Fe Cr Matrix Research Articles

Influence of injected ions on α’ formation under ion irradiation in Oxide Dispersion Strengthened Steels

Oxide Dispersion Strengthened (ODS) steels hold great promise for applications in next generation reactors. Under irradiation, a phase separation α/ α’ can occur within the Fe-Cr matrix of ODS steels that can alter their mechanical properties. This work presents, for the first time, the characteristics of α’ precipitates enhanced by ion irradiation at 400 °C and examines the influence of the implanted ions. Far from the implanted region, α’ is reported in significant density while at the implanted peak, the α’ density is considerably reduced. This suggests that ion implantation either reduces the fraction of α’ phase formed after irradiation or delays considerably its formation. Through atom probe tomography analysis and comparison with existing literature, the low impact of the damage rate and fluence on the α’ formation in ODS steels is highlighted. Interestingly, the efficiency of ballistic mixing of α’ appears to be less pronounced in ODS steels than in Fe-Cr systems.

Chromium agglomeration induced by Fe+ ion irradiation of Fe-10at%Cr

Fe-Cr alloys serve as model alloys for the investigation of radiation induced effects in ferritic-martensitic steels which are candidate structural materials for future fusion reactors. In this work the effect of Cr segregation and/or agglomeration in 490 keV Fe+ ion irradiated Fe-10at%Cr alloys in the form of thin films is investigated. The irradiations took place at 300 °C at doses ranging from 0.5 to 20 displacements per atom (dpa). Polarized Neutron Reflectivity (PNR) measurements were used for the determination of the solute Cr concentration in the Fe-Cr matrix. Cr depletion from the Fe-Cr matrix up to 2.4 at% was found. This is related to solute Cr decrement as the accumulated dose increases. After the damage of 4 dpa, solute Cr reaches the asymptotic value of 8.4 at%, close to that of the thermodynamic equilibrium in Fe-Cr. Atom Probe Tomography (APT) measurements showed that after irradiation Cr accumulates into clusters the majority of which is co-located with oxygen.

Solution heat treatment of electron beam freeform fabricated in-situ TiC-γ'/ Ni–Fe–Cr composite: Strength-toughness synergy by regulating TiC and γ′ precipitates

Nickel matrix composites reinforced by ceramic particles typically exhibit elevated strength but diminished toughness. In this paper, a novel in-situ micron-size TiC and nano-size γ′ phases reinforced Ni–Fe–Cr matrix composite was successfully prepared using the electron beam freeform fabrication (EBF3). Subsequently, the as-built composite was subjected to solution heat treatment at different temperatures. At a solution temperature of 1150 °C, the composite demonstrates superior strength-toughness synergy (ultimate tensile strength = 1109.4 ± 25.4 MPa, elongation = 12.08 ± 0.62%), exhibiting an increase of 17.1% and 104.4%, respectively, compared to the as-built composite. A systematic analysis of the strengthening and toughening mechanisms revealed that the γ′ phase played a predominant role in enhancing strength, while the size, morphology, and content of TiC phase significantly influence fracture toughness. After being solution heat treated at 1150 °C for 2 h, the γ′ phase precipitated uniformly with its size coarsening to 93.85 nm, demonstrating an optimal strengthening effect by shearing mechanism. Meanwhile, the TiC phase partially dissolved into the matrix and transformed from a long-strip shape to a more stable spherical shape. This transformation significantly reduced the formation of microvoids along the TiC/matrix interface during tensile loading, enhancing the fracture toughness. The γ′ and TiC phases were regulated by solution heat treatment to reach a balance of optimising properties, achieving synergistic enhancements in strength and toughness.

