Fe-Cr Phase Research Articles

Al–Cr–Fe phase diagram. Isothermal Sections in the region above 50 at% Al

The Al–Cr–Fe phase diagram was studied in the compositional range of 50–100at% Al and partial isothermal sections were determined at 1160, 1100, 1075, 1042, 1000, 900, 800 and 700°C. In the low-Al part of the studied compositional region the isostructural high-temperature Al–Cr and Al–Fe γ1-phases form a continuous region of solid solutions. Both binary Al13Fe4 and Al5Fe2 were found to dissolve up to 6.5at% Cr while Al2Fe was found to extend up to 4.1at% Cr. The solid solutions based on the Al–Cr γ2 and μ phases were determined to reach 35.2 and 1.3at% Fe, respectively. The dissolution of Cr in the Al–Fe binaries only slightly influences their Al concentrations, while the Al–Cr binaries exhibit decreasing Al concentration with increasing Fe concentration. The Al–Cr η-phase dissolves up to 5at% Fe, which results in a sharp decrease of its Al concentration and increase of melting temperatures. The earlier reported existence of a ternary decagonal D3-phase and three complex periodic phases O1, H and ε was confirmed and their compositions at different temperatures were specified.

α′ precipitation in neutron-irradiated Fe–Cr alloys

A series of model Fe–Cr alloys containing 3–18 at.% Cr was neutron irradiated at a nominal temperature of 563 K to 1.82 dpa. Solute distributions were analyzed by atom probe tomography , which revealed α′ precipitation for alloys containing more than 9 at.% Cr. Both the Cr concentration dependence of α′ precipitation and the measured matrix compositions are in agreement with the recently published Fe–Cr phase diagrams . An irradiation-accelerated precipitation process is strongly suggested.

Atomic-scale compositional characterization of a nanocrystalline AlCrCuFeNiZn high-entropy alloy using atom probe tomography

We have studied a nanocrystalline AlCrCuFeNiZn high-entropy alloy synthesized by ball milling followed by hot compaction at 600°C for 15min at 650MPa. X-ray diffraction reveals that the mechanically alloyed powder consists of a solid-solution body-centered cubic (bcc) matrix containing 12vol.% face-centered cubic (fcc) phase. After hot compaction, it consists of 60vol.% bcc and 40vol.% fcc. Composition analysis by atom probe tomography shows that the material is not a homogeneous fcc–bcc solid solution but instead a composite of bcc structured Ni–Al-, Cr–Fe- and Fe–Cr-based regions and of fcc Cu–Zn-based regions. The Cu–Zn-rich phase has 30at.% Zn α-brass composition. It segregates predominantly along grain boundaries thereby stabilizing the nanocrystalline microstructure and preventing grain growth. The Cr- and Fe-rich bcc regions were presumably formed by spinodal decomposition of a Cr–Fe phase that was inherited from the hot compacted state. The Ni–Al phase remains stable even after hot compaction and forms the dominant bcc matrix phase. The crystallite sizes are in the range of 20–30nm as determined by transmission electron microscopy. The hot compacted alloy exhibited very high hardness of 870±10HV. The results reveal that phase decomposition rather than homogeneous mixing is prevalent in this alloy. Hence, our current observations fail to justify the present high-entropy alloy design concept. Therefore, a strategy guided more by structure and thermodynamics for designing high-entropy alloys is encouraged as a pathway towards exploiting the solid-solution and stability idea inherent in this concept.

Modeling of chromium nanocluster growth under neutron irradiation

This study simulated growth kinetics of Chromium (Cr)-rich nanoclusters in Iron (Fe)–Cr based alloys under irradiation. A radiation-enhanced diffusion model was used to investigate the peculiarities of cluster growth under irradiation. The calculation was based on kinetics methods of intermolecular reactions and on the analysis of near-equilibrium phase Fe–Cr alloy composition under irradiation. The dependence of the basic characteristics of Cr-rich clusters (clusters composition, average size, density) on damage dose (up to 10dpa) were calculated at T=573K. The estimated diffusion coefficient of Cr atoms under irradiation was found to be DCr∗=1.4×10-19cm2/s, which is seven orders of magnitude higher than the corresponding thermal value.

Effect of external stress on the Fe–Cr phase separation in 15-5 PH and Fe–15Cr–5Ni alloys

The effect on Fe-Cr phase separation of a uniaxial stress during thermal ageing at 425 °C is investigated on a Fe-15Cr-5Ni steel, a model alloy of commercial 15-5 PH steel. The applied stress is shown to accelerate the ageing kinetics, and influence the morphology of Cr rich domains. A dependence of the phase separation decomposition kinetics on the relative orientations of the load and the crystal local orientation has also been observed.

