Increasing Cr Content Research Articles

Diffusion in undoped and Cr-doped amorphous UO2

UO2 fuel pellets are often doped with chromium oxide to obtain favourable properties such as higher density, improved thermal stability, large grain sizes, improved pellet-clad interaction margins, and increased fission gas retention during transients. Chromium has a low solubility limit in UO2, with past experimental work reporting solubility limits ranging between 0.004 to 0.06 wt.% Cr. Due to its low solubility, segregation of Cr ions to the grain boundary may occur. Further, the complexity of these boundaries may be high as observed in other ceramics resulting in disordered or amorphous regions along the boundary, affecting a range of material and operational properties of the fuel pellet. To assess these disordered regions, in this work we study amorphous undoped and Cr doped UO2 systems (containing 10–50 at.% Cr3+) that have been modelled using classical molecular dynamics methods incorporating Cr3+ into the well-used CRG potential library. Diffusion coefficients, pre-exponential factors, and activation energies for diffusion were computed for oxygen ions, assessing the impact of structure and extrinsic species on migration. Oxygen diffusion was observed to be much faster in the undoped amorphous system compared to its crystalline counterpart. Oxygen diffusion in doped systems decreased with increasing Cr concentration, highlighting the importance of additives to retain fission products and other migratory species.

Groundwater conceptual pollution model and related human health hazards, the main dilemma of a desert aquifer near ophiolite complex.

Groundwater is a finite resource in Davarzan region which is located between the ophiolite complex mountain in the north and salty playa at the south. The water samples were analyzed to assess the origin of groundwater pollution and explain links between the disturbed heavy metals composition of the earth's surface and the human health risks. The main heavy metal pollutants in the groundwater are Cr, Fe, As and Pb ions. In general, the groundwater salinity and some elements such as Cr and As are increased along with surface topography and groundwater flow directions from the northern ophiolite highlands recharge area to the adjacent desert discharging zone in the south. Despite the ophiolite complexes being the most enriched in Cr element, the lowest Cr concentration in the groundwater was measured near the ophiolite area, which is in the range of its discharged springs. Based on the groundwater conceptual pollution model, bedrock geochemistry controls the composition of soil and hence that of groundwater. The Cr samples show a direct relation with the EC value indicating that intrusion of salinity from the salt pan is probably another reason for the increased Cr concentration. The results of health risk assessment indicated that the groundwater suffered from significant contamination and if used for long-term without pre-treatment may pose serious health risks to human population via drinking water and irrigation of agricultural fields. This is the first attempt to apply hydrogeological setting along with the source of pollution and its health risk in a desert-ophiolitic area.

Stabilizing nanograined Fe-Cr alloy by Si-assisted grain boundary segregation

• Si promotes GB segregation of Cr in the NG Fe-8Cr-1Si treated by SMGT and annealing. • Cr segregation increases with the decrease of grain size, which is related to high- density GND. • NG Fe-8Cr-1Si has enhanced thermal stability and hardness due to Cr segregation. • Additions attractive to solute can stabilize NG binary alloys with weak segregation tendency. Intentional solute segregation at grain boundary (GB) can effectively stabilize the nanograined alloys by reducing the excess energy and mobility of GB, but it usually works for binary alloys with sufficient GB segregation tendency. Here, we found that the segregation of Cr can be enhanced in a nanostructured Fe-8Cr alloy with insufficient GB segregation tendency through the interaction of another solute Si (1.0 wt.%). After surface mechanical grinding treatment and subsequent annealing, the nanograined Fe-8Cr-1Si is more thermally stable than the nanograined Fe-8Cr, which is mainly attributed to the Si-enhanced Cr segregation as observed by Super-X energy-dispersive X-ray spectroscopy (EDS) mapping system. With the grain refinement to nanoscale, the thermal stability is further improved due to the increase of Cr content at GBs and the precipitates formed at appropriate high temperatures. The present finding provides guidance for the development of advanced nanostructured ternary alloys.

