Partitioning Of Cr Research Articles

The effect of Si on metal–silicate partitioning of siderophile elements and implications for the conditions of core formation

We have determined the liquid metal–liquid silicate partitioning of Ni, Co, Mo, W, V, Cr and Nb at 1.5 GPa/1923 K and 6 GPa/2123 K under conditions of constant silicate melt composition with variable amounts of Si in the Fe-rich metallic liquid. Partitioning of Ni, Co, Mo, W and V is sensitive to the Si content of the metal with, in all five cases, increasing Si tending to make the element more lithophile than for conditions where the metal is Si-free. In contrast, metal–silicate partitioning of Cr and Nb is, at constant silicate melt composition, insensitive to the Si content of the metal. The implications of our data are that if, as indicated by the Si isotopic composition of the silicate Earth ( Georg et al., 2007; Fitoussi et al., 2009), the core contains significant amounts of Si, the important siderophile elements Ni, Co, W and Mo were more lithophile during accretion and core formation than previously believed. We use our new data in conjunction with published metal–silicate partitioning results to develop a model of continuous accretion and core segregation taking explicit account of the partitioning of Si (this study) and O (from Ozawa et al., 2008) between metal and silicate and their effects on metal–silicate partitioning of siderophile elements. We find that the effect of Si on the siderophile characteristics of Ni, Co and W means that the pressures of core segregation estimated from these elements are ∼5 GPa lower than those derived from experiments in which the metal contained negligible Si (e.g., Wade and Wood, 2005). The core–mantle partitioning of Cr and Nb requires that most of Earth accretion took place under conditions which were much more reducing than those implied by the current FeO content of the mantle and that the oxidation took place late in the accretionary process. Paths of terrestrial accretion, oxidation state and partitioning which are consistent with the current mantle contents of Ni, Co, W, V, Cr and Nb lead to Si and O contents of the core of ∼4.3 wt.% and 0.15%, respectively.

Distribution of Cd, Ni, Cr, and Pb in Amended Soils from Alicante Province (SE, Spain)

Heavy metals can be transferred from soils to other ecosystem parts and affect ecosystems and human health through the food chain. Today the use of biosolids to improve the nutrient contents of a soil is a common practice. Contamination of soils by potentially toxic elements (e.g., Cd, Ni, Cr, Pb) from amendments of biosolids is subject to strict controls within the European Community in relation to total permissible metal concentrations, soil properties and intended use. In this context, heavy metal concentrations were studied in agricultural soils devoted to vegetable crops in the province of Alicante (SE Spain), where an intensive agriculture takes place. This study is aimed at ascertaining the chemical partitioning of Cd, Ni, Cr, and Pb in agricultural soils repeatedly amended with sludge. Selected soil properties relevant to control the mobility and bioavaibility of heavy metals were analyzed for the general characterisation of these agricultural soils. The distribution of chemical forms of Cd, Ni, Cr, and Pb in five biosolids-amended soils was studied using a sequential extraction procedure that fractionates the metal into soluble-exchangeable, specifically sorbed-carbonate bound, oxidizable, reducible, and residual forms. The biosolids incorporation has modified the soil composition, leading to the increment of heavy metals. The residual, reducible, and carbonate-sorbed forms were dominant. Detailed knowledge of the soil at the application site, especially pH, CEC, buffering capacity, organic matter, clay minerals, and clay content, is essential.

Comparison of sequential extraction and principal component analysis for determination of heavy metal partitioning in sediments: the case of protected Lagoon El Kelbia (Tunisia)

A sequential extraction procedure applied to surface sediments from El Kelbia Lagoon determined the partitioning of Ba, Co, Cr, Cu, Ni, Sr, V and Zn among evaporites, carbonates, Fe–Mn oxides, organic matter and silicates. To validate the procedure, the sequential extraction results (SER) were compared to principal components analysis (PCA) using major and trace element concentrations and mineralogical quantitative data of surface sediments. SER showed that a part of Sr was highly mobile; Cu, Sr, and Zn and a part of Co and Ni were mobile depending on pH conditions; Cr and V were strongly bound to silicate phases; Co and Ni were partitioned between carbonates, oxides and silicates, and a great part of Ba and Sr were bound to organic matter or sulfides. An agreement was found with PCA in terms of partitioning among minerals for most trace elements. Moreover, the absence of correlation between Ca concentrations and the abundances of calcite, gypsum or dolomite could be explained by an important fraction of Ca bound to organic matter. Also, unexpected negative correlations between abundances of smectite and illite could be explained by a transformation of illite into smectite. Thus, SER and PCA were mostly convergent, which enabled: (1) validation of the extraction procedure used, and (2) refinement of interpretations of the origin and relations between minerals.

