Precipitation Of Secondary Phases Research Articles

Corrosion protection and mechanical properties of the electroless Ni-P-MOF nanocomposite coating on AM60B magnesium alloy

Al-based MIL-53 MOF nanostructure was synthesized hydrothermally and then co-deposited in the electroless nickel coating on AM60B magnesium alloy using Zr pretreatment as an eco-friendly underlayer. The MIL-53(Al) nanostructure was synthesized in the form of layered semi-cube crystals with the surface area and mean pore diameter of 985.72 m 2 g −1 and 2.00 nm, respectively. The SEM images captured with two various zooming scales from the surface of the plain and MOF containing electroless layers showed cauliflower-like morphology with even distribution of nodule size. Also, the sub-grains of the plain coating disappeared after incorporation of the MOF. Although, both the normal and nanostructure-containing electroless layers have crystalline-amorphous structure, but the nanocomposite coating showed less crystallinity. The average surface roughness of the plain electroless coating was about 309 nm, which decreased to about 222 nm after incorporation of the MOF. The XRD patterns showed that the characteristic peak of Ni broadened after incorporation of the MOF, probably due to the decreasing of the crystallinity. For the heat-treated normal and MOF containing coatings at 200 °C no phase transition takes place, but new peaks appeared for heat-treated coatings at 400 °C due to the crystallization and second-phase precipitation. The results of the EIS tests showed an increase in the amount of the charge transfer resistance (from 19 to 29 kΩ cm 2 ) after addition of the MOF, which means an improvement in the corrosion resistance. Also, low J corr of the composite coating represents its higher corrosion resistance with respect to the plain coating. The micro-hardness values of the composite coating before and after the heat treatment were higher than the plain coating. Also, the Ni-P-MOF coating has a lower wear rate both before and after the heat treatment due to an improvement in its micro-hardness.

Thermodynamics-guided alloy and process design for additive manufacturing

In conventional processing, metals go through multiple manufacturing steps including casting, plastic deformation, and heat treatment to achieve the desired property. In additive manufacturing (AM) the same target must be reached in one fabrication process, involving solidification and cyclic remelting. The thermodynamic and kinetic differences between the solid and liquid phases lead to constitutional undercooling, local variations in the solidification interval, and unexpected precipitation of secondary phases. These features may cause many undesired defects, one of which is the so-called hot cracking. The response of the thermodynamic and kinetic nature of these phenomena to high cooling rates provides access to the knowledge-based and tailored design of alloys for AM. Here, we illustrate such an approach by solving the hot cracking problem, using the commercially important IN738LC superalloy as a model material. The same approach could also be applied to adapt other hot-cracking susceptible alloy systems for AM.

Precipitation kinetics of secondary phases induced by heat accumulation in the deposit of Inconel 718

The precipitation kinetics were investigated of the secondary phases including γ'', γ', and δ phases formed by the heat accumulation produced in Inconel 718 (IN718) deposition of cold metal transfer (CMT)-based wire arc additive manufacturing (WAAM) process. Heat accumulation from the layer-by-layer deposition causes a difference in the holding time within the aging temperature range for each layer, resulting in a difference in precipitation of secondary phases that enhances hardness. In order to investigate the precipitation kinetics, which lead to establish the WAAM-related heat treatment standard specification unlike that of the wrought or cast IN718, the thermal history was measured during the deposition process and microstructural analysis was performed according to the height of the deposit. In addition, continuous cooling transformation (CCT) and time-temperature transformation (TTT) diagrams were prepared for dendritic and inter-dendritic regions at each height of the deposit. Based on the thermal measurements, the holding time within each precipitation temperature bands for the δ phase and for the strengthening phases were correlated with the ratio of δ phase fraction and hardness depending on the height, respectively. In addition, using the thermal history, Johnson–Mehl–Avrami–Kolmogorov (JMAK) equation was applied to calculate the hardness of the IN718 deposit which was compared with the measured hardness depending on the holding time.

