Al Partitioning Research Articles

Influence of Ta + Al on the microstructure evolution of two Ru-containing Ni-based single crystal superalloys deposited with γ′ + β NiAl coating at extremely high temperature

The effects of γ′ forming elements Ta + Al on the microstructural evolution of two Ru-containing single crystal (SX) superalloys deposited with γ′ + β NiAl coatings were investigated after thermal exposure at 1140 °C. TCP precipitation, recrystallization, and elemental interdiffusion were particularly investigated to illustrate the influence of Ta + Al on the compatibility between the coating and substrate. After exposure at 1140 °C, two distinct zones were formed under the IDZ, where the SRZ in the higher Ta + Al alloy contained a large number of rod-like and needle-like TCP phases. The other was the SDZ in the lower Ta + Al alloy, which possessed fewer and smaller granular TCP phases. Noteworthily, this discrepancy was attributed to the enhanced tendency of recrystallization in alloy with higher Ta + Al, since higher Ta + Al would result in more negative misfit of γ/γ′ and lower Al partitioning in γ phase, which provided more driving force for recrystallization. Meanwhile, the elemental distribution in the diffusion zone of the alloy with higher Ta + Al was more heterogeneous, and even the RuAl phase was observed in the coating. As the thermal exposure time prolonged, the thickness of the SRZ exceeded that of the SDZ, which significantly reduced the effective load-bearing area of alloy with higher Ta + Al. It could be summarized that a worse compatibility between the coating and the substrate would be introduced with the increase of Ta + Al in the substrate. This study provided valuable insights into the design of coatings and alloy compositions, thereby facilitating the development and application of advanced Ru-containing SX superalloys as well as their corresponding coatings.

Atomic diffusion and microstructural evolution at the Nb/TiAl heterogenous interfaces during annealing at elevated temperatures

The relevant phase transformations induced by atomic diffusion at the vicinity of Nb/TiAl heterogenous interface were systematically investigated by establishing a novel system of TiAl-Nb couple via metallurgical bonding of Nb block and γ-TiAl powders by hot pressing sintering and post-annealing treatments. As a result, the resultant diffusion layer, acting as adhesive region linking the Nb and TiAl substrates, thickened with the increase of annealing temperature and the prolongation of holding time, which could be well described by the developed power function. A series of intermediate phases initially nucleated at the Nb/TiAl interface and grew toward both substrates, which was the result of the exchange, partitioning and recombination of solute atoms in the light of vacancy-mediated substitutional diffusion pattern. The phase transformation path on the Nb side was β→η→η + σ→σ→β0→β0 + γn, while that on the TiAl side was γ→γss→γss+ α→(γ + α)+ (α + β0)→α + β0→β0 + γn. Then the disorder-order transition of α→α2 and β0→B2 occurred during cooling. The progress of phase transformation slowed down as the diffusion proceeded, which was attributed to the decrease in atomic substitution efficiency due to the reduction of atomic concentration gradient and the extension of migration distance. In such a case, the banded σba split into spike σsp along the direction of maximum concentration gradient, and then shrunk to isolated σis until completely dissolved in β0 regime, principally dominated by Ti occupying Nb sublattices. The lenticular γn existed mainly in the form of twins, resulting from the lattice reconstruction in accordance with the orientation relationship of 111γ//110β0 and 1̅10γ//11̅1β0 in the Al partitioning region during the spontaneous fluctuation of atomic concentration of β0 regime. These findings filled some fundamental gaps with respect to TiAl-Nb diffusion reaction system, and afforded profound insights into the development of high-performance TiAl/Nb composites with tailorable microstructure.

Atomistic study of Al partitioning and its influence on nanoscale precipitation of Cu-rich nanocluster-strengthened steels

The intrinsic partitioning behavior of Al and its influence on the nanoscale precipitation of Cu-rich nanocluster-strengthened steels were investigated by using atom probe tomography (APT) and first-principles calculations. The APT results reveal that Al partitions to Cu-rich nanoclusters, which results in a slight decrease in the volume fraction of the nanoclusters. The first-principles calculations indicate that the Al substitution in body-centered cubic (bcc) Cu is more energetically favorable as compared with that in bcc Fe. In addition, the Al partitioning has no significant influence on the chemical driving force and interfacial energy but slightly increases the strain energy for nucleation, thereby increasing the critical energy for the formation of Cu-rich nanoclusters. As a result, the nanoscale Cu precipitation is slightly inhibited in the Al-containing ferritic steels. In addition, the effects of Al on the precipitation strengthening response were quantitatively evaluated, and the results indicate that the degree of precipitation strengthening depends majorly on the combined effect of cluster size and inter-cluster spacing.

