Fe Cr Model Alloys Research Articles

Carburisation of ferritic Fe–Cr alloys by low carbon activity gases

Model Fe–Cr alloys were exposed to Ar–CO 2–H 2O gas mixtures at 650 and 800 °C. At equilibrium, these atmospheres are oxidising to the alloys, but decarburising ( a C ≈ 10 −15 to 10 −13). In addition to developing external oxide scales, however, the alloys also carburised. Carbon supersaturation at the scale/alloy interface relative to the gas reflects local equilibrium: a low oxygen potential corresponds to a high p CO / p CO 2 ratio, and hence to a high carbon activity. Interfacial carbon activities calculated on the basis of scale–alloy equilibrium are shown to be in good agreement with measured carburisation rates and precipitate volume fractions, providing support for the validity of the thermodynamic model.

Study of thermal ageing behaviour of Fe-Cr model alloys by magnetic hysteresis loop technique

Fe-Cr alloys with different Cr content (5–20 wt. %) were prepared by arc melting technique. The alloys were solution annealed at 1050 oC/ 2h and air quenched and then tempered at 750 oC/ 1h followed by air cooling. Thermal ageing was carried at 475 oC for various lengths of time up to 1000 hours (hrs) to produce thermal embrittlement. The hardness and coercivity of Fe−20% Cr alloys were increased and the remanence was decreased, being due to the precipitation of Cr-rich a' phase and their subsequent growth. In Fe−15% Cr and Fe−10% Cr alloys, it was found that the coercivity decreased at the initial ageing period and then increased subsequently due to the competing effect of recovery of dislocations and precipitation of Cr rich phase. The change in magnetic properties in Fe−5% Cr alloy was different than other alloys since it was within the solubility limit of Cr in Fe. A linear relationship was found between the coercivity and hardness in Fe−20% Cr alloy, indicating that Magnetic Hysteresis Loop (MHL) technique would be a good tool for the Non-Destructive Evaluatiion (NDE) of embrittlement in Fe-Cr alloys.

Back Cover: Phys. Status Solidi C 6/11

AbstractPositron annihilation, as a well‐established experimental tool in the materials science community, provides unique information about the behavior of the material microstructure after radiation treatment. The experimental simulation of radiation damage by charged particle implantation is, as a first approach, a very reasonable way to study radiation embrittlement. The article presented by Vladimir Kršjak et al. on p. 2339 ff. discusses this timely topic on recent experiments on helium implanted Fe–Cr model alloys. Simultaneous results from positron beam and conventional depth profiled positron lifetime techniques are demonstrated. In the cover picture, the intensity of the positron lifetime component I3 characterizes the presence of large open volume defects (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Effect of water vapor on high-temperature oxidation of FeCr alloys

The suppression of protective chromia scale formation in water vapor containing service environments limits in many cases the upper application temperature of high-Cr martensitic and ferritic steels. The present paper discusses the mechanisms which are responsible for this technologically important effect, using results of oxidation tests with two types of FeCr model alloys in Ar-O2, Ar-O2-H2O, and Ar(-H2)-H2O mixtures. The data shows that in atmospheres with a high ratio of water vapor to oxygen, Cr exhibits a higher tendency to become internally oxidized than in dry Ar-O2, or e.g. air. Contrary to previous studies which showed the presence of water vapor to affect transport processes in the scale and/or to enhance formation of volatile Cr species, the present results thus reveal that the presence of water vapor also affects the transport processes in the alloy, likely by incorporation of hydrogen.

The EU programme for modelling radiation effects in fusion reactor materials: An overview of recent advances and future goals

The EU fusion materials modelling programme was initiated in 2002 with the objective of developing a comprehensive set of computer modelling techniques and approaches, aimed at rationalising the extensive available experimental information on properties of irradiated fusion materials, developing capabilities for predicting the behaviour of materials under conditions not yet accessible to experimental tests, assessing results of tests involving high dose rates, and extrapolating these results to the fusion-relevant conditions. The programme presently gives emphasis to modelling a single class of materials, which are ferritic-martensitic EUROFER-type steels, and focuses on the investigation of key physical phenomena and interpretation of experimental observations. The objective of the programme is the development of computational capabilities for predicting changes in mechanical properties, hardening and embrittlement, as well as changes in the microstructure and phase stability of EUROFER and FeCr model alloys occurring under fusion reactor relevant irradiation conditions.