A study of phase transformation in a TiAlNb alloy and the effect of Cr addition

Abstract The phase transformation paths, transformation temperatures and phase equilibria of Ti–45Al–27Nb and Ti–45Al–22Nb–5Cr (at%) alloys were evaluated over a temperature range from 865 °C to 1600 °C. Both alloys solidified as single β-phase and transformed to γ + σ phases upon slow cooling. The addition of Cr did not affect the β → γ transformation temperature upon slow cooling. In contrast, the temperature, at which the σ-phase formed, was reduced noticeably. Upon heating, the temperature at which the β-phase evolves from the γ + σ microstructure was found to decrease significantly with the addition of Cr. In the ternary alloy the formation of the γ-phase could not be retarded on quenching, however, the substitution of Nb with Cr allowed for the retainment of the β-phase to room temperature. These results are explained by the partitioning of Cr into the β-phase, which in addition to thermodynamic stability reduces the kinetics of transformations at lower temperatures.

Total and extractable heavy metal, phosphorous and sulfur concentrations in slaker grits from the causticizing process of a pulp mill for use as a soil amendment

The utilization of solid residues allows industry to reduce and recycle waste materials and produce beneficial by-products. The total heavy metal concentrations in slaker grits originating from the chemical recovery process of a pulp mill are lower than the current Finnish statutory limit values for fertilizers used in agriculture and in forestry. In addition, slaker grits are strongly alkaline pH (13.1), have elevated total calcium concentration (331 g kg−1; d.w.), contain calcite (CaCO3), as well as a neutralizing value of 39.4% (Ca equivalents, d.w.). Thus, 0.96 tonnes of slaker grits would be required to replace 1 tonne of a commercial ground limestone product. This indicates that slaker grits have great potential as an industrial residue-based fertilizer and liming agent for use in agriculture and forestry. Since, before such use, it is necessary to assess the mobility of elements in the grits, the three-stage BCR sequential extraction procedure was carried out. Results from sequential extractions showed that the partitioning of Mn (47%), Co (47.1%), Ni (43.1%) and S (94%) was highest in the exchangeable fraction, in which acetic acid (CH3COOH) is used as extractant. The partitioning of Fe (96.5%) and Ti (75.6%) was highest in the easily reduced fraction, in which hydroxylamine hydrochloride (NH2OH–HCI) is used as extractant. The partitioning of Cr (90.8%), V (75.4%), Zn (78.3%), Ba (62.1%), Al (99.4%) and P (99.8%) was highest in the oxidizable fraction, in which hydrogen peroxide (H2O2) is first applied to a heated medium (i.e.85°C) for dissolving organic matter, after which ammonium acetate (NH4COOH) is used as extractant.

The applications of inorganic and organic acids for the treatment of heavy polluted sewage sludge and the evaluation of the remaining metal with sequential chemical extraction

Sludge is classified as difficult solid waste that requires special arrangements for disposal because of its noxious properties. The acid extraction method is characterized by fast removal of metals. Following this, this paper examined how the concentration and the contact time of the acids affect the removal of metals from the sludge. The sludge was further treated with HNO3, HCl and CH3COOH in different concentrations and different contact times. 5%, 10% 15%, 20% and 30% v/v of HNO3, HCl and CH3COOH for 10, 20 and 30 min of contact time were applied. The final result indicates that a sufficient metal amount, about 60%, is removed by the application of HNO3 (using 20% v/v HNO3), 50% is removed by the application of HCl (using 20 v/v HCl) and 30% is removed by the application of CH3COOH (using 20% v/v CH3COOH). Also, this paper presents the result of the sequential chemical extraction procedure (SCEP) which was applied in order to observe the partitioning of Cd, Cu, Cr, Fe, Ni, Pb and Zn. SCEP was applied as well in the treated samples in order to observe where the remaining metals load. The final results indicated that the remaining metal is further stabilized.