Strength enhancement of a bi-lamellar PM Ti–6Al–4V alloy without sacrificing ductility by Al partitioning and twinning-induced plasticity effect

Ti–6Al–4V alloy samples with two bi-lamellar microstructures were prepared by cost-effective thermomechanical powder consolidation and two inter-critical annealing treatments; one comprising of annealing at 900 °C followed by air cooling (900-AC alloy) and the other comprising of annealing at 910 °C followed by water quenching and aging (910-Q&A alloy), respectively. Both bi-lamellar microstructures encompass lamellae of primary α phase and β transformed structure which consists of ultrathin secondary α lamellae and β thin layers. Thanks to the higher Al concentration in the primary α lamellae associated with the higher annealing temperature and finer scale of βt structure lamellae, the α and βt lamellae of the 910-Q&A alloy exhibit a higher nano-hardness than that of the 900-AC alloy (5.4 and 4.6 GPa vs. 4.5 and 4.3 GPa) as well as a larger difference in nano-hardness between the α and βt lamellae (0.8 vs. 0.2 GPa). These microstructural and nano-hardness differences lead to a significant enhancement of the yield strength of the 910-Q&A alloy by 200 MPa compared to the 900-AC alloy with almost no decrease of tensile ductility. Quantitative analysis indicates that the hardening of α and βt lamellae accounted for approximately 35% and 28% of the total strength increment, respectively, while the heterostructure induced hardening associated with the inhomogeneity between the α and βt lamellae accounts for approximately 37% of the strength increment. Interestingly, the significantly higher flow stress of the 910-Q&A alloy during tensile deformation induces a significant higher degree of trans-lamellar twinning which is believed to be responsible for mitigating the strain localization and maintaining the good tensile ductility despite of a higher flow stress.

Al partitioning between phase D and bridgmanite at the uppermost lower mantle pressure

Al partitioning between phase D and bridgmanite at the uppermost lower mantle pressure

In situ correlation between metastable phase-transformation mechanism and kinetics in a metallic glass

A combination of complementary high-energy X-ray diffraction, containerless solidification during electromagnetic levitation and transmission electron microscopy is used to map in situ the phase evolution in a prototype Cu-Zr-Al glass during flash-annealing imposed at a rate ranging from 102 to 103 K s−1 and during cooling from the liquid state. Such a combination of experimental techniques provides hitherto inaccessible insight into the phase-transformation mechanism and its kinetics with high temporal resolution over the entire temperature range of the existence of the supercooled liquid. On flash-annealing, most of the formed phases represent transient (metastable) states – they crystallographically conform to their equilibrium phases but the compositions, revealed by atom probe tomography, are different. It is only the B2 CuZr phase which is represented by its equilibrium composition, and its growth is facilitated by a kinetic mechanism of Al partitioning; Al-rich precipitates of less than 10 nm in a diameter are revealed. In this work, the kinetic and chemical conditions of the high propensity of the glass for the B2 phase formation are formulated, and the multi-technique approach can be applied to map phase transformations in other metallic-glass-forming systems.

Phase separation and up-hill diffusion in the ordered α2 compound of a γ- Ti-Al-Nb alloy

γ-TiAl alloys are intermetallic materials that are designed for applications in air craft engines. Oftentimes alloying elements are added to improve the corrosion resistance and mechanical properties. Here we investigate microstructural changes that take place when Nb is added to a binary γ-TiAl alloy. Previous studies of the alloy Ti-42Al-8.5Nb have shown that upon Nb addition the orthorhombic O-phase forms out of the parent α2 phase within the alloy’s microstructure. During annealing Nb segregates to the newly formed O-phase while Al and Ti partition to the remaining α2-like matrix phase. The changes these newly formed O-phase domains undergo with time at elevated temperature are not known. Here we use Transmission Electron Microscopy techniques to monitor the distribution of Nb and the size of O-phase domains during long-term annealing between 1 week and 4 weeks at 550°C. The results show that the size of O-phase domains shrinks with annealing time. At the same time the Nb concentration remains largely constant or possibly shows a small decrease.

