Iron-chromium Alloys Research Articles

Iron–chromium alloy nanoparticles produced by pulsed sonoelectrochemistry: Synthesis and characterization

This study describes synthesis of Fe–Cr alloy nanoparticles by using a method which couples electrodeposition of metals with the employment of high power ultrasound. The production of alloyed nanoparticles was performed using a 20 kHz titanium alloy horn ultrasound generator, a “sonoelectrode” generating short current pulses ( t ON) triggered and followed immediately by ultrasonic pulses ( t US). A rest time ( t r ) restores the initial conditions close to the sonoelectrode. The final product is a suspension of nanoparticles with high purity and high surface/volume ratio, which could be controlled by varying process parameters like time management and current density.

Single-kink relaxation processes in iron–chromium binary alloys

The internal friction spectra in cold-worked pure iron and iron–chromium alloys are measured in the inverted torsion pendulum, operating at 1.8 Hz and in the temperature range from 100 K to 600 K. The addition of chromium, in combination with a plastic deformation, activates the relaxation process producing an internal friction peak at about 210 K. The peak intensity exhibits a strong increase by increasing the concentration of solute atoms. On the basis of the estimated activation energy, E sk ∼ 0.57 eV, and atomistic simulations, this mode is assigned to the single-kink relaxation process corresponding to the recombination of geometrical kinks formed as a result of Cr-dislocation core interaction in the 1 / 2 〈 1 1 1 〉 screw dislocations.

On the effect of micrograin crystallography on creep of FeCr alloys

We use scanning electron orientation imaging microscopy to study the role of micrograin orientation and the role of the crystallographic character of micrograin boundaries during creep of two iron–chromium alloys with similar chromium contents. The ultra fine grained microstructure of a creep resistant tempered martensite ferritic steel (German grade X20) is characterized by carbides which pin micrograin boundaries. Without this type of stabilization, a binary Fe10Cr alloy processed by equal channel angular pressing (ECAP) with the same micrograin size creeps much faster. This difference in creep rate was attributed to the stabilization of micrograin boundaries by carbides [A. Kostka, K.-G. Tak, R.J. Hellmig, Y. Estrin, G. Eggeler, Acta Mater. 55 (2007) 539]. But it was not clear whether a difference in the crystallographic nature of micrograin boundaries also contributes to the difference in creep rates. The results obtained in the present study show that this is not the case. We also consider the formation of macro- and microtextures during processing (X20: pipe making followed by tempering, Fe10Cr: ECAP followed by tempering) and their evolution during creep. Creep deformation results in a decrease of preferential micrograin orientations in tempered martensite ferritic steels and triggers recrystallization events in the ECAP-processed binary Fe10Cr alloy.

Nanoindentation and Micromechanical Testing of Iron-Chromium Alloys Implanted with Iron Ions

The mechanical properties of Fe-Cr alloys were investigated in as-grown and in post-ion-implanted conditions. Sets of specimens were produced using dual implantations of Fe+ ions to give 1µm deep damaged layers with average damage levels of 0.35 displacements per atom and 5.33 displacements per atom. Nanoindentation was used to measure hardness as a function of depth and showed that implanted material had a higher hardness than unimplanted material. Additionally, micron-scale cantilevers were fabricated from the ion-damaged surface of the material and were tested using a nanoindenter for AFM-imaging and loading. The mechanical properties deduced from the controlled loading of these cantilevers pertain only to radiation-damaged material, and for the high-dose material show significant changes in Young’s modulus, yield stress and work-hardening.

Numerical prediction of thermodynamic properties of iron–chromium alloys using semi-empirical cohesive models: The state of the art

In this work the capability of existing cohesive models to predict the thermodynamic properties of Fe–Cr alloys are critically evaluated and compared. The two-band model and the concentration-dependent model, which are independently developed extensions of the embedded-atom method, are demonstrated to be equivalent and equally capable of reproducing the thermodynamic properties of Fe–Cr alloys. The existing potentials fitted with these formalisms are discussed and compared with an existing cluster expansion model. The phase diagram corresponding to these models is evaluated using different but complementary methods. The influence of mixing enthalpy, low-energy states and vibrational entropy on the phase diagram is examined for the different cohesive models.

The hardening of iron–chromium alloys under thermal ageing: An atomistic study

In this work an atomistic multi-scale modelling approach is applied to study: (i) the kinetics of phase separation during thermal ageing of binary Fe–Cr alloys and (ii) the interaction of dislocations with Cr-rich precipitates. The studies were performed varying the Cr content in the range of 12–18at.% Cr and at temperatures from 600 to 900K, which is within the miscibility gap where phase separation occurs. Thermal ageing was simulated using atomistic kinetic Monte Carlo techniques while the interaction of a dislocation with Cr-rich precipitates was studied by molecular dynamics. In both cases the description of atomic interactions was provided by the same semi-empirical interatomic potential, partially fitted to available ab initio data. Both studies are combined to analyse the hardening of Fe–Cr alloys during thermal ageing.