Effects of radiation-induced defects on microstructural evolution of Fe–Cr model alloys

We investigate the role of radiation-induced defects in the microstructural evolution of Fe–Cr model alloys with two approaches; a positron annihilation measurement and computer simulations. We prepared Fe–Cr model alloys with different Cr contents which were irradiated with 2 MeV-electrons at the JAEA accelerator at Takasaki in Japan. The production of vacancy-type defects was verified by positron annihilation lifetime spectroscopy. The average positron lifetimes of the irradiated samples were increased by about 14% when compared to the unirradiated one. The ratio curves, derived from a Doppler broadening measurement, did not reveal a prominent change between the irradiated pure Fe and Fe–Cr alloys, which implies that there was no formation of a vacancy-Cr complex. We performed computer simulations by using the Metropolis Monte Carlo method to predict a possible microstructure composed of Cr and vacancies. As expected, the simulation also revealed that no vacancy-Cr complexes were formed. Both results support a small amount of radiation-induced swelling in the Fe–Cr alloys.

Microstructural characterization of Y 2O 3 ODS–Fe–Cr model alloys

Two Fe–12 wt% Cr alloys, one containing 0.4 wt% Y 2O 3 and the other Y 2O 3-free, have been produced by mechanical alloying followed by hot isostatic pressing. These oxide dispersion strengthened and reference alloys were characterized both in the as-HIPed state and after tempering by transmission electron microscopy and atom-probe tomography. The as-HIPed alloys exhibited the characteristic microstructure of lath martensite and contained a high density of dislocations. Small voids with sizes <10 nm were also observed. Both alloys also contained M 3C and M 23C 6 carbides (M = Cr, Fe) probably as a result of C ingress during milling. After tempering at 1023 K for 4 h the microstructures had partially recovered. In the recovered regions, martensite laths were replaced by equiaxed grains in which M 23C 6 carbides decorated the grain boundaries. In the ODS alloy nanoparticles containing Y were commonly observed within grains, although they were also present at grain boundaries and adjacent to large carbides.

Atomic scale analysis and phase separation understanding in a thermally aged Fe–20 at.%Cr alloy

Fe–Cr model alloys are of interest for understanding of phase separation in structural material for fusion or fission reactors. This motivated the quantitative study of the phase separation in a thermally aged (773 K) Fe–20 at.%Cr alloy using a Tomographic Atom Probe. It is shown that the chromium content in the α′ phase evolves with ageing time synonymous of a non-classical nucleation mechanism. The overlap of the nucleation and coarsening regimes is observed. A non-steady coarsening regime occurs before 1067 h of ageing. The solubility limit at 773 K is found to be 14 at.%. A maximum concentration of (83 ± 1) at.% is observed for the Cr-concentration in α′ precipitates.

Heavy-ion irradiations of Fe and Fe–Cr model alloys Part 2: Damage evolution in thin-foils at higher doses

A study of heavy-ion damage in Fe and Fe–Cr alloys started in Part 1 1 was continued with an investigation of damage development in UHP Fe and Fe–8%Cr at higher doses up to 2 × 1019 ions m−2 (∼13 dpa). In thin-foil irradiations with 150 keV Fe+ ions at 300°C and room temperature (RT), more complex microstructures started to develop in thicker regions of the foils at doses greater than about 2 × 1018 ions m−2, apparently involving cooperative interaction, alignment and coalescence of smaller loops. First strings of loops all with the same ½⟨111⟩ Burgers vectors formed. In UHP Fe irradiated at 300°C the damage then developed into colonies of resolvable interstitial loops with ½⟨111⟩ Burgers vectors. By a dose of 2 × 1019 ions m−2, large (several hundred nanometre) finger-shaped loops with large shear components had developed by the growth and subsequent coalescence of smaller loops. Similar but finer-scale damage structures developed in UHP Fe irradiated at RT and in Fe–8%Cr irradiated at both RT and 300°C.