Trace-element geochemistry of diamond-hosted olivine inclusions from the Akwatia Mine, West African Craton: implications for diamond paragenesis and geothermobarometry

Trace-element concentrations in olivine and coexisting garnets included in diamonds from the Akwatia Mine (Ghana, West African Craton) were measured to show that olivine can provide similar information about equilibration temperature, diamond paragenesis and mantle processes as garnet. Trace-element systematics can be used to distinguish harzburgitic olivines from lherzolite ones: if Ca/Al ratios of olivine are below the mantle lherzolite trend (Ca/Al < 2.2), they are derived from a harzburgitic mantle source, and syngenetic garnets are without exception subcalcic G10 garnets. For harzburgitic olivines that cannot be identified this way, Na and Ca contents can be used: olivine inclusions with < 60 µg/g Na and Na/Al < 0.7 are all harzburgitic, whereas those with > 300 µg/g Ca or > 60 µg/g Na are lherzolitic. Conventional geothermobarometry indicates that Akwatia diamonds formed and resided close to a 39 mW/m2 conductive geotherm. A similar value can be derived from Al in olivine geothermometry, with TAl-ol ranging from 1020 to 1325 °C. Ni in garnet temperatures is on average somewhat higher (TNi-grt = 1115–1335 °C) and the correlation between the two thermometers is weak, which may be not only due to the large uncertainties in the calibrations, but also due to disequilibrium between inclusions from the same diamond. Calcium in olivine should not be used as a geothermobarometer for harzburgitic olivines, and often gives unrealistic P–T estimates for lherzolitic olivine as well. Diamond-hosted olivine inclusions indicate growth in an extremely depleted (low Ti, Ca, Na, high Cr#) environment with no residual clinopyroxene. They are distinct from olivines from mantle xenoliths which show higher, more variable Ti contents and lower Cr#. Hence, most olivine inclusions in Akwatia diamonds escaped the refertilisation processes that have affected most mantle xenoliths. Lherzolitic inclusions are probably the result of refertilisation after undergoing high-degree melting first. Trivalent cations appear to behave differently in harzburgitic diamond-hosted olivine inclusions than lherzolitic inclusions and olivine from mantle xenoliths. Some divalent chromium is predicted to be present in most olivine inclusions, which may explain high concentrations up to 0.16 wt% Cr2O3 observed in some diamond inclusions. Strong heterogeneity of Cr, V and Al in several inclusions may also result in apparent high Cr contents, and is probably due to late-stage processes during exhumation. However, in general, diamond-hosted olivine inclusions have lower Cr and V than expected compared to mantle xenoliths. Reduced Na activity in depleted harzburgites limits the uptake of Cr, V and Sc via Na–M3+ exchange. In contrast, Al partitioning in harzburgites is not significantly reduced compared to lherzolites, presumably due to uptake of Al in olivine by Al–Al exchange.

High-P amphibolite-facies metamorphism in the Adrar–Souttouf Metamafic Complex, Oulad Dlim Massif (West African Craton margin, Morocco)

The Oulad Dlim Massif represents the northern segment of the Mauritanide belt that thrusts over the western margin of the Reguibat Shield, north of the West African Craton (WAC). This belt includes various metamorphic units of Archean, Neoproterozoic and Palaeozoic ages that were stacked and thrust eastward during the Variscan orogeny. The core of the Oulad Dlim Massif comprises the Adrar–Souttouf Metamafic Complex that represents a large tectonic unit made of high-grade mafic rocks and vast exposures of amphibolites. A characterisation of the metamorphism in these amphibolites is essential to understand the relationships of the Oulad Dlim Massif with the southern segments of the Mauritanide belt and to provide constraints on the geodynamic evolution of the western margin of the WAC. Here we determine the P–T conditions of metamorphism of two samples of garnet amphibolites collected at the northernmost end of the Adrar–Souttouf Metamafic Complex. The samples show a main mineral assemblage of garnet+low-Ti pargasite+oligoclase+phengite+epidote+quartz+rutile±paragonite±K-feldspar. We calculated their P–T conditions using the amphibole–plagioclase NaSi–CaAl exchange thermometer, and the garnet–amphibole–plagioclase–quartz and the amphibole–plagioclase Si–Al partitioning barometers. The thermobarometric results indicate that this mineral assemblage was formed at high-P amphibolite-facies conditions at 650–700°C and 10–13 kbar. The observed stability of paragonite and phengite reveals fluid-absent conditions or the presence of a fluid phase with reduced H2O activity during the peak of metamorphism. We found no relicts of eclogite-facies mineral assemblage in the garnet amphibolites. This contrasts with the eclogite-facies metamorphism found due south in the Tarf Magneïna unit. This suggests that the northernmost end of the Adrar–Souttouf Metamafic Complex may have been buried to shallower depths than the units further south, probably during the Variscan orogeny. However, precise absolute radiometric dating of the high-P amphibolite-facies metamorphism is required to confirm these findings.