Mineralogy and Distribution of Cd, Ni, Cr, and Pb in Biosolids-Amended Soils From Castellón Province (NE, Spain)

Heavy metals can be transferred from soils to other portions of the ecosystem, with effects at both the ecosystem level and on human health from transmission through the web chain. For several years, this environmental issue has concerned scientists. In this context, heavy metal concentrations were studied in agricultural soils devoted to vegetable crops in the province of Castellón (NE Spain), in areas of intensive agriculture. Currently, the use of biosolids to improve the nutrient contents of the soil is a common practice. Contamination of soils by potentially toxic elements (e.g., Cd, Ni, Cr, Pb) from amendments of biosolids is subject to strict controls in relationship to total permissible metal concentrations, soil properties, and intended use within the European Community. This study is aimed at determining the chemical partitioning of Cd, Ni, Cr, and Pb in agricultural soils repeatedly amended with sludge. The distribution of chemical forms of Cd, Ni, Cr, and Pb in five biosolids-amended soils was studied using a sequential extraction procedure that fractionated the metal into soluble-exchangeable, specifically sorbed-carbonate bound, oxidizable, reducible, and residual forms. The biosolids incorporation has modified the soil composition, leading to the increment of heavy metals. The residual, reducible, and carbonate-sorbed forms were dominant. The mineralogical association of the clay fraction was illite, kaolinite, and chlorite.

Fractionation of Cu, Pb, Cr, and Zn in a Soil Column Amended with an Anaerobic Municipal Sewage Sludge

The objective of this study was to investigate the changes in the chemical partitioning of Cu, Pb, Cr and Zn within a column of soil incubated with an anaerobic sewage sludge (ANSS) for 2.5 months. The soil was irrigated during the incubation period. A sequential extraction method was used to fractionate these metals into exchangeable, weakly adsorbed, organic, Al oxide, Fe–Mn oxide, and residual, respectively. ANSS was applied at a loading rate of 69 Mg ha−1. The soil is a Dystric Cambisol with low pH (<3.8), low CEC [<10 cmol(+) kg−1 below the first 4 cm depth], and low base saturation (<7%). The addition of the ANSS caused a decrease in concentrations of Cu, Pb, and Cr in the A1 horizon, and an increase in the concentrations with depth. Below the A1 horizon, concentrations of Cu increased uniformly (~1 mg cm−1), and the greatest increases were observed in the residual, Fe–Mn oxides, and weakly adsorbed fractions. Maximum increases in Pb occurred at 4–9 cm of depth (1.6 mg cm−1), and mainly affected the weakly adsorbed fraction. Chromium essentially accumulated at the limit between the A2 and the Bw horizons (1.1–1.5 mg cm−1) as residual and organic bound forms, probably through particulate transport. Zinc mainly accumulated in the A1 horizon (2.9 mg cm−1) as exchangeable Zn. At depth, Zn increments were predominantly observed in the residual fraction. The results of this study thus demonstrate the redistribution of contaminants into different chemical pools and soil layers after sludge amendment.

Solid speciation and mobility of potentially toxic elements from natural and contaminated soils: A combined approach

The study area (Szklary Massif, SW Poland) comprises three sites of different soil provenance: (1) natural serpentine Cambisol, (2) anthroposol situated on waste dump and (3) cultivated Inceptisol developed on glacial tills next to the dump. Potentially toxic elements (PTE) have either lithogenic or anthropogenic origins in these sites. The chemical partitioning of Co, Cr, Cu, Ni, Pb and Zn among solid forms was determined by sequential extractions coupled with direct mineral identifications (SEM, electron microprobe analysis – EMPA, and XRD). Examination of solid residues after several extraction steps was conducted in order to discuss the indirect speciation obtained by the extraction method. Total concentrations of PTE having anthropogenic origin greatly exceed those of lithogenic origin. Mobility of studied PTE is variable in the different environments except for Cr which is always mostly found in residual fractions of extractions. Cu and Pb are more mobile than Cr and Co in all soils. Zn is more stable (Cu > Pb > Ni > Co > Zn >> Cr) in the serpentine soil and cultivated epipedon (Pb > Cu > Zn > Ni > Co >> Cr) than in the anthroposol (Zn > Cu ⩾ Pb > Ni > Co >> Cr). PTE of lithogenic origin are generally less mobile than those from anthropogenic origin except Ni which is more mobile in the serpentine soil. Nonetheless, mineral forms of metals better determine their mobility than metal origin. Identification by direct methods of the PTE mineral form was not possible for metals present at low concentrations (Cu, Pb). However, direct mineralogical examinations of the solid residues of several extractions steps improved the assessment of the PTE solid speciation and mobility, particularly for Cr, Ni and Zn.