Heavy-ion irradiations of Fe and Fe–Cr model alloys Part 1: Damage evolution in thin-foils at lower doses

The evolution of radiation damage in Fe and Fe–Cr alloys under heavy-ion irradiation was investigated using transmission electron microscopy. Thin foils were irradiated with 100 or 150 keV Fe+ and Xe+ ions at room temperature (RT) and 300°C. Dynamic observations followed the evolution of damage and the early stages in damage development are reported. Small (2–5 nm) dislocation loops first appeared at doses between 1016 and 1017 ions m−2 in all materials. Loop number densities depended strongly on the foil orientation in pure Fe but not in Fe–Cr alloys. Number densities did not depend strongly on Cr content. For a given material, defect yields were higher for Xe+ ions than for Fe+ ions, and were higher at RT than at 300°C. Loops with both ⟨100⟩ and ½⟨111⟩ Burgers vectors were identified. The proportion of ⟨100⟩ loops was larger, especially in pure Fe. Dynamic observations showed that: the contrast of some new loops developed over intervals as long as 0.2 s; hopping of ½⟨111⟩ loops was induced by the ion and electron beams and was pronounced in ultra-pure iron; and many loops were lost during and after ion irradiation by glide to the foil surface. The number of loops retained was strongly dependent on the foil orientation in Fe, but less so in Fe–Cr alloys. This is due to lower loop mobility in Fe–Cr alloys, probably due to pinning by Cr atoms. Reduced loop loss probably explains the higher loop number densities in Fe–Cr alloys compared with pure Fe.

Effect of Cr on the mechanical properties and microstructure of Fe–Cr model alloys after n-irradiation

High-chromium ferritic–martensitic steels are candidate structural materials for high-temperature applications in fusion reactors and accelerator driven systems (ADS). Cr concentration has been shown to be a key parameter which needs to be optimized in order to guarantee the best corrosion and swelling resistance, together with the minimum embrittlement. The behavior of Fe–Cr model alloys with different Cr concentrations (0, 2.5, 5, 9 and 12 wt%Cr) has been studied. Tensile tests have been performed in order to characterize the flow properties in the temperature range from −160 °C to 300 °C. The trend of the yield strength with temperature shows that the strain hardening is the same for all materials at low temperatures, even though they have different microstructures. The same materials have been neutron-irradiated at 300 °C in the BR2 reactor of SCK·CEN, up to three different doses (0.06, 0.6 and 1.5 dpa). The results obtained so far indicate that even at these low doses, the Cr content affects the hardening behavior of Fe–Cr binary alloys. Using the Orowan mechanism, the TEM observed microstructure provides an explanation of the obtained hardening but only at the very low dose, 0.06 dpa. At higher doses, other hardening mechanisms are needed.

Magnetic origin of nano-clustering and point defect interaction in Fe–Cr alloys: an ab-initio study

To understand the behaviour of point defects generated in irradiated FeCr ferritic/martensitic steels and to identify the kinetic pathways of micro-structural evolution of binary model Fe–Cr alloys, we use a combination of density functional theory (DFT) with statistical approaches involving cluster expansion and Monte Carlo simulations. This makes it possible to generate in a systematic way the low-energy configurations required for the subsequent DFT study of intrinsic defects (vacancies, interstitials) and impurity-defect interactions over the entire range of Fe–Cr alloy compositions. In the limit of low Cr concentration, DFT calculations predict that an intermetallic compound Fe15Cr has the lowest negative heat of formation. At higher Cr concentrations, simulations performed using a 4 × 4 × 4 super-cell show that magnetism is responsible for the nano-segregation of the ferromagnetic Fe15Cr and anti-ferromagnetic (α′-Cr) phases giving rise to the formation of clusters characterised by a very low positive heat of formation. We perform a systematic investigation of formation energies of point defects and their energies of interaction with Cr atoms. Further investigation of interaction of interstitial and vacancy defects with impurities (V, Nb, Ta, Mo, W, Al, Si, P, S) also shows a complex picture of interplay between magnetism and short-range ordering that affects the interaction between defects and impurities in the presence of chromium in Fe-rich alloys.

Microstructure and Mechanical Properties of n-Irradiated Fe-Cr Model Alloys

AbstractHigh chromium ( 9-12 wt %) ferritic/martensitic steels are candidate structural materials for future fusion reactors and other advanced systems such as accelerator driven systems ADS). Their use for these applications requires a careful assessment of their mechanical stability under high energy neutron irradiation and in aggressive environments. In particular, the Cr concentration has been shown to be a key parameter to be optimized in order to guarantee the best corrosion and swelling resistance, together with the least embrittlement.In this work, the characterization of the neutron irradiated Fe-Cr model alloys with different Cr % with respect to microstructure and mechanical tests will be presented. The behavior of Fe-Cr alloys have been studied using tensile tests at different temperature range ( from -160°C to 300°C). Irradiation-induced microstructure changes have been studied by TEM for two different irradiation doses at 300°. The density and the size distribution of the defects induced have been determined. The tensile test results indicate that Cr content affects the hardening behavior of Fe-Cr binary alloys. Hardening mechanisms are discussed in terms of Orowan type of approach by correlating TEM data to the measured irradiation hardening.