Aluminum fractionation in acidic soils and river sediments in the Upper Mero basin (Galicia, NW Spain).

This study aims to determine aluminum fractions in the fine earth of acidic soils under different land uses (forest, pasture and cultivation) and in the river bed sediments of the headwater of the Mero River in order to identify and quantify Al-bearing phases to assess Al mobility and potential bioavailability (environmental availability) in the monitoring area. Sequential extraction is used to evaluate the Al partitioning into six fractions operationally defined: soluble/exchangeable/specifically adsorbed, bound to manganese oxides, associated with amorphous compounds, aluminum bound to oxidizable organic matter, associated with crystalline iron oxides, and residual fraction (aluminum within the crystal lattices of minerals). The mean concentration of total aluminum (24.01gkg-1) was similar for the three considered uses. The mean percentage of the aluminum fractions, both in soils and sediments, showed the following order: residual fraction≫amorphous compounds≈crystalline iron oxides>water-soluble/exchangeable/specifically adsorbed>bound to oxidizable organic matter≈Mn oxides. However, in the soils, the amorphous compounds and water-soluble/exchangeable/specifically adsorbed fraction showed considerable differences between some types of uses, the percentage of aluminum linked to amorphous compounds being higher in forest soils (16% of total Al) compared to other uses (mean about 8% of total Al). The highest values of water-soluble/exchangeable/specifically adsorbed Al were also found in forest soils (mean 8.6% of the total Al versus about 4% of pasture and cultivation), which is consistent with the lower pH and higher organic matter content in forest soils. Nevertheless, the potentially bioavailable fraction (sum of the first three fractions) is low, suggesting very low geoavailability of this element in both soils and sediments; hence, the possibility to affect the crops and water quality is minimal.

Solidification interface transformation and solute redistribution of FeCrAl stainless steel

The solid–liquid interface structures and the Al segregation in the solidifying front were investigated by directional solidification and electron probe microanalysis. The results show that the interface structure would vary greatly with solidification velocity, the morphology of which changed from planar at 1 µ m/s, dendrite at 25 µ m/s, cellular–dendrite at 50 µ m/s, to hexagon cell at 100 and 150 µ m/s and finally high-velocity cell at 200 µ m/s. The stability of the planar interface was mainly influenced by Al segregation. The narrow solidification temperature range benefited the transformation of dendrite to cell-dendrite. Regarding Fe, Cr as solvents and Al as solute, the secondary dendrite arm spacing when solidification velocity is 25 µ m/s could be predicted by the formula which was widely adopted in binary alloy system, and the predicted values fit well with the experimental results. The Al partition coefficient was found to be 0.96, and it indicated that there existed slight Al segregation when solidification velocity is 1 µ m/s, which was the main stabilizing factor of the planar interface. Al segregation is not obvious at velocities above 25 µ m/s and the interface transformed to various structures when solidification velocities were higher than 25 µ m/s.

Effects of AL addition on microstructure and mechanical properties of AlxCoCrFeNi High-entropy alloy

The effects of Al on microstructure and mechanical properties of AlxCoCrFeNi (x=0.1, 0.75 and 1.5) high-entropy alloys were systematically studied by using various characterization methods. It was found that the crystalline structure of AlxCoCrFeNi high-entropy alloy varies markedly with Al content, which changes from the initial single face-centered cubic (fcc) to fcc plus ordered body-centered cubic (bcc) structure (B2) and then to a duplex bcc structure (A2+B2) as the Al content is increased. The chemical composition analysis reveals that Al primarily partitions to B2 phase, suggesting Al is a stabilizer of B2 structure. With increasing Al content, more Ni and Al partition to the B2 phase due to the very negative mixing enthalpy of Ni and Al, and another phase enriched in Cr and Fe transforms from fcc to disordered bcc. Nano indentation measurements show that the hardness of AlxCoCrFeNi high-entropy alloy increases with Al content, accompanied by the decrease of ductility. The stability of single-phase solid solution in AlxCoCrFeNi HEAs is deduced from various criteria. Combined with the experiment results of other similar HEA systems, such as AlxCoCrFeNiCu, the effects of Al addition on the microstructure of AlxCoCrFeNi HEAs are discussed based on the Gibbs free energy of all competing phases and the fundamental properties of constituent elements. The aim of current study is to provide experimental evidence to establish a correlation between the microstructure and mechanical properties to search for high-entropy alloys with higher performances.