Influence of Spartina and Juncus on saltmarsh sediments. II. Trace element geochemistry

Sequential sediment extractions are used to assess the partitioning of Fe, Mn, Cr, Co, Ni, Cu, Zn and Pb, as a function of depth, in the sediments of two pairs of saltmarsh sites, chosen to represent a gradient of macrophyte and macrofaunal density. One pair is located adjacent to a large tidal creek, and includes a nearly unvegetated site and a site densely vegetated by tall-form Spartina alterniflora. The other two sites, located higher in the marsh, are densely vegetated by Juncus roemarianus and sparsely vegetated by short-form S. alterniflora. At each site, trace element distributions among four operationally-defined solid fractions (exchangeable, carbonate, reducible and oxidizable) were measured at 4–5 cm intervals from the sediment water interface to a depth of up to 50 cm. The operationally-defined extractions suggest that in suboxic sediments, Fe, Mn, Cr, Ni, Co and Zn occur in association with Fe and Mn oxyhydroxides. In sulfidic sediments, Fe, Co and Ni probably occur mostly in pyrite, Mn is likely pyritized and may also form rhodochrosite at two of the sites, Cr is likely bound mostly to organic matter, and Zn, Cu and Pb are inferred to occur in association with both sulfides and organic matter. The depth- and site-dependent trace element distributions are in surprisingly good agreement with variations in redox geochemistry inferred from pore water data in a companion study [Koretsky, C.M., Haveman, M., Cuellar, A., Beuving, L., Shattuck, T., Wagner, M., 2008. Influence of Spartina and Juncus on saltmarsh sediments. I. Pore water geochemistry. Chemical Geology]. This demonstrates that, in spite of their inherently operational nature, sequential sediment extractions can yield useful insights into metal speciation, especially when measured in conjunction with pore water geochemistry. Furthermore, these data demonstrate that lateral and vertical variations in saltmarsh pore water chemistry lead to distinct variability in solid phase trace metal distributions.

A Comparative Study of Metal Readsorption in the Application of a Three-Stage Sequential Extraction Scheme and Two Accelerated Versions (Ultrasonic and Single Extractions)

Accelerated methods for partitioning of Cd, Cr, Cu, Ni, Pb and Zn, such as the use of single and ultrasonic ex- tractions are assessed in terms of readsorption and compared with the three-stage sequential extraction scheme (SES) of the Standards Measurements and Testing Programme (SM&T). The standard addition approach was employed for charac- terising the readsorption artefact by applying the above fractionation methods over different certified reference materials (CRMs), BCR 701, BCR 601 (lake sediments), and BCR 141R soil. Ultrasonic extractions provided higher readsorption, mainly for BCR 141R and 601 as compared to conventional SES, the role of ultrasound in the activation of adsorptive sites being significant. The single extraction approach seemed to be inadequate with samples containing large amounts of carbonates such as BCR 141 R but worked well with both lake sediments. The readsorption phenomena are mainly occurring in the most labile fraction (i.e. acid soluble) and has been observed to be matrix dependent. The extent of such phenomenon is also dependent on the extraction methodology, i.e conventional vs accelerated.

Core formation and the oxidation state of the Earth: Additional constraints from Nb, V and Cr partitioning