High-temperature oxidation properties of economical and lightweight Fe-Cr-Ni-Al medium-entropy alloy

This study investigated the microstructure, high-temperature oxidation properties and mechanism of non-equiatomic Fe-Cr-Ni-Al medium entropy alloy (MEA). Microstructural observation of MEA identified coarse grains of mm scale, and cuboidal precipitates with an average size of 125 nm were evenly distributed within grains. Phase analysis confirmed that the alloy consisted of a Fe-Cr matrix (BCC structure) and B2 Ni-Al precipitate. High temperature isothermal oxidation tests measured oxidation weight of 0.08 mg/cm2 at 800 ℃, 0.17 mg/cm2 at 900 ℃, 0.22 mg/cm2 at 1000 ℃, and 0.47 mg/cm2 at 1100 ℃. Compared to other alloys tested with similar conditions, Fe-Cr-Ni-Al MEA presented by this study achieves economic benefits with relatively excellent high-temperature oxidation resistance. The outstanding high-temperature oxidation resistance of the MEA presented by this study is due to the evenly distributed dense stable Al2O3 layer formed regardless of temperature conditions. In addition, the alloy did not have breakaway oxidation, which can occur as oxidation progresses.

Effect of Nano-Y2O3 Content on Microstructure and Mechanical Properties of Fe18Cr Films Fabricated by RF Magnetron Sputtering.

In this work, FeCr-based films with different Y2O3 contents were fabricated using radio frequency (RF) magnetron sputtering. The effects of Y2O3 content on their microstructure and mechanical properties were investigated through scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), inductive coupled plasma emission spectrometer (ICP) and a nanoindenter. It was found that the Y2O3-doped FeCr films exhibited a nanocomposite structure with nanosized Y2O3 particles uniformly distributed into a FeCr matrix. With the increase of Y2O3 content from 0 to 1.97 wt.%, the average grain size of the FeCr films decreased from 12.65 nm to 7.34 nm, demonstrating a grain refining effect of Y2O3. Furthermore, the hardness of the Y2O3-doped FeCr films showed an increasing trend with Y2O3 concentration, owing to the synergetic effect of dispersion strengthening and grain refinement strengthening. This work provides a beneficial guidance on the development and research of composite materials of nanocrystalline metal with a rare earth oxide dispersion phase.

On a new Ti-carbooxinitride redistribution driven microcrack healing mechanism in an annealed 14YWT nanostuctured ferritic alloy

FCRD NFA-1 is a larger best practice heat of Fe-14Cr nanostructured ferritic alloy (NFA) dispersion strengthened by a large population of nano-scale Y2Ti2O7 nano-oxides. NFA-1 was produced by ball milling FeO with argon atomized Fe-14Cr-3W-0.35Ti-0.25Y (wt.%) powders, followed by hot extrusion at 850 °C, followed by 1 h annealing and multipass cross-rolling at 1000 °C to form an ≈ 10 mm thick plate. Cross-rolling results in a large population of {001}<110> cleavage system microcracks (MCs) running on planes parallel to the plate faces. A series of high temperature vacuum anneals were carried out from 1100 to 1300 °C for 1 and 5 h to explore the possibility of self-healing, not only of the MCs, but also macroscopic fatigue cracks. Most notably, a new crack self-healing mechanism was discovered, involving the redistribution and coarsening of initially fine matrix Ti, C, O and N precipitates (TiCONs) to bridging sites on the crack faces. The relocated TiCONs grow and coarsen, thereby consuming a volume of the adjoining Fe-Cr matrix. The displaced matrix atoms diffuse to further fill the crack. Extensive microstructural, fractographic and mechanical property characterization measurements show that MC self-healing begins at 1100 °C/1 h, increasing with annealing time and temperature, up to completion at 1300 °C. The NFA-1 grains and texture are relatively stable. However, at 1300 °C the nano-oxides and dislocations coarsen or recover, respectively, leading to significant softening.