Solid-Solution Metal Partitioning for the Upper Mero River Basin (NW Spain)

This study determines the seasonal variability of metal partition coefficients [aluminium (Al), iron (Fe), manganese (Mn), copper (Cu), and zinc (Zn)] and analyses the importance of suspended sediments (SS), dissolved organic carbon (DOC) concentrations, pH, and discharge (Q) on the seasonal variability of metal partition coefficients (KDs) in the headwaters of the Mero River catchment, which drains an agroforestry area in northwestern Spain. Metal partition coefficients were used as an approach to relate dissolved and particulate fractions. Water samples were collected over 3 years (2005–2008) at the catchment outlet. The mean metal dissolved concentrations were: Fe (43.5 μg L−1) > Al (23.3 μg L−1) > Zn (1.8 μg L−1) > Mn (1.2 μg L−1) > Cu (0.3 μg L−1). Partition coefficients followed the order Mn > Al > Fe > Zn > Cu, and their values exhibited low variability. Al, Cu, and Zn partition coefficients presented the greatest values in summer, except during 2007–2008, when the greatest KDs value was observed in autumn, whereas the KDs of Fe showed the greatest values in winter. The KD of Mn has no seasonality. For Al, Cu, and Zn, the seasonal SS concentrations were closely related to Kd. For Fe, Kd was more closely related to DOC concentration than to SS concentration.

The high-pressure stability of chlorite and other hydrates in subduction mélanges: experiments in the system Cr2O3–MgO–Al2O3–SiO2–H2O

The solubility of chromium in chlorite as a function of pressure, temperature, and bulk composition was investigated in the system Cr2O3–MgO–Al2O3–SiO2–H2O, and its effect on phase relations evaluated. Three different compositions with X Cr = Cr/(Cr + Al) = 0.075, 0.25, and 0.5 respectively, were investigated at 1.5–6.5 GPa, 650–900 °C. Cr-chlorite only occurs in the bulk composition with X Cr = 0.075; otherwise, spinel and garnet are the major aluminous phases. In the experiments, Cr-chlorite coexists with enstatite up to 3.5 GPa, 800–850 °C, and with forsterite, pyrope, and spinel at higher pressure. At P > 5 GPa other hydrates occur: a Cr-bearing phase-HAPY (Mg2.2Al1.5Cr0.1Si1.1O6(OH)2) is stable in assemblage with pyrope, forsterite, and spinel; Mg-sursassite coexists at 6.0 GPa, 650 °C with forsterite and spinel and a new Cr-bearing phase, named 11.5 A phase (Mg:Al:Si = 6.3:1.2:2.4) after the first diffraction peak observed in high-resolution X-ray diffraction pattern. Cr affects the stability of chlorite by shifting its breakdown reactions toward higher temperature, but Cr solubility at high pressure is reduced compared with the solubility observed in low-pressure occurrences in hydrothermal environments. Chromium partitions generally according to $$X_{\text{Cr}}^{\text{spinel}}$$ ≫ $$X_{\text{Cr}}^{\text{opx}}$$ > $$X_{\text{Cr}}^{\text{chlorite}}$$ ≥ $$X_{\text{Cr}}^{\text{HAPY}}$$ > $$X_{\text{Cr}}^{\text{garnet}}$$ . At 5 GPa, 750 °C (bulk with X Cr = 0.075) equilibrium values are $$X_{\text{Cr}}^{\text{spinel}}$$ = 0.27, $$X_{\text{Cr}}^{\text{chlorite}}$$ = 0.08, $$X_{\text{Cr}}^{\text{garnet}}$$ = 0.05; at 5.4 GPa, 720 °C $$X_{\text{Cr}}^{\text{spinel}}$$ = 0.33, $$X_{\text{Cr}}^{\text{HAPY}}$$ = 0.06, and $$X_{\text{Cr}}^{\text{garnet}}$$ = 0.04; and at 3.5 GPa, 850 °C $$X_{\text{Cr}}^{\text{opx}}$$ = 0.12 and $$X_{\text{Cr}}^{\text{chlorite}}$$ = 0.07. Results on Cr–Al partitioning between spinel and garnet suggest that at low temperature the spinel- to garnet-peridotite transition has a negative slope of 0.5 GPa/100 °C. The formation of phase-HAPY, in assemblage with garnet and spinel, at pressures above chlorite breakdown, provides a viable mechanism to promote H2O transport in metasomatized ultramafic melanges of subduction channels.