We have combined metal–silicate partitioning data from the literature with new experimental results at 1.5–8 GPa and 1480–2000 °C to parameterize the effects of pressure, temperature and composition on the partitioning of V, Cr and Nb between liquid Fe metal (with low S and C content) and silicate melt. Using information from the steelmaking literature to correct for interactions in the metal phase, we find that, for peridotitic silicate melts, metal–silicate partition coefficients are given by: log D Nb met/sil = 2.5 log D Fe met/sil + 1.57 - 11 , 930 T - 114 ( ± 43 ) P T ± 0.41 log D V met/sil = 1.5 log D Fe met/sil + 0.582 - 6493 T - 62 ( ± 19 ) P T ± 0.08 log D Cr met/sil = log D Fe met/sil + 0.643 - 4232 ( ± 538 ) T - 22 ( ± 13 ) P T where D i met/sil is the partition coefficient for element i. Partitioning of V and Cr is insensitive to silicate melt composition, but Nb shows a considerable compositional effect. The new data enable us to examine different models of terrestrial accretion and core formation. If we fix the Fe content of the mantle at the current value and use the Ni content of the mantle to estimate pressure of equilibration then temperatures about 1200 °C above the silicate liquidus are required to match vanadium partitioning to the current concentration of V in the mantle. Under these conditions the core concentrations of Si (∼15%) and Nb (>60% of Earth’s budget) are implausibly high. A more realistic approach is to assume that the metal of accreting planetesimals equilibrated at the base of a deep magma ocean whose temperature was close to the silicate liquidus. As the magma ocean deepened in proportion to the size of the Earth the metal was continuously extracted to the core without further re-equilibration in the lowermost mantle. In this case the V, Cr and Nb contents of core and mantle can easily reproduce the expected values provided the Earth became more oxidized as it accreted (O’Neill H. S. (1991) The origin of the Moon and the early history of the Earth—a chemical model. 2. The Earth Geochim. Cosmochim. Acta 55(4), 1159–1172; Wade J. and Wood B. J. (2005) Core formation and the oxidation state of the Earth. Earth Planet. Sci. Lett. 236, 78–95; Wänke H. and Dreibus G. (1988) Chemical-composition and accretion history of terrestrial planets. Philos. Trans. Roy. Soc. Lond. A-Math. Phys. Eng. Sci. 325(1587), 545–557). Increasing oxidation requires the oxidized Fe content of the mantle to increase from 0.5% to 1.0% to the current value of 6.26% as the Earth grew. This model , with magma ocean thickness corresponding to 35% of mantle depth, reproduces the calculated core–mantle partitioning of Ni, and Co and yields a Si content of the core of approximately 6%, in good agreement with cosmochemical estimates.

Valence state partitioning of Cr between pyroxene-melt: Effects of pyroxene and melt composition and direct determination of Cr valence states by XANES. Application to Martian basalt QUE 94201 composition

D Cr augite/melt is approximately double that of D Cr pigeonite/melt in synthetic Martian basaltic samples equilibrated at the same f O 2 . This increase is not related to changing f O 2 and the valence of Cr, but rather to the increased availability of elements for coupled substitution with the Cr 3+ ion, namely Na and Al. The availability of Al and Na to partition into pyroxene is due to delayed nucleation of plagioclase for the QUE 94201 Martian basalt composition. Direct valence state determination by XANES shows that Cr 3+ is the dominant valence state in pyroxene at IW-1, IW, and IW+1. Trivalent Cr is apparently much more compatible in the pyroxene structure than divalent Cr, and thus an increasing D Cr for both augite/melt and pigeonite/melt with increasing f O 2 is a function of the increased activity of Cr 3+ in the crystallizing melt.

Valence state partitioning of Cr and V between pyroxene-melt: Estimates of oxygen fugacity for martian basalt QUE 94201

Based on the partitioning of Cr and V between pigeonite cores and bulk composition, we estimate that martian basalt QUE 94201 crystallized at an fO₂ between IW+0.2 and IW+0.9. These estimates are based on calibration curves for DCr, DV, and DCr/DV (pyroxene/melt) derived from experimental charges that were synthesized at fO₂ conditions of IW-1, IW, and IW+1. We believe our fO₂ estimate is robust because (1) the fO₂ is measured in the earliest crystallizing pyroxenes; (2) the calibration curves are based on the same bulk composition as the natural sample; and (3) that bulk composition represents a melt from the martian mantle, so an accurate DCr and DV are measured. Presently, the two best candidates for martian melts, Y 980459 and QUE 94201, indicate an fO₂ of IW to IW+1 for the upper martian mantle.