CoS2를 이용한 6가 크롬 폐수 처리에서 pH 및 온도의 영향

The effective treatment of wastewater containing toxic substances produced by human activity continues to be a pressing issue, particularly given the increasingly serious water shortages that occur globally due to development pressure and climate change. In particular, Cr(VI) has been identified by many governments worldwide as a priority pollutant for treatment due to its wide distribution and toxic effects. Thus, there is a need to develop efficient removal methods for this pollutant, including the design of materials with water purification functions. In this study, cobalt sulfide (CoS₂) was used to remove Cr(VI) from water. After 120 min at room temperature (25℃), 30 mg of CoS₂ removed 4.24, 2.68, 2.44, 1.28, and 1.80 mg/L Cr(VI) at a pH of 2, 4, 6.5, 9, and 11, respectively. At pH 2, the Cr(VI) removal efficiency of CoS₂ increased with a higher reaction temperature (19.91 mg/L after 90 min at 55℃), while an increase in the initial concentration of Cr(VI) facilitated the greater removal of Cr(VI) by CoS₂ at pH 11. The results of total chromium testing indicated that there were two mechanisms for the removal of Cr(VI) by CoS₂ at pH 2 and pH 11. At pH 2-9, HCrO₄ ^- ions attract electrons on the surface of CoS₂ and react with hydrogen ions to form water-soluble Cr(III) cations and water. At pH 11, Co³⁺ on the CoS₂ surface reacts with hydroxide ions to generate Co(OH)₃ flocs that adsorb Cr(VI) ions from the solution. The results of this study collectively demonstrate that CoS₂ is effective in the treatment of Cr(VI) at pH 2 and pH 11 and has broad potential for use in a range of applications.

Microstructure and Mechanical Properties of Autogeneous GTA Weldments of Reduced-Activation Ferritic/Martensitic Steel and the Effects of Alloying Elements

Five reduced-activation ferritic/martensitic (RAFM) steels containing different contents of Cr, Ta, and Ti were prepared and the weldability of these alloys were evaluated by autogeneous gas tungsten arc (GTA) welding. After welding, post weld heat treatment (PWHT) was performed at temperatures of approximately 730-750 ℃. The microstructures of all alloys after PWHT consisted of a tempered martensite matrix and two types of precipitates, namely, M<sub>23</sub>C<sub>6</sub> and MX. Both the tensile and yield strength of the GTA welds increased with an increase in Cr and Ta contents due to the enhanced solid solution and precipitation strengthening. An increase in Ti content promoted the precipitation of fine MX particles and thereby suppressed the precipitation of M<sub>23</sub>C<sub>6</sub> carbide. This improved the impact toughness without a considerable loss of strength.

Investigation on storage stability and rheological properties of environment-friendly devulcanized rubber modified asphalt

The application of conventional rubber modified asphalt was limited to its high viscosity, poor storage stability, and environmental pollution. This study proposed a preparation process of environment-friendly devulcanized rubber modified asphalt (DRMA) with superior performance. The storage stability and rheological properties of base asphalt, rubber modified asphalt (RMA), and DRMA with different modifier content were evaluated by segregation test, frequency scanning test, multiple stress creep recovery test (MSCR), linear amplitude scanning (LAS) test, and bending beam rheometer (BBR) test. Modification mechanisms of DRMA were detected by scanning electron microscope (SEM), Fourier-transform infrared spectroscopy (FTIR), and four-component analysis. Test results indicated that interaction between crumb rubber and asphalt was mainly physical and has slight chemical reaction. The swelling process of devulcanized rubber (DVR) was adequate and absorbs more light components, which was crucial to improve rheological properties. The storage stability of DRMA was less susceptible to the content of DVR. Besides the crack resistance of DRMA with 18% DVR dosage, the storage stability, high-temperature performance, fatigue resistance, and low-temperature performance of DRMA were markedly superior to that of RMA if the modifier dosage is the same. The rutting and fatigue resistance of RMA was reinforced and then weakened slightly with the increase of CR content, while that of DRMA was enhanced continuously, implying that more waste crumb rubber can be added to asphalt after devulcanization. DRMA was expected to be widely applied owing to its superior performance and environmental features.