Oxidation resistance of nanocrystalline vis-à-vis microcrystalline Fe–Cr alloys

This paper investigates the hypothesis that nanocrystalline structure may confer considerable oxidation resistance to iron–chromium alloys at much lower chromium contents. Discs of nanocrystalline Fe–10%Cr alloy were produced by ball milling of Fe and Cr powders and compaction of the powder without considerable grain growth (by processing within a suitable time–temperature window). Corrosion resistance of discs of nanocrystalline and microcrystalline alloys was compared by subjecting them to oxidation in air and post-oxidation characterisation of the oxide scales by secondary ion mass spectroscopy (SIMS). Nanocrystalline Fe–10%Cr alloy showed considerably greater oxidation resistance than the microcrystalline alloy of the same composition. Chromium content of the inner scale formed over the nanocrystalline alloy was detected to be nearly five times greater than that in the case of microcrystalline alloy, facilitating formation of the passive layer and providing an explanation for the greater oxidation of the former.

Dependence of the grain boundary energy on the alloy composition in the bcc iron–chromium alloy: A molecular dynamics study

Dependence of the grain boundary energy on the composition ratio in the bcc iron–chromium alloy was investigated by classical molecular dynamics simulation. Symmetric tilt boundaries with the alloy–alloy interface and alloy–iron interface were examined under various composition ratios. In all cases, large energy cusps were observed at the (1 1 2) Σ 3 plane and small cusps were observed at the (2 2 1) Σ 9 plane. In the case of the alloy–alloy interface, the grain boundary energies decreased with increasing proportion of chromium. On the other hand, no correlation was observed between the grain boundary energies and alloy composition in the case of the alloy–iron interface.

Ductile–brittle transition of polycrystalline iron and iron–chromium alloys

Fracture toughness of polycrystalline Fe, Fe–3%Cr and Fe–9%Cr was measured by four-point bending of pre-cracked specimens at temperatures between 77 K and 150 K and strain rates between 4.46 × 10 −4 and 2.23 × 10 −2 s −1. For all materials, fracture behaviour changed with increasing temperature from brittle to ductile at a distinct brittle–ductile transition temperature ( T c), which increased with increasing strain rate. At low strain rates, an Arrhenius relation was found between T c and strain rate in each material. At high strain rates, T c was at slightly higher values than those expected from extrapolation of the Arrhenius relation from lower strain rates. This shift of T c was associated with twinning near the crack tip. For each material, use of an Arrhenius relation for tests at strain rates at which specimens showed twinning gave the same activation energy as for the low strain rate tests. The values of activation energy for the brittle–ductile transition of polycrystalline Fe, Fe–3%Cr and Fe–9%Cr were found to be 0.21, 0.15 and 0.10 eV, respectively, indicating that the activation energy for dislocation glide decreases with increasing chromium concentration in iron.

Performance of NiB Inhibitor for PbSCC of Alloy 600 in Pb-Contaminated Water and Caustic Solutions

Nickel-based iron-chromium alloys are used as steam generator (SG) tubing materials in nuclear power plants (NPPs) but experience intergranular stress corrosion cracking (IGSCC) and lead stress corrosion cracking (PbSCC) due to the harsh operating conditions of NPPs. In order to improve the integrity of SGs, many studies have sought suitable inhibitors for IGSCC, while those for PbSCC have been sought to a limited extent. In this study, the performance of nickel boride (NiB) as an inhibitor for PbSCC was evaluated. Nickel boride has been shown to be a good inhibitor for IGSCC of Alloy (or Inconel) 600 in a caustic aqueous solution containing no lead (Pb). No significant SCC was observed to occur in Alloy 600 tested in pure water under the slow strain rate test (SSRT) condition in this study. A mixed mode of IGSCC and TGSCC, however, occurred relatively easily in Alloy 600 tested in pure water containing lead oxide (PbO) at 315 °C. The susceptibility of Alloy 600 to SCC decreased with the addition of NiB, implying that NiB can be used as an inhibitor for PbSCC in Pb-contaminated water.

Grain growth behaviour and consolidation of ball-milled nanocrystalline Fe–10Cr alloy

Nanocrystalline iron–chromium alloys may provide considerable corrosion resistance, even at low chromium contents. However, processing of such alloys could be a challenge. This paper describes successful synthesis of nanocrystalline Fe–10%Cr alloy by ball-milling route. In the absence of suitable hot compaction facility, the alloy powder could be successfully compacted close to the desired density, by employing a step of prior annealing of the powder. Grain growth behaviour of Fe–10%Cr nanocrystalline alloy was investigated at 500, 600 and 700°C. At 500°C, no appreciable grain growth was observed, after the initial grain growth. However, sudden and rapid grain growth was observed after 90min at 600°C, and 30min at 700°C.