INFLUENCE OF SIGMA PHASE ON GENERAL CORROSION OF AUSTENITIC STAINLESS STEEL

Austenitic stainless steel is mostly used at high temperatures. It is known that during heating of the austenitic stainless steel with increasing temperature and time of annealing a microstructure is changed i.e. there is the precipitation of carbides followed by precipitation of secondary phases as a sigma phase. The presence of the delta ferrite in austenitic stainless steel enhances the formation of the sigma phase. This study represents the influence of delta ferrite on the sigma phase formation and the effect of the sigma phase on general corrosion. Corrosion tests were conducted in the corrosion cell according to Standard ASTM G5, on instrument potentiostat/galvanostat, Princeton Applied Research, model 263A-2, with the software PowerCORR® (Standard, ASTM G5-94). Examinations were carried out in 1% HCl and 10% FeCl3 solutions. The Tafel extrapolation method was used for the investigation of general corrosion. The results of corrosion testing indicated that the intensity of the general corrosion increases with the increase of the delta ferrite and sigma phase contents.

Effect of high-energy ball milling on the microstructure, phase composition and microhardness of the Al–Mn–Cu alloy

X-ray diffraction and scanning electron microscopy methods were used to study the effect of the planetary ball mill treatment time on the morphology, phase composition and microstructure of the Al–Mn–Cu-based alloy granules with and without nanodiamond particles. The phase composition of the alloy was determined by X-ray diffraction after casting and milling for 5–20 h. It was shown that nanodiamond particles promote granule coarsening, and this is especially noticeable with an increase in the milling time up to 20 h. At the same time, the size of initial alloy granules weakly depends on the processing time. Cu-bearing phases of crystallization origin dissolve during mechanical alloying. The lattice constant of the aluminum solid solution decreases after 5-hour treatment to 0.4028–0.4030 nm, and increases with further increasing milling time. Exothermic effects associated with the precipitation of secondary phases were revealed for mechanically alloyed granules during heating. An increase in the milling time reduces the intensity of peaks. The solidus temperature of samples decreased after mechanical alloying. For the nanodiamond-bearing sample, an exothermic effect is observed which can be ascribed to the aluminum carbide formation or oxidation reactions in nanodiamond particles. The maximum microhardness is achieved after 5–10 h of mechanical alloying, and the nanodiamond particles slightly increase the maximum microhardness from 316 to 330 HV. The results indicate the dissolution of copper and manganese in the aluminum solid solution after 5 h of treatment and their precipitation with the increasing milling time. Nanodiamond particles have no effect on the dissolution of elements but accelerate the solid solution decomposition with the increasing treatment time.

Effect of Sr Deficiency on Electrical Conductivity of Yb-Doped Strontium Zirconate.

The effect of Sr-deficiency on microstructure, phase composition and electrical conductivity of SrxZr0.95Yb0.05O3-δ (x = 0.94–1.00) was investigated via X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy and impedance spectroscopy. The samples were synthesized by a chemical solution method and sintered at 1600 °C. According to X-ray diffraction data, the samples with x = 0.96–1.00 were single-phase oxides possessing an orthorhombic perovskite-type structure; while zirconia-based minor phases arouse at x = 0.94, which was confirmed by the electron microscopy. Sr stoichiometry was shown to influence the electrical conductivity. The highest total and bulk conductivities, 6–10−4 Scm−1 and 3–10−3 Scm−1, respectively, at 600 °C in humid air (pH2O = 3.2 kPa), were observed for the x = 0.98 composition. In the temperature range of 300–600 °C, the conductivity of the samples with x = 0.96–1.00 increased with the increase in humidity, which indicates a significant contribution of protonic defects to the charge transport. Electrical conductivity of SrxZr0.95Yb0.05O3-δ was discussed in terms of the defect formation model and the secondary phases precipitation.