First-principles based thermodynamic model of phase equilibria in bcc Fe-Cr alloys

The complete and accurate thermodynamic and kinetic description of any systemis crucialfor understanding and predicting its properties. A particular interest is in systemsthat are used for some practical applications and have to be constantly improved usingmodification of their composition and structure. This task can be quite accuratelysolved at a fundamental level by density functional theory methods. Thesemethods areapplied to two practically important systems Fe-Cr and TiC-ZrC.The elastic properties of pure iron and substitutionally disordered Fe-Cr alloy are investigatedas a function of temperature and concentration using first-principles electronicstructurecalculations by the exact muffin-tin orbitals method. The temperature effectson the elastic properties are included via the electronic, magnetic, and lattice expansioncontributions. It is shown that the degree of magnetic order in both pure iron andFe90Cr10 alloy mainly determines the dramatic change of the elastic anisotropy of thesematerials at elevated temperatures. A peculiarity in the concentration dependence ofthe elastic constants in Fe-rich alloys is demonstrated and related to a change in theFermi surface topology.A thermodynamic model for the magnetic alloys is developed from first principles andapplied to the calculation of bcc Fe-Cr phase diagram. Various contributions to the freeenergy (magnetic, electronic, and phonon) are estimated and included in the model. Inparticular, it is found that magnetic short range order effects are important just abovethe Curie temperature. The model is applied for calculating phase equilibria in disorderedbcc Fe-Cr alloys. Model calculations reproduce a feature known as a Nishizawahorn for the Fe-rich high-temperature part of the phase diagram.The investigation of the TiC-ZrC system includes a detailed study of the defect formationenergies and migration barriers of point defects and defect complexes involvedin the diffusion process. It is found, using ab initio atomistic simulations of vacancymediateddiffusion processes in TiC and ZrC, that a special self-diffusion mechanism isoperative for metal atom diffusion in sub-stoichiometric carbides. It involves a noveltype of a stable point defect, a metal vacancy ”dressed” in a shell of carbon vacancies.It is shown that this vacancy cluster is strongly bound and can propagate through thelattice without dissociating.

Ab Initio Study of Lattice Site Occupancies in Binary Sigma Phases Using a Single-Site Mean Field Model

The site occupation of binary Fe-Cr, Co-Cr, Re-W and Fe-V sigma phases is studied in the present work with a first-principles-based single-site mean field theory. We show that the alloy components in these systems exhibit similar site preferences except for the Re-W system, where the occupation of two sites is reversed in agreement with previously published works. In case of the FeV sigma phase, for which the size mismatch between the alloy components is large, we also include into our consideration the effect of local lattice relaxations. The obtained results are found in good agreement with the experimental data and previous theoretical studies.

Structural and hyperfine comparative study of ball-milled and arc-melted Fe50 Cr50 alloys

In this work we make a structural and hyperfine comparative study of the Fe50 Cr50 stoichiometric alloy obtained for two different techniques, mechanical alloying and arc-melting. Both alloys were heated to 650 ◦ C for 7 days, quenched into ice water and then mechanically alloyed for different milling times. Before heat treatment the two alloys exhibit Fe-Cr BCC structure and ferromagnetic behavior similar, after treatment, in the alloy obtained by mechanical alloying, the BCC structure is completely transformed into the ordered structure σ Fe-Cr, whereas that in the alloy obtained by melted this transformation is partial. The mechanical alloying realized in the last stage affect notoriously the structure σ Fe-Cr, for 2 hours of milling the structure is completely disordered, subsequently transformed into BCC Fe-Cr phase, and disappears at 16 hours of milling.

Enhancement of local superconductivity in ferromagnetic FeCrB metallic glass by Ar+ ion irradiation

We have reinforced local superconductivity in ferromagnetic Fe67Cr18B15 metallic glasses by ion irradiation. Superconductivity in this medium appears due to the presence of large-scale layered clusters of metallic Fe–Cr phase, 150–230 Å in size, with a ferromagnetic (or superparamagnetic) Fe-rich core and nonmagnetic Cr-rich superconducting shell. Here we show that due to the intensification of concentration phase separation in the Fe–Cr clusters under ion (Ar+) irradiation, the volume of the superconducting phase increases from the initial 0.4–0.5% up to 7–8%. After irradiation, the resistivity jump Δρ/ρ in the temperature range T = 3.1–3.6 K increases ∼14 times, reaching 19%, as compared to 1.36% for the initial sample. In the interval of T = 3.1–3.6 K, the rate of resistance change reaches 79% K−1 for the irradiated sample instead of 3.6% K−1 for the initial sample. In the same temperature interval, the rate of magnetoresistance change increases from 3% K−1 for the initial sample up to 70% K−1 after irradiation.