Effect of Short-Range Ordering on the Grain Boundary Character Distribution Optimization of FCC Metals with High Stacking Fault Energy: A Case Study on Ni-Cr Alloys

The critical roles of short-range ordering (SRO) in the grain boundary character distribution (GBCD) optimization of Ni-Cr alloys with high stacking fault energies were experimentally studied by thermomechanical treatments. It is found that, with the enhancement of the SRO degree (or the increase in Cr content), the dislocation slip mode changes from wavy slip to planar slip, and even deformation twins (DTs) appear in the cold-rolled Ni-40at.%Cr alloy. Within the lower level of Cr content (≤20 at.%), the optimized result of GBCD is conspicuous with the increase in Cr content. As the Cr content is higher than 20 at.%, the GBCD optimization of Ni-Cr alloys cannot be further enhanced, since the cold rolling induced DTs would hinder the growth of twin related domains during subsequent annealing.

High temperature corrosion resistance of FeCr(Ni, Al) alloys as bulk/overlay weld coatings in the presence of KCl at 600 °C

The long-term high temperature corrosion behavior of FeCr(Ni, Al) alloys, both in bulk form and as overlay welds, was investigated in a new experimental setup, in the presence of KCl at 600 °C. All the alloys experienced breakaway oxidation, with the bulk materials exhibiting better corrosion resistance than the overlay weld coatings with similar compositions. All results indicate diffusion-controlled kinetics, and the analysis revealed an improved corrosion behavior connected to an increased Cr content in the inward-growing scale.

High-temperature Oxidation Properties of TiC-reinforced Mo Matrix Composites with Cr Element Added

This work studied Mo-TiC-xCr (x = 0.0, 0.5, 1.0 wt.%) matrix composites with different component gradients prepared by Spark Plasma Sintering (SPS). The high-temperature oxidation test was conducted at 1200 ℃ in the atmosphere to study the influence of Cr on the high-temperature oxidation behavior of TiC-reinforced Mo matrix composites. The phase composition and morphology of the oxide film were analyzed using X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS), and the goal was to determine the effect of the Cr content on the high-temperature oxidation properties of the Mo-TiC-xCr (x = 0.0, 0.5, 1.0 wt.%) matrix composites. The results showed that with increasing oxidation time, the oxide film on the surface of Mo alloy will crack and fall off easily, and the porosity gradually increases. Oxygen underwent vigorous oxidation through the pores and the inside of the matrix, and a protective MoO2 internal oxide film disappeared. With increasing Cr content, the high-temperature oxidation performance of the Mo alloy improved, the thickness of the oxide film decreased, warping of the surface oxide film was inhibited, the porosity decreased, and volatilization of MoO3 was inhibited. After oxidation, the surface oxide film was mainly composed of TiO2, MoO3, Cr2O3 and Cr2(MoO4)3 phases. The TiC particles dispersed in the matrix were oxidized to TiO2 to inhibit the oxidation of the alloy. The Cr formed a protective oxidation film of Cr2O3, which effectively reduced the porosity and delayed the volatilization of the MoO3, thus improving its oxidation performance.

Effect of Cr on Isothermal Structure Transformation of Iron Oxide Scale

Herein, the effect of different contents of Cr on the isothermal structure transformation of FeO is systematically studied by a thermogravimetric analyzer, and it is found that the temperature range of eutectoid transformation of FeO in Fe–Cr alloy is 300–500 °C. Combined with the experimental data, the kinetic model of isothermal phase transformation of FeO is established based on Johnson–Mehl–Avrami–Kolmogorov equation, and the time–temperature–transformation curve of isothermal transformation of FeO is predicted, which follows the law of C curve, and the C curve shifts backward with the increase of Cr content. Therm‐calc thermodynamic software calculates the Fe–O phase diagrams of different Cr contents. Combined with the experimental results, the effect mechanism of Cr content on FeO eutectoid transformation is clarified. With the increase of Cr content, the stability of FeO decreases, and under the same isothermal temperature and time, high Cr content has greater undercooling, a higher driving force of phase transition, and the starting time of eutectoid transformation is earlier than that of lower Cr content.