Phase composition, crystal structure, and microwave dielectric properties of Nb-doped and Y-deficient yttrium aluminum garnet ceramics

Nb-doped and Y-deficient yttrium aluminum garnet ceramics were designed and synthesized using the solid-state reaction method according to the chemical equation Y3−xAl5NbxO12+x (0 ≤ x ≤ 0.16). The phase composition, sintering behavior, microstructure, and microwave dielectric properties were investigated as functions of the composition and sintering temperature. A single-phase solid solution of yttrium aluminum garnet structure formation was observed in the range of 0 ≤ x ≤ 0.1. Further increments in x prompted the precipitation of the YNbO4 secondary phase at the grain boundary of Y3Al5O12. The complexity of the phase composition degrades the micromorphology and dielectric properties of the ceramics to varying degrees. Transmission electron microscopy results show that the lattice exhibits additional symmetry, which is closely related to the ultrahigh Q×f values of the ceramics. Effectively improving the sintering behaviour and suppressing the secondary phase by simultaneously doping with Nb5+ and reducing the yttrium stoichiometry. Finally, excellent microwave dielectric properties of εr ~ 10.99, Q×f ~ 280,387 GHz (13.5 GHz), and τf ~ − 34.7 ppm/°C can be obtained in x = 0.1 (Y2.9Al5Nb0.1O12.1) sintered at 1700 °C for 6 h.

Distinct Au and Ag precipitation mechanism in the Xiayingfang Au–Ag deposit, North China Craton

The Yan-Liao Au polymetallic ore belt, located in the northern margin of the North China Craton, hosts a number of sizeable hydrothermal Au–Ag deposits with abundant Pb-, Sb-, Bi-minerals. These minerals are closely linked to the Au and Ag precipitation, but are rarely studied. The Xiayingfang deposit (22.5 Mt @ 1.9 g/t Au and 9.9 g/t Ag) is a typical Au–Ag deposit in this ore belt, and has characteristic Pb–Sb–Bi–Au–Ag-mineral assemblages. We conducted an integrated study on the mineral assemblages and geochemical components and sulfur isotopes to constrain the Au–Ag precipitation mechanism at Xiayingfang.The Xiayingfang mineralization can be divided into three stages: (I) quartz-pyrite, (II) quartz-polymetallic sulfide, and (III) quartz-sphalerite-galena-electrum (main Au–Ag ore stage), which comprises the sphalerite-galena-native bismuth-pyrite-electrum (IIIA) and galena-freibergite-bournonite-pyrrhotite-electrum (IIIB) substages. The δ34SV-CDT of the paragenetic pyrite and galena pairs reveals a general temperature drop from 300 to 330 °C (Stage I), through 250–290 °C (Stage II), to 170–250 °C (Stage III). The Stage I and II fluid temperature is higher than the melting point of Bi-melt (271 °C) and Au–Bi eutectic (241 °C), which probably caused the “liquid bismuth collector model”. Bi-minerals are widely distributed in later Stage II, and are represented by the coexisting native bismuth and maldonite with dissolution pits on the surface (occurring as droplet-shape or eutectic texture in Stage II galena), which indicates the temperature evolved to lower than 241 °C. Such inference is also supported by fluid inclusion data (216–286 °C for Stage II). The pH generally shifts from the felspar-illite buffer (∼7.0) in Stages I and II to the illite-calcite buffer (ca. 5.2–5.6) in Stage IIIA, which could lead to Au solubility decrease (but minor impact on Ag solubility) in the fluid, favoring electrum precipitation (19.0–31.2 wt% Ag). Moreover, oxygen fugacity drop is revealed by pyrite stable from the Stage IIIA (–41 to –46) to pyrrhotite stable in Stage IIIB (≤–46), which may have markedly decreased both Au and Ag solubility in the fluid. Meanwhile, freibergite coexisting with bournonite and galena in Stage IIIB is highly heterogeneous under EPMA element map, suggesting that it was mainly derived from retrograde and solid-state reactions. These reactions could effectively remobilize Ag from galena and Pb–Sb–Ag-sulfosalts via decomposition and exsolution during fluid cooling. These processes may have contributed to the second-phase precipitation of electrum with a steep rise of Ag (57.5–77.8 wt%) in Stage IIIB. The different mechanisms of Au and Ag precipitation at Xiayingfang are also consistent with the different Au-/Ag- mineral assemblages, i.e., electrum-pyrite-chalcopyrite-enargite assemblages in Stages I and II, and electrum-maldonite-freibergite in Stage III.