Alternating Pulsed Electrolysis for Fe-Cr Surface Alloying of Conventional Carbon Steel

Alternating pulsed electrolysis was investigated for the surface modification of carbon steel substrates with carbon contents of 0.2 wt.%, 0.6 wt.% and 0.8 wt.%. This process involves the anodic periodic dissolution of substrates to provide ferrous, or iron(II), ions near the substrate as an electroactive component to form the objective alloy during subsequent cathodic times. The carbon steel substrates dissolved heterogeneously since the substrates had nonuniform texture composed of ferrite, cementite, and pearlite. Although the heterogeneous dissolution tended to provide a rough surface of the iron-chromium alloy layers, which resulted in poor adhesion of the layers, relatively flat and smooth alloy layers were obtained by reducing the amount of dissolved iron during each anodic pulse. Pits and defects that were originally recognized on the carbon steel substrate were gradually infilled and covered with iron-chromium alloy by the pulsed electrolysis.

The morphology of nitrided iron–chromium alloys; influence of chromium content and nitrogen supersaturation

Abstract Different iron–chromium alloys (4, 8, 13 and 20 wt.% Cr) were nitrided in NH3/H2 gas mixtures at 580°C. The nitrided microstructure was investigated by X-ray diffraction, light microscopy, hardness measurements and scanning electron microscopy. Composition depth-profiles of the nitrided zone were determined by electron probe microanalysis. Various microstructures develop, depending on the nitriding conditions and the alloy composition (chromium content). The initial development of coherent, sub-microscopical CrN nitrides leads to a state of hydrostatic stress allowing the uptake of excess nitrogen dissolved in the ferrite matrix. It is shown that the outcome of the subsequent discontinuous coarsening process, which leads to a lamellar microstructure, has a close relation to the nitrogen supersaturation. As a result, the occurrence of a distinct gradient in hardness across the nitrided zone can be understood.

Influence of the microstructure on the residual stresses of nitrided iron–chromium alloys

Different iron–chromium alloys (4, 8, 13 and 20 wt.%Cr) were nitrided in a NH 3/H 2 gas mixture at 580 °C for various times. The nitrided microstructure was characterized by X-ray diffraction, light microscopy and hardness measurements. Composition depth profiles of the nitrided zone were determined by electron-probe microanalysis. Residual stress–depth profiles of the nitrided specimens were measured using the (X-ray) diffraction sin 2 ψ method in combination with cumulative sublayer removals and correction for corresponding stress relaxations. Unusual, nonmonotonous changes of stress with depth could be related to the microstructure of the nitrided zone. A model description of the evolution of the residual stress as function of depth and nitriding time was given.

Electrochemical Iron-Chromium Alloying of Carbon Steel Surface Using Alternating Pulsed Electrolysis

Alternating pulsed electrolysis was investigated for the surface modification of carbon steel substrates with carbon contents of 0.2 mass%, 0.6 mass% and 0.8 mass%. This process involves the anodic periodic dissolution of substrates to provide ferrous or ferric ions near the substrate as an electroactive component to form the objective alloy during subsequent cathodic times. The carbon steel substrates dissolved heterogeneously since the substrates had nonuniform texture composed of ferrite, cementite, and pearlite. Although the heterogeneous dissolution tended to provide a rough surface of the iron-chromium alloy layers, which resulted in poor adhesion of the layers, relatively flat and smooth alloy layers were obtained by reducing the amount of dissolved iron during each anodic pulse. Pits and defects that were originally recognized on the carbon steel substrate were gradually filled in and covered with iron-chromium alloy by the pulsed electrolysis. [doi:10.2320/matertrans.MRA2008028]

Monte Carlo simulations of Fe-Cr solid solution

Iron-chromium alloys are characterised by a complex phase diagram, the small negative heat of formation at low Cr concentrations in bcc α-structure of Fe and by the inversion of short-range order parameter. We present Monte Carlo simulations of Fe-Cr alloy based on cluster expansion (CE) approximation for the enthalpy of the system. The set of cluster expansion coefficients is validated versus the DFT results on small clusters in bcc structure. The enthalpy of mixing is negative at small Cr concentrations up to high temperatures. Also, at small concentrations chromium atoms are well separated from each other. Clustering of Cr atoms begins at concentrations of about 10% at 800 K and 20% at 1400 K. Short-range order parameters were calculated and it was confirmed that negative values of the first and second parameters at low Cr concentrations change sign at about 10.5% Cr, in agreement with experiment. We demonstrate that complex ordering reactions in Fe-Cr and its properties may be described by 12 concentration-independent expansion coefficients.