The role of Cr content on the corrosion resistance of carbon steel and low-Cr steels in the CO2-saturated brine

The selection of appropriate materials for the transportation pipelines is of vital importance to ensure the safety operation in Carbon Capture, Utilisation and Storage (CCUS). To clarify the effects of Cr content in steel on the resistance against general and localised corrosion, electrochemistry methods combined with pH measurements and various surface analysis techniques were implemented on X65, 1Cr, 3Cr and 5Cr steel samples in a CO2-saturated solution at 60 °C and pH 6.6 during 192 h of immersion. Additionally, thermodynamic and kinetic analyses of the formation of the corrosion products on carbon steel and low-Cr steels were performed. The results show that the general corrosion resistance increased with rising Cr content without the presence of significant corrosion products formation. However, with the formation and development of the corrosion products, the general corrosion resistance reduced with the increase in Cr content. The formation of the compact crystalline FeCO3 on X65 and 1Cr steel surfaces offered superior general corrosion protection, while cannot provide enough localised corrosion protection. By contrast, the double-structural corrosion product layers on 3Cr and 5Cr steels notably suppressed the localised corrosion, but providing poor protection against general corrosion over long immersion periods. This study reveals the contributions of Cr content on general and localised corrosion resistance at various periods, providing references for material selection and evaluation in CO2 environments relevant for CCUS.

Microstructure and oxidation behavior of Co–Cr–Ta ternary alloys

The improvement of oxidation resistance of alloys is one of the key points to develop components for ultra-high temperature applications. The formation of a CrTaO4 oxide layer has attracted attention because of its thermal stability and good protection against oxidation above 1000 °C where more commonly used protective oxide layers such as Cr2O3 show their limits. This study investigates the microstructure evolution and oxidation resistance of the Co-Cr-Ta ternary alloys in comparison to the Calphad predictions. The Co-Cr-Ta samples tested here are composed of an FCC Co-rich phase and a C15 laves phase, and an increase of Cr content was linked to the formation and growth of an additional σ phase. Thermogravimetric analysis, together with the isothermal oxidation tests in air at 1200 °C proved that alloys exhibited good oxidation resistance formed a continuous CrTaO4 layer. Also, optimization of Co-Cr-Ta composition with improved oxidation resistance was attempted using data-driven strategy and the results demonstrated the potential of such approach for the future of alloy design.

Leaching and geochemical evaluation of oxyanion partitioning within an active coal ash management unit

Field monitoring combined with laboratory leaching characterization and geochemical speciation modeling were used to identify important geochemical parameters and controlling mechanisms for leaching of oxyanions from coal ash in a disposal site. The site consisted of dry stacked ash on top of a former surface impoundment. Constituents leached from the dry stacked ash served as sources for the soluble fractions of constituents in underlying impoundment ash. Weathering of field ash was evidenced by the identification of calcite and ettringite. Porewater concentrations of As, B, and V in the field pH range (9–11) and liquid-to-solid ratio (L/S) condition (∼0.6 L/kg-dry) were primarily controlled by Ca-bearing precipitates (Ca-arsenate, B-substituted carbonate solid solution, Ca-vanadate, and V-substituted ettringite solid solution). The leachable fraction of Mo as a function of pH was limited by the available content in the solid, with aqueous concentrations of Mo being a function of L/S. Leaching behavior of Cr and Se was sensitive to the redox changes (from suboxic to oxic conditions) during field ash collection and laboratory leaching tests, including oxidative dissolution of insoluble Cr(III)-oxides and conversion of Se(0) in the solid to dissolved Se(IV)/Se(VI). Oxidation resulted in increased Cr concentrations from < 0.00005 (in porewater) to 0.001–0.1 mg/L (in laboratory eluates). The identified geochemical parameters and leaching-controlling mechanisms provide the bases for understanding and estimating the partitioning of oxyanions in other disposal sites with alkaline and suboxic conditions. This case study also provides a typical example of laboratory test results interpretation and field leaching behavior estimation for other studies.

Clarifying the effect of a small amount of Cr content on the corrosion of Ni-Mo steel in tropical marine atmospheric environment

The effect of Cr content on the corrosion behavior of Ni-Mo steel in the tropical marine atmospheric environment is systematically investigated. The increase of Cr content accelerates the aggregation and the enrichment of Ni element, and thus promotes the formation of an inner electronegative film which attracts positively charged ions including Mo6+, Mo4+, Ni2+ and Cr3+ ions, and repels negatively charged Cl- ions. Cr promotes the formation of more nano-FeOOH and the conversion of γ-FeOOH into α-FeOOH. A small amount of chromium promotes the formation of micro-pits at the initial stage, and with time, those micro-pits gradually accumulate into pieces.