Three-sublattice model that takes into account the spin density anisotropy, short-range order, and dimensioned factor for the binary Fe-Cr(V, Mo) systems

The diffusion properties of the elements entering into the composition of austenitic and ferritic steels are analyzed as functions of the temperature and the time evolution of induced radioactivity. As compared to austenitic steels, ferritic steels are shown to have better diffusion properties and swelling resistance during operation at temperatures up to 650°C under irradiation conditions and can be used as structural materials in the core of a fast neutron reactor. Experimental and calculated (Calphad method) data on the Fe-Cr phase diagram and the thermodynamic properties and the experimental data on the short-range order in Fe-Cr alloys are analyzed, and it is concluded that they are conflicting and that models taking into account the short-range order should be developed. The results of quantum-mechanical calculations of the average magnetic moment for ferromagnetic (FM) iron as a function of the volume are analyzed and used to introduce a concept of partial magnetic moments of the iron atoms located in the first four coordination spheres (1–4 CS). The values of these moments are calculated. The concept of the partial magnetic moments of iron atoms agrees qualitatively with the experimental data on the spin-density anisotropy of the bcc lattice of pure iron. This concept is used to formulate a three-sublattice model for binary FM alloys of Fe-M systems (M is an alloying paramagnetic element). An extended cell whose sites contain 8 bcc cells and 16 atoms per cell and that is isomorphic to a DO3-type crystal lattice is considered. A functional is constructed for the internal energy on the extended 16-atom cell. The dimensioned factor is taken into account in a self-consistent manner, by the expansion of the interaction energies of atoms of both components located in different CSs in the atom displacement with respect to ideal lattice sites. The dimensioned factor is taken into account in the three-sublattice model to obtain a set of equations of state for bcc FM binary iron-rich alloys in a 1–3 CS approximation. The obtained estimates demonstrate that the anisotropy of the distribution of magnetic moments in the bcc iron lattice is responsible for the appearance of a short-range order in bcc FM iron-rich Fe-Cr (V, Mo) binary alloys.

Cu-8.6wt%Al 삽입금속을 사용한 페라이트계 스테인리스강의 아크 브레이징 접합부의 미세조직과 인장성질

Microstructures and tensile properties in arc brazed joints of ferritic stainless steel, 429EM using Cu-8.6%Al insert metal was investigated as function of brazing current. The brazing speed was fixed at 800mm/min and brazing current varied in the range of 80A to 120A. The initial phase of filler metal was Cu single phase. However, the insert metal structures of brazed joints was composed of Cu matrix and intermetallic compound such as γ1(Al4Cu9), and flower-shape Fe-Cr. The fraction of γ1(Al4Cu9) phase was similar with 80A and 100A brazing currents while that of brazed with 120A was decreased. On the other hand, the fraction of Fe-Cr phase increased with increasing of the brazing current. A reaction layer at the base metal/insert metal interface was observed and this reaction layer was thickened with increasing of the brazing current. In the brazed joints with the current lower than 100A, crack was grew up along the interface which was perpendicular to the tensile stress, and then, passed through the insert metal in the final stage of fracture. As the brazing current increased to 120A, fracture occurred at the base metal.

Effect of raw materials on microstructure and bending strength of porous in situ MgO/Fe–Cr–Ni composites

Abstract Porous in situ MgO (17.0–34.9 wt.%)/Fe–Cr–Ni composites were prepared from the mixtures of Mg and the powder compositions, such as nano-Fe 2 O 3 + Cr + Ni, micro-Fe 2 O 3 + Cr + Ni, micro-Cr 2 O 3 + Fe + Ni or micro-Ni 2 O 3 + Fe + Cr, via pressureless reactive sintering at 600–800 °C for 2 h. The influence of raw materials on microstructure and bending strength of the resultant porous composites were studied. The results showed that the porous composites, fabricated at 700 °C for 2 h, consisted of MgO, Cr 0.7 Fe 0.36 Ni 2.9 and CrFe phases generally. The porous composite fabricated from the mixture of nano-Fe 2 O 3 , Mg, Cr and Ni, whose fracture mode was a mainly ductile manner, had uniform pore size and possessed higher bending strength than other resultant porous composites.