Microstructure of Oxide Dispersion Strengthened Steel With Cr Content Variation

Microstructure and phase distribution of innovative Oxide Dispersion Strengthened (ODS) steel based on Fe-Cr-ZrO2 particularly for application at high temperature reactor with variation of Cr content was analysed. The alloy was synthesized with Cr composition variation of 15, 20 and 25 wt.% Cr, while zirconia dispersoid kept constant at 0.50 wt.%. The samples was synthesized by mechanical alloying comprising of high energy milling for 3 hours followed by vibrated compression with iso-static load at 20 ton. The final consolidation was performed via sintering process for 4 minutes using the Arc Plasma Sintering (APS) technique, a new method developed in BATAN especially for synthesizing high temperature materials. The samples were then characterized by means of scanning electron microscopy (SEM) with energy dispersed X-ray (EDX) analysis capability and X-ray diffraction. The mechanical property of hardness was measured using standard Vickers micro hardness tester to confirmed the microstructure analysis. The results show that the microstructure of the ODS alloy samples in all variation of Cr content consists generally of cubic Fe-Cr matrix phase with small of porosity and Zirconia particles distributed homogenously in and around the matrix grains. The achievable hardness was between 142 and 184 HVN dependent consistently on Cr content in which Cr element may cause grain refining that in turn increase the hardness.

Microstructure and High-Temperature Wear Performance of FeCr Matrix Self-Lubricating Composites from Room Temperature to 800 °C.

FeCr matrix high-temperature self-lubricating composites reinforced by Mo, Ag, and CuO were fabricated by the powder metallurgy technique. The tribological behaviors of composites were studied at temperatures up to 800 °C. The CuO content was optimized according to the tribological results. Mo showed an obvious lubricating effect when it converted into MoO3. The bimetallic oxide system formed high-temperature solid lubricants with low shear strength. CuO reacted with MoO3 and formed CuMoO4 and Cu3Mo2O9. The composites showed an increase in the friction coefficient with the increase of CuO. However, the wear rates decreased with the increase of CuO. The critical threshold at which there was a transition of friction coefficients and wear rates from room temperature (RT) to 800 °C was 10 wt.% CuO. The Fe(Cr)-14% Mo-10.5% Ag-10% CuO composite showed the most reasonable high-temperature tribological behaviors. This was ascribed to the synergistic effects of silver, Mo, in situ formed solid lubricants (metal oxides and salt compounds), and the stable oxide film on the worn surfaces. At elevated temperatures, the dominant wear mechanism was oxidation wear.

Determination of curie temperatures and thermal oxidation behavior of Fe-Cr matrix composites produced by hot pressing

Fe-Cr matrix composite alloys were produced by hot pressing (HP) method then their microstructure, mechanical, and oxidation properties were examined in detail. In composite alloys produced with Fe-Cr and Fe-Cr-C alloys, the main phases were Fe, M(Cr, Fe)7C3 phases. In addition, Fe2B/Cr7BC4 phases were formed by addition of B, and Cr, Fe and FeCr-rich regions were found in the microstructure. When the Fe-Cr alloy showed the lowest hardness (average 218 HV0.025), the hardness value was recorded as 286 HV0.025 by the addition of alloying elements. With the help of Thermal Gravimetry (TG) it was determined that the alloys exhibit two different oxidation behaviors. It was also found that the Curie temperatures of the ferromagnetic alloys in operation were decreased with the addition of B to the FeCr alloy.

High temperature oxidation behavior of a high Al-containing ferritic heat-resistant stainless steel

The high temperature oxidation behavior of a high Al-containing ferritic heat-resistant stainless steel at temperatures of 800, 900, and 1000°C in air were studied in isothermal oxidation tests. The results showed that the isothermal oxidation kinetic curves obtained at different temperatures followed the parabolic law, and the weight gain per unit at 1000°C was significantly higher than that at 800 and 900°C. The oxidation rate at 1000°C was about three times faster than that at 800 and 900°C. Continuous and compact multicomponent oxide films mainly composed of Cr2O3, Al2O3, spinel MnFe2O4, and MnCr2O4 were obtained at 800 and 900°C. The oxide film started delaminating at 1000°C; the outer layer was composed of Cr2O3, spinel MnCr2O4, and MnFe2O4, the middle layer was composed of Fe2O3 and Fe-Cr matrix, and the inner layer was composed of Al2O3 and SiO2. Oxidation resistance at 1000°C was reduced mainly because of porous Fe2O3 and inner oxidation of Al and Si. In addition, the oxidation mechanism was discussed based on kinetic and morphological observations.