Examination of high temperature FeCrAl(RE) alloys using FEG-SEM and STEM techniques

The recrystallisation, microstructure and oxidation behaviour of high purity FeCrAl model alloys and commercial FeCrAl alloys have been investigated using advanced analytical techniques including field emission gun scanning electron microscopy (FEGSEM) with electron backscattered diffraction (EBSD) with resolution of ∼ 50 nm and field emission gun scanning transmission electron microscopy (FEGSTEM with probe size ranging from 0.2–1 nm). The alloys investigated contained controlled additions of impurities such as phosphorus and carbon and beneficial elements such as yttrium, zirconium and titanium. Samples were studied in the as-received state (hot rolled sheets annealed at 1000 °C) and after oxidation at temperatures ranging from 600 °C to 1300 °C for times up to 5 h in air. In particular the role of metal substrate recrystallisation, impurity segregation and the formation of chromium-rich carbides during oxidation were all studied. Analysis showed that recrystallisation commenced after oxidation at 600–900 °C, and complete recrystallisation was achieved well before 1 h of oxidation at 1000 °C and appeared to have little effect on the oxidation behaviour of the alloys. Chromium-rich carbides, voids and defective regions were all observed during studying plan view studies and cross-sectional samples of the alloys using the FEGSTEM and their influence on the oxidation behaviour of Fe-20Cr-5Al will be discussed in this paper.

Microstructural evolution in Fe and Fe–Cr model alloys after He + ion irradiations

Thermal desorption spectra (TDS) analysis and transmission electron microscopy (TEM) observation were performed to investigate microstructural evolution in Fe and Fe–X at.% Cr ( X=5.0, 15) model alloys during thermal annealing after irradiation with 8 keV He + ions at room temperature. TDS analysis showed that there is not so much difference in He desorption during post-irradiation annealing between Fe and Fe–Cr alloys, but the amount of He desorption depended on the Cr concentration of the samples. As for TEM observation, a number of interstitial dislocation loops was observed in all the samples even before post-irradiation annealing, while He bubbles were observed in all the samples only after annealing at temperatures greater than 773 K. Microstructural evolution observed by TEM also depended on Cr concentration. The effect of solute Cr addition on the mobility of interstitial loop and He bubbles is discussed.

Effect of Trace Amounts of Carbon and Nitrogen on the High Temperature Oxidation Resistance of High Purity FeCrAl Alloys

The growth rate and adhesion of the protective alumina scales on the heat resistant FeCrAl alloys are known to depend on minor metallurgical additions of the reactive elements such as Y, Ti, Zr, etc. The present study of high purity model FeCrAl alloys illustrates that the oxidation behaviour of these materials is strongly affected by trace amounts (up to 300 mass ppm) of carbon and/or nitrogen impurities. During long term high temperature oxidation testing of high purity model alloys it has been found that the carbon and nitrogen can interact with the reactive elements, such as Zr and Ti in the ferritic alloy matrix, resulting in a number of important effects on the scale growth and adherence. These effects can be beneficial as well as detrimental, depending on the exact amount and/or distribution of the mentioned minor alloying elements and C/N-impurities.

The Measurement of Composition During the Early Stages of the Spinodal Decomposition of Fe-Cr Alloys

AbstractThe properties of many advanced alloys are derived from extremely fine-scale microstructures. This poses interesting questions about the measurement of composition on this scale. The phase separation of model Fe-Cr alloys has been been studied with the atom-probe. Statistical techniques have been used to estimate the composition and compare the results with the predictions of linear and non-linear theories of spinodal decomposition and the distributions obtained from Monte-Carlo calculations.

High-temperature oxidation of Fe-2 1 4 Cr-1Mo in oxygen

The oxidation of a 2 1 4 Cr-1Mo steel in dry flowing oxygen has been studied in the temperature range 550–700°C for periods of up to 100 hr. A detailed low-resolution microstructural investigation revealed a layered oxide consisting of a very fine-grained and finely pored innermost layer of doped spinel, a central columnar-grained relatively coarsely pored layer of magnetite, and an outer fine-grained hematite layer with fine pores and covered with whiskers of α-Fe2O3. This structure is compared with previous results on Fe and model Fe-Cr alloys, as are the kinetics of the oxidation reaction.