Porous nano-Al 2O 3/Fe–Cr–Ni composites fabricated by pressureless reactive sintering

Porous in situ nano-Al 2O 3 reinforced Fe–Cr–Ni metal matrix composites (MMCs) were fabricated from a mixture of Al/nano-Fe 2O 3/Cr/Ni powders via pressureless reactive sintering at 700–800 °C for 1–2 h. X-ray diffraction (XRD) demonstrated that the porous composites produced at 800 °C for 2 h consisted of Al 2O 3, Cr 0.7Fe 0.36Ni 2.9 and Fe–Cr phases generally. Scanning electron microscopy (SEM) showed that the Al 2O 3 particles were ultrafine and uniformly distributed in the metal matrix. The pore sizes ranged from 1 μm to 20 μm for the sintered porous composites fabricated at 800 °C for 2 h. The resultant porous composite prepared under prepressing pressure of 200 MPa before sintering had more homogeneous and finer pore structure, lower porosity and higher bending strength and fracture toughness than those prepared under prepressing pressure of 5–100 MPa.

Microstructure and wear characteristics of high-carbon Cr-based alloy claddings formed by gas tungsten arc welding (GTAW)

High-carbon Cr-based alloy claddings with distinct carbon contents are fabricated on medium-carbon steel substrates by the GTAW process utilizing various pure Cr and CrC (Cr:C=4:1) alloy fillers. The results show that the microstructures of hypoeutectic, near-eutectic, and hypereutectic structures with various pro-eutectic Cr–Fe phases, (Cr,Fe)23 C6 and (Cr,Fe)7 C3 carbides, form in the coated surface depending on the carbon contents. The wear resistance is determined by the type, mechanical properties, and volume fraction of pro-eutectic phases, as well as by the matrix microstructure. The cladding with 2.3%C possesses the worst wear resistance, which can be attributed to great amounts of soft pro-eutectic Cr–Fe phases. The cladding with 5.9%C has the best wear resistance. The improvement of wear resistance results from the high volume fraction of hard and wear-resistant pro-eutectic (Cr,Fe)7 C3 carbides, which are distributed homogeneously in the fine and strong [α+(Cr,Fe)7 C3] eutectic colonies.

New Contribution to the Thermodynamics of Fe-Cr Alloys as Base for Ferritic Steels

High chromium ferritic-martensitic steels are commonly used for industrial applications requiring high strength at elevated temperature. Such steels typically contain 9-14 at.% Cr and a few percents of minor alloying elements. In recent studies it has been shown that the Cr solubility limit of the standard Fe-Cr phase diagram in that composition range is significantly underestimated. For the purpose of more reliable engineering and out of physical considerations, we reparameterize the Gibbs free energy so that the correct Cr solubility at low temperature (<700 K) is reproduced, while leaving the rest of the phase diagram changed only slightly. The mixing enthalpy and heat capacity resulting from the new parameterization are also compared with experiments and found to be in good agreement.

Low-temperature anomalies in resistance and magnetoresistance of amorphous FeCrBribbons. Coexistence of ferromagnetism and local superconductivity?

Investigation of the conductivity mechanisms in ferromagneticFe67Cr18B15 metallic glasses with clusterized structure reveals anomalies in the behaviour ofresistance and magnetoresistance (MR) in a narrow temperature interval,T = 3.6–3.1 K.The anomalies are seen as a sharp decrease of the sample resistivity in this range, with a rate equalto 3.6% K − 1, i.e. 200–500 times more than the rate 0.008–0.021% K − 1 in the range of 300–4 K. MR in the same range increases with a rate 1000 times larger (4% K − 1 atT ∼ 3.1–3.6 K) than in the300–4 K range (<0.0015% K − 1). We explain this result by the appearance of local superconductivity in thelarge-scale layered clusters of metallic Fe–Cr phase, 150–200 Å in size, with ferromagneticFe2Cr core andnonmagnetic FeCr2 superconducting shell. The superconducting phase, which occupies0.4–0.5% of the sample volume, provides a resistance jumpΔρ/ρ≈1.5% that corresponds to calculation. The superconducting state of the clusters collapses if themagnetic field exceeds 20 kOe.