Feedbacks and non-linearity of silicate glass alteration in hyperalkaline solution studied by in operando fluid-cell Raman spectroscopy

Mechanisms, feedbacks and resulting non-linearity during silicate glass alteration in a hyperalkaline carbonate solution were studied through hyperspectral Raman imaging of heated fluid-cells. Our experimental setup enabled in operando visualization and rate measurements of glass dissolution and secondary phase precipitation, complemented by spectral characterization of the phases involved and semi-quantitative monitoring of the ionic strength of the solution close to the glass interface. After initial congruent dissolution of the Ba-bearing soda-lime boroaluminosilicate glass, the formation of a crystalline, saponite-based surface alteration layer (SAL), as well as subsequent zeolite precipitation, witherite coating, and carbonate precipitation within pore spaces of the saponite layer were observed. Two in operando experiments were conducted at ∼ 90 °C for 180 and 260 h that otherwise solely differed in the solution volume (SV) while keeping the surface area constant. The high SV experiment exhibited a transient upward excursion of initial dissolution rates, followed by continuously rapid glass dissolution along with slow SAL growth and sustained oscillations in ionic strength. Contrastingly, in the low SV experiment, glass dissolution monotonically decreased after the onset of rapid SAL growth and no sustained oscillations were observed. We find that growth conditions and resulting properties of the SAL exert dominant, non-linear effects on the evolution of glass dissolution rates. In turn, SAL formation depends on nucleation/growth kinetics and the accumulation of glass-derived solutes at the reaction front. Both, dissolution and precipitation, feedback with solution chemistry and transport processes, together controlling the evolution of the corrosion process. Additionally, fracturing, delamination, and the evolution of surface morphology may affect glass dissolution rates and transport pathways. Such interpretations of decelerating reaction rates in response to the growth of a protective layer are consistent in micro-scale experiments and in outcrop- to global-scale observations, as is the accelerating effect of surface area creation by physical disruption and morphology. Thus, these µm-scale mechanistic insights could help elucidating local to global environmental feedbacks (e.g., erosion or weathering patterns) as well as process dynamics in engineered environments (e.g., nuclear waste disposal) and may assist the improvement predictive models.

Fluid-rock interaction during low-salinity water flooding of North Sea chalks

The injection of low-salinity water into carbonate oil-reservoirs has been the subject of much interest as a potential method to enhance oil recovery (EOR). Low salinity water injection has been shown to alter the geochemical equilibrium in the reservoir, and consequentially potentially change the surface wettability of the reservoir rocks. Nevertheless, the geochemical reaction pathways during low salinity water flooding in carbonates remain poorly understood.In order to provide a more comprehensive insight into the geochemical reaction pathways, a series of batch and core-flooding experiments have been performed at 70 °C. The experiments made use of analogous and authentic reservoir material from the Danish North Sea oil reservoirs, and brines with chemical composition similar to the formation water and injection water available in offshore flooding operations.Results show that both mineral dissolution and the precipitation of new secondary phases can accurately describe the geochemical changes observed in the experimental effluents during the course of a low-salinity water core-flooding scenario.The dissolution of amorphous silica (SiO2) present in the chalk, and the precipitation of a Mg–Si clay mineral, identified here as sepiolite, are key reactions that take place during low-salinity water core-flooding in chalks. The suggested geochemical pathway was successfully fitted by a simplified geochemical simulation which can accurately describe the experimental observations.Surface reactions which take place during modified-salinity water flooding were put into perspective by quantifying their contribution to the overall effluent composition.

Oxidative Dissolution of Arsenic-Bearing Sulfide Minerals in Groundwater: Impact of Hydrochemical and Hydrodynamic Conditions on Arsenic Release and Surface Evolution.