Alternating Pulsed Electrolysis for Iron-Chromium Alloy Coatings with Continuous Composition Gradient

To develop a surface-finishing technique, an Fe–Cr surface alloying of an iron substrate was investigated using alternating pulsed electrolysis in an aqueous solution containing only Cr(III) ions as a single metal component. In this electrochemical process, the iron substrate was slightly dissolved during the anodic pulse, providing iron ions into the solution in the vicinity of the substrate, while Fe and Cr were both electrodeposited on the substrate surface during the subsequent cathodic pulse. Crack-free or fracture-free Fe–Cr alloy layers with continuous composition gradients were formed on the iron surface under appropriate pulse conditions. Both the thickness and Cr content of the Fe–Cr alloy layer increased with increasing anodic pulse time, suggesting that the Fe(II) ions which dissolved and stayed near the substrate induced the electrodeposition of chromium atoms. The time-dependent concentration distribution of Fe(II) ions in the vicinity of the substrate was digitally simulated through a simple diffusion model using a set of experimental data on current density and current efficiency. The effect of the Fe(II) ion concentration near the substrate surface was discussed in terms of the layer properties, such as chromium content and its anticorrosion behaviors.

Characterization of dislocation loops and chromium-rich precipitates in ferritic iron–chromium alloys as means of void swelling suppression

We have used both ab initio and empirical potential based molecular dynamics and static calculations to parameterize a model accounting for the mobility of self-interstitial clusters and small dislocation loops in Fe–Cr alloys depending on Cr content. Particularly, we have considered the interval of 5–15% of Cr, corresponding to the typical concentration of Cr in ferritic/martensitic steels. The mobility of self-interstitial clusters and small dislocation loops was found to be strongly reduced in the presence of Cr due to attractive interaction between self-interstitials and Cr atoms, whereas Cr-rich precipitates act as repulsive obstacles. The results obtained allow the experimental trend of swelling measurements in neutron and high-energy electron irradiated Fe–Cr alloys to be interpreted and explained.

Chromium vaporization of bare and of coated iron–chromium alloys at 1073 K

Suppression of chromium vaporization from oxidation scales formed on ferritic stainless steel, when used on the cathode side of Solid Oxide Fuel Cells (SOFC), is a most important issue, since gaseous chromium species cause deposition of chromium oxide on electrolyte or electrode materials. This phenomenon leads to deterioration of the cell performance. The chromium vaporization rates of bare and of coated iron–chromium alloys were measured at 1073 K. The effect of ceramic coating layers, deposited by low-cost aerosol spraying or by dip coating, was evaluated and the morphology of the coating layer was observed. Lanthanum strontium manganese oxide (LSM: La 0.65Sr 0.3MnO 3), lanthanum strontium cobalt iron oxide (LSCF: La 0.6Sr 0.4Co 0.8Fe 0.2O 3) and manganese cobalt oxide (MCO: MnCo 2O 4) coatings, formed from submicron powders, decreased chromium vaporization by a factor of as much as 21–40 at 1073 K. Manganese cobalt oxide and lanthanum strontium chromium oxide (La 0.7Sr 0.3CrO 3) coating, formed from powders prepared by a glycine nitrate combustion synthesis, reduced chromium vaporization by factors of only 2–3 because the coating layers were porous.

Monte Carlo study of thermodynamic properties and clustering in the bcc Fe-Cr system

Iron-chromium alloys are characterized by a complex phase diagram, by the small negative enthalpy of mixing at low Cr concentrations in the bcc $\ensuremath{\alpha}$-phase of Fe, and by the inversion of the short-range order parameter. We present Monte Carlo simulations of the binary Fe-Cr alloy based on the cluster expansion approximation for the enthalpy of the system. The set of cluster expansion coefficients is validated against density functional calculations of energies of small clusters of chromium in bcc structure. We show that in the limit of small Cr concentration the enthalpy of mixing remains negative up to fairly high temperatures, and individual Cr atoms remain well separated from each other. Clustering of Cr atoms begins at concentrations exceeding approximately 10% at $800\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and 20% at $1400\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, with Cr-Fe interfaces being parallel to the [110] planes. Calculations show that the first and the second short-range order parameters change sign at approximately 10.5% Cr, in agreement with experimental observations. Semi-grand-canonical ensemble simulations used together with experimental data on vibrational entropy of mixing give an estimate for the temperature of the top of the $\ensuremath{\alpha}\text{\ensuremath{-}}{\ensuremath{\alpha}}^{\ensuremath{'}}$ miscibility gap. We find that the complex ordering reactions occurring in Fe-Cr, as well as the thermodynamic properties of the alloy, can be reasonably well described using a few concentration-independent cluster expansion coefficients.