The effect of He + irradiation on hardness and elastic modulus of Fe-Cr–40 wt.% TiB 2 composite rod designed for neutron absorbing

Abstract A composite rod, designed for the neutron absorption in nuclear research reactors or Nuclear Power Plants (NPPs), was synthesized by ball milling and subsequent Hot Isostatic Pressing (HIP-ing), similarly to the one reported in Ref. [3]. Its core consists of the oxide dispersion strengthened ferritic Fe-Cr matrix and 40 wt.% of TiB 2 reinforcement, surrounded by a Ti tubing/cladding with a functionally graded interface layer. The Specific Surface Area (SSA) for the ball-milled pre-alloyed powders increases from 0.64 to 2.92 m 2 /g and XRD shows that the crystallite/grain size of TiB 2 is ∼38 nm. Berkovich nanoindentation study after irradiation with 160 keV He + ions at a fluence up to 1 × 10 17 He + /cm 2 shows an initial hardening effect at fluences of 1 × 10 15 and 1 × 10 16 ions/cm 2 . Hardness and elastic modulus of the Fe-Cr -TiB 2 core rapidly drops when the fluence reaches 1 × 10 17 ions/cm 2 . The Ti cladding seems to be relatively impervious to increased radiation fluence since its hardness and elastic modulus change very slightly with increasing ion fluence. The observed changes in the mechanical properties are discussed in terms of vacancy/dislocation loops and He bubble formation in the irradiated microstructure. Although the He-vacancy complexes are widely regarded as being immobile, it is hypothesized here, based on the grazing incidence XRD (GI XRD), that interstitial helium diffuses outward through the boundaries of the (100) hcp-TiB 2 nanograins. As a result, the relaxation of compressive strains due to high concentration of vacancies in a nanograin crystalline lattice finally leads to the hcp-TiB 2 unit cell contraction.

Corrosion Resistance of the Metal Vibration Deposited from Flux-Core Wires Based on the FE–CR–B System

We study the corrosion resistance of coatings obtained by submerged-arc built-up welding. The fused metal is obtained from Fe-based flux-core wires of the Kh10R4G2S grade. On the basis of the results of phase analysis, it was discovered that the fused metal is formed by a FeCr matrix and a solid solution of Fe2B and (FeCr)B borides. This observation was confirmed by the data of spectral analysis. It is demonstrated that the application of horizontal vibration with an amplitude of 300 μm in the course of surfacing leads to the dispersion of the Fe2B and (FeCr)B boride phases down to 2–5 μm2, which increases the resistance of fused metal to abrasive wear. The mass losses in the cases of wear with fixed and nonfixed abrasives are made 2–2.5 times lower. It is also shown that the procedure of surfacing performed in the presence of vibration negatively affects the corrosion resistance of the welded metal obtained from Kh10R4G2S flux-core wires due to the increase in the fracture of cathodic inclusions.

Direct comparison of Fe-Cr unmixing characterization by atom probe tomography and small angle scattering

The fine microstructure obtained by unmixing of a solid solution either by classical precipitation or spinodal decomposition is often characterized either by small angle scattering or atom probe tomography. This article shows that a common data analysis framework can be used to analyze data obtained from these two techniques. An example of the application of this common analysis is given for characterization of the unmixing of the Fe-Cr matrix of a 15-5 PH stainless steel during long-term ageing at 350°C and 400°C. A direct comparison of the Cr composition fluctuations amplitudes and characteristic lengths obtained with both techniques is made showing a quantitative agreement for the fluctuation amplitudes. The origin of the discrepancy remaining for the characteristic lengths is discussed.