The dissolution of sulfide minerals can lead to hazardous arsenic levels in groundwater. This study investigates the oxidative dissolution of natural As-bearing sulfide minerals and the related release of arsenic under flow-through conditions. Column experiments were performed using reactive As-bearing sulfide minerals (arsenopyrite and löllingite) embedded in a sandy matrix and injecting oxic solutions into the initially anoxic porous media to trigger the mineral dissolution. Noninvasive oxygen measurements, analyses of ionic species at the outlet, and scanning electron microscopy allowed tracking the propagation of the oxidative dissolution fronts, the mineral dissolution progress, and the change in mineral surface composition. Process-based reactive transport simulations were performed to quantitatively interpret the geochemical processes. The experimental and modeling outcomes show that pore-water acidity exerts a key control on the dissolution of sulfide minerals and arsenic release since it determines the precipitation of secondary mineral phases causing the sequestration of arsenic and the passivation of the reactive mineral surfaces. The impact of surface passivation strongly depends on the flow velocity and on the spatial distribution of the reactive minerals. These results highlight the fundamental interplay of reactive mineral distribution and hydrochemical and hydrodynamic conditions on the mobilization of arsenic from sulfide minerals in flow-through systems.

The formation mechanism of micro-nano secondary phase in solid-liquid doped TZM alloy

Among the various secondary phases distributed in the TZM (Titanium-Zirconium-Molybdenum) alloy matrix, the ones that can play a strengthening role are often those with a particle size smaller than 1 μm. In this study, titanium sulfate, zirconium nitrate, and fructose were used as raw materials. The TZM alloy with uniform distribution and an average secondary phase particle size of 940 nm (hereinafter referred to as MN-TZM alloy) was prepared through a solid-liquid doping preparation process. The thermodynamic calculation reveals the formation mechanism of the micro-nano-level secondary phase in the MN-TZM alloy. Thermogravimetric analysis (TG-DSC, TG-MS) and transmission electron microscope (TEM) confirmed the calculation results. The generation process of the micro-nano secondary phase evolution can be divided into four parts. The first stage is the process of solute dissolution at room temperature. In the second stage, doping and drying process take place below 100 °C. Third, reduction and secondary phase precursor incubation processes occur from 100 to 700 °C. Fructose decomposes (100 °C) to obtain organic carbon and water. The decomposition of zirconium nitrate (100 °C) yields NH 3 , H 2 O, ZrO 2 and Zr, while the titanium sulfate decomposes directly at 200– 242 °C to obtain SO 2 , H 2 O, TiO 2 and Ti. In the last stage, the secondary phase nucleates and grows up (700– 1800 °C). A large variety of micro-nano-scale and even nano-scale secondary phase particles have been discovered, including TiO 2 , ZrO 2 , ZrC and TiC. These observations provide necessary information for tracing back the secondary phase precipitation mechanism in the manufacturing process of high-performance molybdenum alloys. • The average grain size of MN-TZM alloy is 940 nm. • The micro-nano secondary phase reduces the average grain size of TZM alloy by 26%. • TZM alloy was prepared from titanium sulfate, zirconium nitrate, fructose and molybdenum powder for the first time. • The formation mechanism of micro-nano secondary phase in TZM alloy was clarified.

Effect of V Content and Heat Input on HAZ Softening of Deep-Sea Pipeline Steel.

In this paper, the welding thermal cycle process of deep-sea pipeline steel was investigated by welding thermal simulation. The microstructure evolution, crystallology and second-phase precipitation behavior of the soft zone of the heat-affected zone (HAZ) were characterized and analyzed by combining scanning electron microscopy, electron back-scattered diffraction, transmission electron microscopy and hardness testing. The results show that HAZ softening appeared in the fine-grained zone with a peak temperature of 900–1000 °C for deep-sea pipeline steel, the base metal microstructure of which was the polygonal ferrite and acicular ferrite. Using V microalloying and low welding heat input could effectively decrease the softening of the HAZ fine-grained region, which was achieved by reducing the effective grain size, increasing the proportion of the dislocation substructures, and precipitating the nanoscale second-phase particles.