Speciation and Environmental Fate of Chromium in Rivers Contaminated with Tannery Effluents

AbstractRedox and size speciation of chromium in rivers contaminated with tannery wastewater was carried out to provide insight into its transport and removal mechanisms. Total chromium was determined with Inductively Coupled Plasma‐Mass Spectrometry and Cr (VI) with Catalytic Adsorption Stripping Voltammetry. For the size speciation, particles were retained with a cartridge filter (cut‐off 1.2 μm) and the total filterable fraction was further fractionated with Tangential Flow Filtration to determine the concentrations of chromium associated with the High Molecular Weight Colloidal (HMWC), Low Molecular Weight Colloidal (LMWC) and Truly Dissolved (TD) fractions. Two fluvial systems of similar sizes, but located in contrasting climatic zones, were selected for comparison: the Sebou‐Fez system in Morocco and Dunajec River‐Czorsztyn Reservoir system in Poland. Particulate Cr dominated in the Sebou‐Fez system (about 90 %); while in the Dunajec‐Czorsztyn system, it represented only 17–53 % of the total chromium in raw water. Still, the partition coefficients [Kd] were of the same magnitude. Chromium (III) was the only form detected in Sebou‐Fez, whereas in Dunajec‐Czorsztyn Cr (VI) was also present with its proportion increasing downstream from the input of tannery wastewater due to the preferential removal of Cr (III). In the filtered water in Morocco a large fraction of Cr occurred in the HMWC fraction (50–70 %) at the two most contaminated sites, while the LMWC and TD forms prevailed at the non‐contaminated sites in the Sebou River. At a very high concentration, in the water in the proximity of tanneries (well above the theoretical saturation level) Cr precipitated as polynuclear Cr‐hydroxide. In Dunajec‐Czorsztyn, the partition of Cr (III) was approximately equal between the HMWC, LMWC and TD fractions, in contrast to Cr (VI) which occurred almost exclusively in the TD fraction. In both systems, Cr (III) was rapidly removed from the water to the sediments. The confluence of the Sebou with the Fez and the Czorsztyn reservoir trapped efficiently Cr (III) preventing its spreading over long distances. Cr (VI) showed conservative behavior and bypassed the Czorsztyn Reservoir. This study provides a first set of data on the partitioning of Cr (III) and Cr (VI) between the particulate, the colloidal and truly dissolved fractions in fluvial systems contaminated with tannery effluents. It also suggests that, in these systems, truly dissolved Cr (III) can be adequately modeled from the total filterable concentrations.

Elemental partitioning in Ru-containing nickel-base single crystal superalloys

Single crystal nickel-base superalloys with spherical, intermediately shaped and cuboidal γ′ precipitates have been observed in Ru-containing superalloys with substantial amounts of Re and W. The γ′ precipitates experienced stress-induced directional coarsening (rafting) during creep deformation at 950 °C and 290 MPa indicative of a wide range of γ–γ′ lattice misfit, which vary from negative to near-zero to positive. The precipitate morphology, lattice misfit and compositions of the γ and the γ′ phases are interrelated, and therefore, the elemental partitioning between the phases were investigated to determine the origin of the unusual microstructural features. TEM-based EDS analysis of the γ and γ′ phases indicate that the combination of Ru and Cr influences the partitioning of Re, which displays a wide range of partitioning behavior within the investigated alloy compositions. In alloys containing 5.7–9.6 wt.% Ru and no Cr, Re partitioned to both the γ and γ′ phases. With high levels of Cr (6.7 wt.%) the Re strongly partitioned to the γ matrix, while in alloys with intermediate Cr and Ru levels, the Re displayed intermediate partitioning behavior. The Ru and Co partitioned nearly equally between the γ and the γ′ phase in all of the experimental Ru-containing alloys investigated. The degree of Cr partitioning was also similar among the alloys investigated, however, it partitioned more strongly to the γ phase. These investigations demonstrate that Ru and Cr control the lattice misfit and precipitate shape, through the associated changes in system thermodynamics that alter the γ and γ′ phase compositions.

Overall kinetics and morphology of the products of austenite decomposition in a Fe-0.46 Pct C-5.2 Pct Cr alloy transformed isothermally above the bay

The overall transformation kinetics of austenite isothermal decomposition above the bay of the time-temperature-transformation (TTT) curve and the eutectoid morphology of the resulting products have been studied in a Fe-0.46 pct C-5.2 pct Cr alloy. Classical lamellar pearlite was formed at high temperatures while complex ferrite plus carbide morphologies, sometimes described as spiky pearlite, arborescent structures, or nonclassical decompositions products of austenite in Fe-Cr-C alloys, formed at low temperatures. While X-ray diffraction of extracted carbides and selected area diffraction-transmission electron microscopy (TEM) showed evidence for a mixture of M3C and M7C3 carbides, thermodynamic calculations results obtained only M7C3 as the equilibrium carbide at the temperatures studied. A tentative explanation for the arborescent morphology is presented, based on the hypothesis of the existence of a drag force or free energy dissipation term that is locally relaxed by the partition of Cr into the carbides at the reaction front, consequently removing Cr from the interface.