Synthesis of Ruddlesden-Popper LaSrFe1-Cr O4 phases (x = 0.0, 0.2, 0.4, 0.6) by glycine-nitrate combustion process: Effect of Cr doping on magnetic, optical and photocatalytic properties

This work presents the study of new Ruddlesden-Popper phases, LaSrFe 1- x Cr x O 4 ( x = 0.0, 0.2, 0.4 and 0.6), synthesized by glycine-nitrate combustion method. In this research, we are for the first time reporting on a complete Ruddlesden-Popper series ( n = 1) for the photocatalytic degradation of a dye. The effect of Cr 3+ doping on the crystal structure, optical and magnetic properties of the LaSrFeO 4 phase has also been systematically explored via different characterization techniques. Rietveld refinement of PXRD data indicates the formation of single phase compounds with tetragonal symmetry in the space group I 4/ mmm . The results show a decrease in the lattice parameters a and c , as well as the unit cell volume V , with increase in Cr 3+ doping. The energy band gap ( E g ) values calculated using the DRS spectra decreases with increase in Cr concentration. The magnetic measurements revealed dominant anti-ferromagnetic (AFM) interactions in all the phases and the appearance of weak ferromagnetic (FM) interactions with Cr 3+ doping. The results are discussed by considering the increase in Fe 3+ ─O─Cr 3+ FM interaction in addition to the Fe 3+ ─O─Fe 3+ and Cr 3+ ─O─Cr 3+ AFM super-exchange interactions induced due to random distribution of Fe 3+ and Cr 3+ ions at the octahedral site. Rhodamine B, as a dye pollutant model, was used for evaluation of photocatalytic activity of the prepared phases. Enhanced photocatalytic activity was observed with the addition of Cr 3+ substitution. The results show ~99 % degradation for x = 0.6 while only ~76 % degradation is shown for x = 0.0 sample in 40 min under visible light irradiation. • New RP-type LaSrFe 1- x Cr x O 4 phases synthesized by glycine-nitrate combustion method. • E g values decreased with increase in Cr concentration. • Dominant AFM interactions in all the phases and the appearance of weak FM interactions with Cr doping. • Enhanced photocatalytic activity was observed by Cr doping.

Impact of different Cr contents on microstructural evolution and mechanical behaviour of CoCrxCuFeMnNiV high-entropy alloys

The CoCrxCuFeMnNiV (x = 0, 0.25, 0.5, 0.75, 1) high-entropy alloy was prepared via vacuum arc smelting, and the effect of different Cr contents on the microstructure evolution and mechanical properties of it was evaluated. The results show that the CoCuFeMnNiV alloy is composed of FCC1+FCC2 phases, while Cr-added CoCrxCuFeMnNiV alloy is composed of FCC1+FCC2+sigma phases; moreover, the amount of sigma phase increases with the Cr content. Through calculations, a phase formation criterion suitable for the CoCrxCuFeMnNiV alloy was established. It was observed that adding Cr does not affect the early solidification behaviour of the FCC1 and FCC2 phases; however, it affects the stability of the FCC1 phase and promotes the transformation of this phase into the sigma phase, as well as secondary precipitation of a nanoscale copper-rich phase on this phase. The formation of sigma phase has a significant strengthening effect on the alloy but reduces its plasticity. The CoCr0.5CuFeMnNiV alloy has favourable comprehensive properties; specifically, its yield strength, compressive strength, microhardness, and fracture strain are 820 MPa, 1050 MPa, 320 HV, and 22.5%, respectively. With an increase in Cr content, the alloy fracture mode changes from ductile to brittle.

Investigations on the Phase Transformations, Equilibria and Athermal ω in Ni-Ga-Cr Ternary System.