GEMAS: adaptation of weathering indices for European agricultural soil derived from carbonate parent materials

Carbonate rocks are very soluble and export elements in dissolved form, and precipitation of secondary phases can occur on a large scale. They leave a strong chemical signature in soil that can be quantified and classified by geochemical indices, and which is useful for evaluating chemical weathering trends (e.g. the Chemical Index of Alteration (CIA) or the Mafic Index of Alteration (MIA)). Due to contrasting chemical compositions and high Ca content, a special adaptation of classical weathering indices is necessary to interpret weathering trends in carbonate-derived soil. In fact, this adaptation seems to be a good tool for distinguishing weathering grades of source-rock types at the continental scale, and allows a more robust interpretation of soil parent-material weathering grade and its impact on the current chemical composition of soil. An increasing degree of weathering results in Al enrichment and Mg loss in addition to Fe loss and Si enrichment, leaching of mobile cations such as Ca and Na, and precipitation of Fe-oxides and hydroxides. The relation between soil weathering status and its spatial distribution in Europe provides important information about the role played by climate and terrain. The geographical distribution of soil chemistry contributes to a better understanding of soil nutritional status, element enrichment, degradation mechanisms, desertification, soil erosion and contamination.

Effect of the nanostructuring by high-pressure torsion process on the secondary phase precipitation in UNS S32750 Superduplex stainless steel

In this work, the precipitation and the morphology of secondary phases after severe plastic deformation (SPD) processing followed by an isothermal treatment was investigated. High-pressure torsion (HPT) was the SPD process carried out on superduplex 2507 (UNS S32750) stainless steel material under P = 6 GPa at room temperature. At this high strain levels (ε up to 170) samples have shown grain size decrease and strained microstructure with high dislocation density and nanostructure features. After a short isothermal treatment at 830 °C, the sigma phase and chromium nitrides were revealed as the main secondary phases identified by scanning and transmission electron microscopy and element analysis by energy dispersive spectroscopy. Scanning precession electron diffraction and automated crystal orientation mapping have been carried out in order to confirm the precipitation of the secondary phases. In fact, the results provide evidence that the precipitation of chromium nitrides seems to be the preferred nucleation site for sigma phase at higher deformation strain, in addition to the intergranular precipitation of sigma. Both the sigma phases nucleated integranularly and besides chromium nitrides are randomly orientated.

Dissimilar linear friction welding of selective laser melted Inconel 718 to forged Ni-based superalloy AD730™: Evolution of strengthening phases

The continuous growth in the manufacture of aerospace components such as blisks has led to an increase in the application of different hybrid materials fabricating methods, and thus the requirements for joining and strengthening of dissimilar welds. According to this goal, selective laser melted (SLM) Inconel 718 was joined with forged AD730™ Nickel-based superalloy through linear friction welding (LFW) in this study. Microstructure variation, specifically with respect to secondary phases precipitation was investigated. The microhardness and strengthening mechanisms of the weldment were also studied. The precipitation (volume fraction and size of particles) at different regions of both sides of the weld line was characterized. Close to the weld line, the dissolution of γ'/γ" and Laves phases and grain refinement occurred which reveals the effects of both compression strain and high temperature on recrystallization and high degree of elemental diffusion in the weld zone (WZ). It is shown that the size, volume fraction, and shape of secondary phases increased and changed (from spherical to long-striped for Laves particles) as we went from the WZ toward the base metal. However, the measured microhardness indicated that the strength of AD730™ alloy depends significantly on the grain size, while strength in SLM Inconel 718 was dominated by shape (or size) and the presence of secondary phases (γ'/γ" and Laves).

The effect of homogenization treatments on the microstructure and mechanical properties of Thermo-Span alloy

The effect of homogenization treatments on the microstructure and tensile properties at room temperature and 650 °C of Thermo-Span alloy are systematically investigated. Laves phases with different sizes and morphologies have been obtained by using three types of homogenization treatments. The results show that the precipitation of small granular secondary Laves phases and γ' phases are inhibited effectively by large blocky primary Laves phases, resulting in high plasticity and low strength of the alloy. The precipitations of secondary Laves phases and γ' phases increase with the dissolution of primary Laves phases, leading to low plasticity and high strength of the alloy. The secondary Laves phases continue to increase and γ' phases slightly decrease with the almost elimination of primary Laves phases, resulting in high plasticity and strength of the alloy. Hence, fewer large blocky primary Laves phases are more favorable for the best match between strength and ductility. The factors affecting the yield strength at room temperature and 650 °C tensile are discussed in detail. Moreover, it is first found the deformation mechanism of Laves phases i.e. the stacking faults shearing in Thermo-Span alloy. In terms of ductility, small granular secondary Laves phases are more favorable than large blocky primary Laves phases.