The microstructure and mechanical properties of Al-containing 9Cr ODS ferritic alloy

In this study, a 9Cr oxide-dispersion strengthened (ODS) alloy with additional corrosion resistant element Al was fabricated by mechanical alloying (MA) and hot pressing (HP) to explore the impact of Al on the microstructure and mechanical property of a 9Cr ODS alloy. It is found that the Al completely dissolved into the Fe–Cr matrix after milling for 30 h. The minor phases in the Al-containing 9Cr ODS ferritic alloy were investigated by a high-energy X-ray, and were identified to be orthorhombic-YAlO3 (YAP), bcc-Y3Al5O12 (YAG), monoclinic-Al2Y4O9 (YAM), and hexagonal-YAlO3 (YAH). These phases were further confirmed by selected area diffraction pattern (SADP), energy dispersive spectroscopy (EDS), and high resolution transmission electron microscopy (HRTEM). In addition, their volume fractions were also calculated from the integrated intensities. According to the analysis of the particles and their formation sequences, the larger particles (greater than 100 nm) are identified as mainly YAG and Al2O3 particles, while the particles with small size (less than 30 nm) are likely primarily YAM, YAH, and YAP particles. The yielding strength (YS) and ultimate tensile strength (UTS) at RT are 563 MPa and 744 MPa, respectively, while the YS and UTS at 700 °C are 245 MPa and 276 MPa, respectively. Although the addition Al in ODS alloys decreases the strength at RT, the values at high temperature are similar to those obtained for 9Cr ODS alloys strengthened by fine Y–Ti–O particles.

Wear behavior of Fe-Cr-B alloys under dry sliding condition

Purpose – The purpose of this study is to prepare new Fe-Cr-B alloys which have better wear resistance and investigate the wear mechanisms of these alloys tribotesting against SiC ceramic balls under dry sliding process. Design/methodology/approach – Fe-Cr matrix alloys were fabricated using powder metallurgy route. The tribological behaviors of Fe-Cr alloys tribotesting against SiC ceramic balls using a ball-on-disc tribotester were studied at different testing conditions. Meanwhile, microstructure, phases and morphology of worn surfaces were investigated. Findings – The element boron improved mechanical properties and tribological behavior of alloys. The friction coefficients of Fe-Cr matrix alloys did not show obvious difference. The specific wear rates of alloys decreased and then increased because of the brittleness of alloys with the increase of boron content. Fe-21 weight per cent Cr-7 weight per cent B alloy showed the best tribological properties in dry sliding. The wear mechanism of Fe-Cr alloy was plastic deformation and abrasive wear. However, the Fe-Cr-B alloys showed fatigue spalling characteristics. Originality/value – This paper reported a new, cheap and wear-resistant Fe matrix material to prepare mechanical parts in food and mining industrial fields.

Structure and magnetic properties of Fe nanoparticles embedded in a Cr matrix

Abstract The structure of 2 nm diameter (340 atoms) Fe nanoparticles embedded in a Cr matrix was determined using X-ray Absorption Fine Structure (EXAFS) and the magnetic properties studied by Superconducting Quantum Interference Device (SQUID) magnetometry. The thin films were produced by the co-deposition of pre-formed gas-phase Fe clusters synthesised by a gas aggregation source with an atomic vapour of Cr produced by an MBE source. The behaviour was studied as a function of Fe nanoparticle volume fraction in the range 5–20% and was compared to previous results on ferromagnetic nanoparticles in antiferromagnetic matrices. EXAFS showed that the atomic structure in the Cr-embedded Fe nanoparticles is the same as the bulk bcc structure. Whereas alloying between the nanoparticles and matrix material has previously been shown to be very pronounced for Co nanoparticles in antiferromagnetic Mn, it was found that any alloying between Fe nanoparticles and Cr matrix material is limited. For dilute samples of Fe nanoparticles in Cr the measured saturation magnetisation ( M S ) was 1 µ B /Fe atom, which is significantly less than the bulk M S value of 2.22 µ B /Fe atom indicating that the surface of Fe nanoparticles is either antiferromagnetic or non-magnetic. An increase in the volume fraction produces an increase in the value of M S and at a volume fraction of 20%, M S exceeds the value of bulk Fe showing that some Cr spins provide a ferromagnetic contribution. After field cooling below 30 K, all films show Exchange Bias (EB) and an increase of coercivity, which are both much larger for the most concentrated sample. The Cr spins at the surface of the Fe particles play a key role in determining the overall magnetic behaviour.