In the present work, the phase equilibria of the Ni-Ga-Cr ternary system at 850, 1000 and 1150 °C were experimentally investigated to provide the essential data for developing the high-entropy shape memory alloys (HESMAs) containing Ni, Ga and Cr. At 850 °C, in the Ni-rich portion, the B2 phase shows equilibrium with the L12 phase when the Cr content is less than 10.49 at. %, while displaying the equilibrium with L12 and BCC phases when the Cr content increases. The B2 + L12 + BCC changes into B2 + FCC + BCC three-phase equilibria from 850 to 1150 °C, as the L12 phase region becomes narrow with rising temperature. The two-phase equilibrium, B2 + BCC, was found at all the isothermal sections investigated. Other three-phase equilibria were also discovered: B2 + α-Cr3Ga + BCC and Ni2Ga3 + α-Cr3Ga + L at 850 °C, and B2 + α-Cr3Ga + L at 1000 °C. Significantly, an athermal ω intermetallic compound with the space group of P3¯m1 was observed distributing at the B2 phase in the quenched Ni45.98-Ga25.50-Cr28.52, Ni42.23-Ga15.70-Cr42.07 and Ni16.54-Ga13.63-Cr69.83 (at. %) alloys after being annealed at 1150 °C for 10 days. The high-resolution transmission electron microscopy (HRTEM) results reveal that the ω shows a crystallographic orientation of [11¯0]B2//[112¯0]ω; (111)B2//(0001)ω with the B2 parent phase.

Molecular dynamics-based characterisation of early oxide in Fe/Cr alloys

The addition of Cr is known to work as an effective prevention against oxidation in Fe-based alloys. This can be attributed to the peripheral oxide, the structure of which is dependent on the composition of the alloy. Using Molecular Dynamic (MD) calculations with a Reactive Force Field (ReaxFF) potential, we successfully identify several oxide structures arising during initial oxide formation in [100] Body-Centered Cubic (BCC) Fe1−xCrx alloys. This structure was found to differ between the surface and bulk. Further, by gradually increasing the Cr content, we manage to track the structural Cr-dependence of both the bulk- and the surface oxide. Both in the surface and bulk, phase changes in the oxide are observed as the Cr content increases. At the surface, this takes place at around 30%–50%Cr. In the bulk, it takes place at around 30% and 70% Cr.

Investigation of Cr content, second phase, and sintering temperature on characteristics of WMoVTiCr refractory high entropy alloys

Refractory high-entropy alloys (RHEAs) are important candidate materials for the design of advanced high-temperature structural materials. In this work, WMoVTiCr RHEAs were synthesized by mechanical alloying. The effect of Cr variation, second phase formation, and sintering temperature on microstructural evolution and mechanical properties of the model alloys were investigated. The results demonstrate that the alloy mainly consists of BCC solid solution phase, Ti-rich oxides, and a trace of Cr-rich phases. The hardness and compressive strength increase significantly with increasing Cr content, which could be ascribed to the dispersion strengthening of second-phase particles and the effect of Cr on the solid solution strengthening. However, the presence of a high amount of Cr content can lead to the reduction of compressive strain due to an increase in the brittle Cr-rich phases. Moreover, a lower sintering temperature caused an incomplete solid solution, thereby reducing material performance.

Evolution of morphology and magnetic properties of α-Fe2−xCrxO3 nanoparticles in dependence on Cr concentration

Hematite, α-Fe2O3, one of the most widely studied magnetic compounds known to mankind since ancient times, has amazing properties and continues to puzzle researchers to this day. In this work, we found exceptionally strong changes in the morphology and magnetic properties of hematite nanoparticles upon doping with rather low concentrations of chromium. Nanoparticles α-Fe2−xCrxO3 with x varied from zero to 0.15 were prepared by solvothermal method using FeCl3·6H2O, CrCl3·6H2O, and 1,2-diaminopropane, (CH2)3(NH2)2, as precursors and temperature 180 °C for 6 h. The X-ray diffraction data showed the presence of only one crystal phase α-Fe2O3 for all x meanings with growing average crystalline size as the x increases. For the parent α-Fe2O3 powdered samples, the transmission electron microscopy revealed very long narrow nanorods with rounded ends that transform into spindles of increasing volume and cross section diameter as the Cr concentration increases. Vibrating sample magnetometry (VSM) and magnetic circular dichroism (MCD) study of the nanoparticles behavior in dependences on temperature and magnetic field revealed many unusual peculiarities. Among them there are strong increase of NPs magnetization with x increase (usually opposite picture is observed), the Morin transition temperature TM dependence on magnetic field, VSM sees the Morin transition in samples with small x only while MCD – in all samples, magnetization field dependences also are different when measured with VSM and MCD, and so on. Qualitative considerations were proposed to explain the observed phenomena.