Microstructural evolution in austenitic heat-resistant cast steel 35Cr25Ni12NNbRE during long-term service

The microstructural evolution of austenitic heat-resistant cast steel 35Cr25Ni12NNbRE during aging and long-term service was investigated using optical microscope (OM), X-ray diffraction (XRD), scanning electron microscope (SEM) and transmission electron microscope (TEM). The microstructure of the as cast steel consists of the dendritic austenite, the block-like eutectic carbide M 7C 3 spreaded among austenitic dendrite, and a small quantity of M 23C 6 carbide. The microstructure of the steel aged at 600 °C consists of eutectic carbide M 23C 6 transformed from eutectic carbide M 7C 3 and dendritic austenite in which fine secondary carbide particles M 23C 6 precipitated. The precipitated carbide M 23C 6 kept a cubic–cubic orientation relationship (OR) with austenite matrix. There existed a carbide precipitation free zone (PFZ) around the eutectic carbide. For the long-term serviced samples, the secondary carbide precipitated in the austenite strikingly increased and the PFZ disappeared. Part of the M 23C 6 transformed into M 6C, which always kept a twin OR, [ 1 1 4 ] M 6 C / / [ 1 1 0 ] A / / [ 1 1 0 ] M 23 C 6 , with the austenite and the M 23C 6 secondary carbide. In addition, a small quantity of σ phase FeCr and ɛ-Cr 2N were also identified. The effects of alloy composition and service condition on the microstructural evolution of the steel were discussed.

Thermodynamic assessment of the Cu–Fe–Cr phase diagram

Thermodynamic assessment of the Cu–Fe–Cr phase diagram

Microstructural and abrasive characteristics of high carbon Fe–Cr–C hardfacing alloy

A series of high carbon Fe–Cr–C hardfacing alloys were produced by gas tungsten arc welding (GTAW). Chromium and graphite alloy fillers were used to deposit hardfacing alloys on ASTM A36 steel substrates. Depending on the four different graphite additions in these alloy fillers, this research produced hypereutectic microstructures of Fe–Cr phase and (Cr,Fe) 7C 3 carbides on hard-facing alloys. The microstructural results indicated that primary (Cr,Fe) 7C 3 carbides and eutectic colonies of [Cr–Fe+(Cr,Fe) 7C 3] existed in hardfacing alloys. With increasing the C contents of the hardfacing alloys, the fraction of primary (Cr,Fe) 7C 3 carbides increased and their size decreased. The hardness of hardfacing alloys increased with fraction of primary (Cr.Fe) 7C 3 carbides. Regarding the abrasive characteristics, the wear resistance of hardfacing alloys were related to the fraction of primary (Cr,Fe) 7C 3 carbides. The wear mechanism was also dominated by the fraction of primary (Cr,Fe) 7C 3 carbides. Fewer primary carbides resulted in continuous scratches worn on the surface of hardfacing alloy. In addition, the formation of craters resulted from the fracture of carbides. However, the scratches became discontinuous with increasing fraction of the carbides. More primary carbides can effectively prevent the eutectic colonies from the damage of abrasive particles.

The effects of alloying and pressing routes in equal channel angular pressing of Cu-Fe-Cr and Cu-Fe-Cr-Ag composites

Equal channel angular pressing (ECAP) was carried out on Cu-Fe-Cr and Cu-Fe-Cr-Ag composites at room temperature. ECAPed Cu-Fe-Cr and Cu-Fe-Cr-Ag exhibited ultrafine-grained microstructures with the shape and distribution of Fe-Cr phase were dependent on the processing routes. In route A, the initial dendrites of Fe-Cr phase were elongated along the shear direction and developed into filaments, whereas in route Bc the initial dendrites became finer by fragmentation with no pronounced change of the shape. The hardness of ECAPed Cu-Fe-Cr-Ag is greater than that of ECAPed Cu-Fe-Cr. The higher hardness in Cu-Fe-Cr-Ag is ascribed to the more effective matrix strengthening due to the dislocation storage and the precipitation hardening. The hardness of ECAPed Cu-Fe-Cr was lower than that of the drawn Cu-Fe-Cr at the same deformation strain because of the less effective refinement and elongation of the two-phase filamentary microstructure. The addition of silver was found to increase the hardness of the ECAPed composite above the strength level of heavily drawn Cu-Fe-Cr, rendering the processing method of applying alloying and ECAP to Cu-Fe-Cr composite an attractive approach to producing bulky high strength Cu base composites.