Dry erosion wear performance of Inconel 718 microwave clad

This paper reports on the dry erosive wear performance of Inconel 718 clads deposited on SS-304 substrates through microwave hybrid heating technique. Clads were deposited using a domestic microwave applicator at 2·45 GHz and 900 W. The microstructural observations of the Inconel 718 clad indicate good metallurgical bonding with the substrate and revealed no visible interfacial cracking. The microhardness of the clads was assessed using a Vicker’s microhardness tester and the average microhardness in the clads was 564±22 HV. Erosive wear performance of the clads was evaluated using an air jet erosion test setup as per ASTM G76 standard. The effect of varying impact angle was studied; the results have been discussed with the help of SEM images of the worn surfaces. It was observed that the improved erosive performance of the Inconel 718 clads was due to presence of strengthening intermetallic phases (Ni3Ti, Ni3Al) in the tough Ni–Fe–Cr matrix.

Energetics, structure and composition of nanoclusters in oxide dispersion strengthened Fe–Cr alloys

Extensive first principle calculations on embedded clusters containing few O, Y, Ti and Cr atoms, as well as vacancies, are performed to obtain the interaction parameters to be applied in Metropolis Monte Carlo simulations, within the framework of a rigid lattice model. A novel description using both pair and triple parameters is shown to be more precise than the commonly used pair parameterization. Simulated annealing provides comprehensive data on the energetics, structure and stoichiometry of nanometer-size clusters at T = 0. Additionally, Metropolis Monte Carlo simulations are carried out at high temperature in order to investigate the dependence of nanocluster composition on temperature. The absolute value of the binding energy per O atom unit increases with cluster size and approaches a constant for large clusters. The presence of Ti and/or vacancies increases the value of this quantity. In alloys without vacancies, clusters show a planar structure, whereas the presence of vacancies leads to three-dimensional configurations. Cr is not part of the nanoclusters, except in alloys without Ti but with vacancies. In the latter case, clusters consist of a core containing O, vacancies, as well as Y and a Cr shell, which have also been observed experimentally. A good agreement between the existing experimental data on the ratios (Y + Ti) : O, Y : Ti, (Y + Cr) : O, and Y : Cr and the simulation results is found. The comparison of experimental data with those obtained by simulations demonstrates that the assumption of nanoclusters consisting of nonstoichiometric oxides that are essentially coherent with the bcc lattice of the Fe–Cr matrix leads to reasonable results.

Chromium enrichment on the habit plane of dislocation loops in ion-irradiated high-purity Fe–Cr alloys

Reduced activation high-chromium ferritic/martensitic steels are candidate materials for Generation IV fission and fusion reactors. To gain knowledge about the radiation resistance of these steels in such environments, the first step is to study the Fe–Cr matrix of this material. For that purpose and to understand ballistic damage by neutrons, self-ion irradiations, with and without simultaneous He injection, were performed on a series of high-purity Fe–Cr binary alloys at 773K. Transmission electron microscopy (TEM) analysis revealed “displacement fringe contrast” inside the dislocation loops. This was attributed to the presence of Cr-enriched zones on their habit plane, which is a defect-free region for body-centered cubic Fe-based alloys. A plausible mechanism is discussed to explain the phenomenon, the first step of which would be the radiation-induced segregation of Cr atoms on the dislocation loop core. Energy-dispersive X-ray spectroscopy in scanning TEM mode and atom probe tomography (APT) gave a coherent quantitative estimate of the Cr concentration in these enriched areas. APT study showed that the enrichment was heterogeneous on the loop plane. Upon in situ annealing up to 900K, the loops and the fringes disappeared completely, without leaving a secondary-phase particle, such as carbide, at their position. Fringes were present until the loop disappeared.

Microstructural evolution of MgAl2O4 oxide-dispersion-strengthened alloy by mechanical milling and hot isostatic pressing

Abstract