Effects of Ru additions on the microstructure and phase stability of Ni-base superalloy, UDIMET 720LI

The effects of Ru additions on the microstructure and phase stability of the Ni-base superalloy U720LI were examined. The results show that the alloys with Ru contents of 0 to 6 wt pct had a dendritic structure, and the volume fraction of the primary γ solid-solution phase in the dendritic core increased with the increase in the Ru content. The Ru additions to U720LI had little effect on suppressing the formation of the σ phase, because the elemental partitioning of Cr, Mo, and Co in the γ solid-solution phase was not influenced by the Ru additions. However, the coarsening of the γ′ phase was restrained by the Ru additions in the U720LI.

Ni基単結晶超合金の組織因子に及ぼすCoの影響

Effect of Cobalt on micro-structural parameters, such as the lattice misfit and partitioning behaviors of alloying elements, of Re-bearing Ni-base single crystal superalloys have been investigated. The second-generation single crystal superalloys, TMS-82+(Ni-7.8Co-4.9Cr-1.9Mo-8.7W-5.3Al-6.0Ta-2.4Re-0.1Hf, in mass%), was compared to a Co-free (0Co) alloy which had the same chemical composition as TMS-82+ except that Co was removed. The lattice misfit of 0Co alloy was found to be more negative value than that of TMS-82+ at any temperature. Additionally, when 0Co alloy was equilibrated at 1100°C, Cr, Mo and W partitioned more into the γ matrix; in contrast, Al and Ti partitioned into the γ′ precipitates. Increase in partitioning of Cr, Mo and W to the γ phase resulted in an increase in the lattice parameter of γ phase. Thus, lattice misfit of 0Co tend to more negative value than that of TMS-82+.

Emissions of Chromium, Copper, Arsenic, and PCDDs/Fs from Open Burning of CCA-Treated Wood

Aged and weathered chromated copper arsenate (CCA) treated wood was burned in an open burn research facility to characterize the air emissions and residual ash. The objectives were to simulate, to the extent possible, the combustion of such waste wood as might occur in an open field or someone's backyard; to characterize the composition and particle size distribution (PSD) of the emitted fly ash; to determine the partitioning of arsenic, chromium, and copper between the fly ash and residual ash; and to examine the speciation of the CCA elements. This work reports preliminary air emission concentrations and estimated emission factors for total particulate matter, arsenic (As), chromium (Cr), copper (Cu), and polychlorinated dibenzodioxins/dibenzofurans (PCDD/F) totals and toxic equivalents (TEQs). The partitioning of As, Cr, and Cu between the emitted fly ash and residual ash is examined and thermochemical predictions from the literature are used to explain the observed behavior. Results indicate a unimodal fly ash PSD between 0.1 and 1.0 microm diameter. In addition to a large carbonaceous component, between 11 and 14% of the As present in the burned CCA treated wood was emitted with the air emissions, with the remainder present in the residual ash. In contrast, less than 1% of both the Cr and Cu present in the wood was emitted with the air emissions. PCDD/F levels were unremarkable, averaging 1.7 ng TEQ/kg of treated wood burned, a value typical for wood combustion. Scanning electron microscopy (SEM) was unable to resolve inorganic particles consisting of Cu, Cr, or As in the wood samples, but X-ray absorption fine structure (XAFS) spectroscopy confirmed that the oxidation states of the CCA elements in the wood were Cu2+, Cr3+, and As5+. SEM examination of the fly ash samples revealed some inorganic microcrystals within the mostly carbonaceous fly ash, while XAFS spectroscopy of the same samples showed that the oxidation states after combustion were mixed Cu+ and Cu2+, Cr3+, and mixed As3+ and As5+. Estimates of the ratios of the mixed oxidation states based on the XAFS spectra were As3+/(total As) = 0.8-0.9 and Cu+/(total Cu) = 0.65-0.7. The Cu and Cr present in the fly ash were determined to coexist predominantly in the two oxide phases CuCrO2 and CuCr2O4. These results indicate that the open burning of CCA-treated wood can lead to significant air emissions of the more toxic trivalent form of As in particle sizes that are most respirable.