Mechanisms and kinetics of citrate-promoted dissolution of a uranyl phosphate mineral

The formation of uranyl phosphate precipitates is a remediation strategy to limit the mobility of uranium in contaminated soils. However, exposure to organic ligands, like the plant exudate citrate, can remobilize the uranium. The purpose of this study is to provide a more thorough comprehension of citrate-promoted dissolution of a uranyl phosphate mineral, chernikovite [(H3O)(UO2)(PO4)·3H2O], by determining the extent of uranium release from chernikovite at a wide range of citrate concentrations. We have quantified the kinetics of dissolution and proposed potential mechanisms of chernikovite dissolution to gain a better understanding of the fate of uranyl phosphate precipitates in the environment. Batch dissolution and continuously stirred tank reactor (CSTR) experiments indicate that increasing citrate concentrations from 0.1 mM to 50 mM increases the concentration of dissolved uranium. However, at citrate concentrations of 10 mM and greater, the effectiveness of the ligand to enhance the dissolution decreases. Density functional theory modeling and Raman spectroscopy indicate a strong interaction between citrate and the uranyl ions at the surface of the uranyl phosphate. The interaction between citrate and uranyl suggests the formation of an alteration layer of uranyl-citrate surface complexes, which could impede uranium release at citrate concentrations of 10 mM or higher. Together, these results indicate that the citrate-promoted dissolution of chernikovite is a fast reaction that is hindered by a combination of surface saturation and a secondary-phase precipitation reaction at higher concentrations of citrate and reaction times greater than or equal to 72 min based on the hydraulic residence time of CSTR experiments. These findings improve our understanding of the stability of uranyl phosphate for use in sequestration of uranium groundwater plumes at contaminated sites.

Corrosion and slurry erosion wear performances of coaxial direct laser deposited CoCrFeNiCu1-xMox high-entropy coatings by modulating the second-phase precipitation

Developing high-entropy alloy (HEA) coatings with preferable properties for severe environments is a promising and cost-efficient way. In this work, an original HEA CoCrFeNiCu1-xMox coatings were designed on-demand and successfully prepared on duplex stainless steel (DSS) through coaxial direct laser deposition technique. The first-principles calculation based on the density functional theory was employed to calculate the characteristics of materials. The results showed that the uniformly distributed Cu could stabilize the face-centered cubic phase structure, hinder the transfer of point defects, and activate the cathode. The synergistic effect of Mo and other HEA elements was beneficial in reducing the point defect density of the passive film, and an appropriate amount of Mo was contributed to grain refinement, solid solution strengthening, and precipitation strengthening of the coating. When the ratio of Cu to Mo is 1:3, a small number of nanoscales second sigma phase (Fe-Cr-Mo) appeared between ordered and disordered, which balanced the coating's corrosion and slurry erosion wear performances. Compared with DSS, the corrosion current density of CoCrFeNiCu0.25Mo0.75 coating was reduced by nearly an order of magnitude, the impedance was increased by 2.5 times, and the erosion wear resistance was increased by 4 to 5 times.

Crystallization and dielectric properties of oxyfluoride aluminosilicate glasses added with Na2O

Mineralizers can efficiently promote the transition of barium aluminosilicate (BaAl2Si2O8, BAS) from hexacelsian to celsian (H-M). However, the detailed mechanism regarding the phase transformation of BAS is still unclear, especially in different glass systems. Herein we systematically studied the crystallization behaviors of SiO2-Al2O3-BaF2 glass in supercooled oxyfluoride aluminosilicate liquids with increasing Na2O concentration, and the influence of crystal phases precipitated from the precursor glass on the dielectric properties of oxyfluoride aluminosilicate glass ceramics was analyzed. We observed that the high addition of Na2O promoted no transformation of H-M, but resulted in the precipitation of second phases (e.g., NaAlSi2O6, Ba3AlF9). Our results provide guidance for designing glass ceramics containing celsian (Monoclinic BAS) nanocrystals with superior thermodynamically stable and dielectric characters.