the Estimation of Gasoline Deterioration

The exoelectron glow curves for oxide surface on Fe-Cr alloy were measured as a function of Cr content. It was seen that with increasing Cr content the total counts of thermally stimulated exoelectron emission (TSEE), threshold temperature for measuring the glow curve (starting temperature of the glow curve) and the activation energy clearly indicate a good coincidence with the oxidation process of the surface. The interaction of Fe-Cr alloy with gasoline was examined by applying the test for the oxidation induced period (ASTM D525). The induction period became minimum at about 4% Cr, and the deposit of gasoline gum was maximum at its composition. The results were compared with that of TSEE, and it was found that the catalytic effect of Fe-Cr alloy on the deterioration of gasoline is explained from the starting temperature of the glow curves.