The β'-α' Interaction: a Study of early Stages of Phase Separation in a Fe-20%Cr-6%Al-0.5%Ti Alloy

The temporal evolution of the microstructure resulting from phase separation into Fe-rich (α), Cr-rich (α¢), and Fe(Ti,Al) (β¢) phases of a Fe-20Cr-6Al-0.5Ti alloy has been analyzed by thermoelectric power measurements (TEP). The early stages of decomposition and the evolution of the three-dimensional microstructure have been analyzed by atom probe tomography (APT). The roles of Cr, Al, and Ti during the decomposition process have been investigated in terms of solute partitioning between the phases. Analysis of proximity histograms revealed that significant Al and Ti partitioning occurs, which is consistent with theoretical calculations. The results indicate that as the α-α¢ phase separation proceeds, Al and Ti are rejected into the α phase, which causes the β¢ phase to nucleate on the surface of the α¢ phase.

The Stability of Plagioclase in the Upper Mantle: Subsolidus Experiments on Fertile and Depleted Lherzolite

Plagioclase peridotites are important markers of processes that characterize the petrological and tectonic evolution of the lithospheric mantle in extensional tectonic settings. Studies on equilibrated plagioclase peridotites have documented continuous chemical changes in mantle minerals in response to plagioclase crystallization, potentially tracing the re-equilibration of mantle peridotites up to very low pressure.This experimental study provides new constraints on the stability of plagioclase in mantle peridotites as a function of bulk composition, and the compositional and modal changes in minerals occurring within the plagioclase stability field as a function of P^T^bulk composition. Subsolidus experiments have been performed at pressures ranging from 0·25 to 1·0 GPa, and temperatures ranging from 900 to 12008C on fertile and depleted anhydrous lherzolites modelled in the system TiO2^Cr2O3^Na2O^CaO^FeO^ MgO^Al2O3^SiO2 (Ti,Cr-NCFMAS). In the fertile lherzolite (Na2O/CaO 1⁄4 0·08; XCr 1⁄4 0·07) a plagioclase-bearing assemblage is stable up to 0·7 GPa, 10008C and 0·8 GPa, 11008C, whereas in the depleted lherzolite (Na2O/CaO 1⁄4 0·09; XCr 1⁄4 0·10) the upper limit of plagioclase stability is shifted to lower pressure.The boundary between plagioclase lherzolite and spinel lherzolite has a positive slope in P^T space. In a complex chemical system, the plagioclase-out boundary is multivariant and sensitive to the XCr value [XCr 1⁄4 Cr/(Cr þAl)] of spinel.This latter is controlled by the reaction MgCr2O4 þ CaAl2Si2O8 1⁄4 MgCrAlSiO6 þ CaCrAlSiO6, which is a function of the Cr^Al partitioning between spinel and pyroxenes, and varies with the XCr value and chromite/ anorthite normative ratio of the bulk composition. Within the plagioclase stability field, the Al content of pyroxenes decreases, coupled with an increase in the anorthite content in plagioclase, and Ti and XCr in spinel with decreasing pressure; these chemical variations are combined with systematic changes in modal mineralogy governed by a continuous reaction involving both plagioclase and spinel. As a consequence, the composition of plagioclase varies significantly over a rather narrow pressure range and is similar at the same P^T conditions in the investigated bulk-rocks.This suggests the potential application of plagioclase composition as a geobarometer for plagioclase peridotites.

Aluminum partitioning during phase separation in Fe–20%Cr–6%Al ODS alloy

Phase separation in a commercial Fe–20 wt.%Cr–6%Al oxide dispersion-strengthened PM 2000 steel has been characterized with a local-electrode atom probe after isothermal aging at 708 K and 748 K for times up to 3,600 h. A progressing decrease in the Al content of the Cr-rich α′ phase was observed with time at both aging temperatures. The Al partitioning trend was consistent with theoretical calculations. However, the experimentally observed Al partitioning factor was significantly lower than the predicted equilibrium value. A ∼10 nm diameter, roughly spherical, Al- and Ti-enriched β′ Fe(AlTi) phase was also observed.

Mechanisms of Al 3+ incorporation in MgSiO 3 post-perovskite at high pressures

Aluminum is the fifth most abundant element in the Earth's mantle, yet its effect on the physical properties of the newly found MgSiO 3 post-perovskite ( PPv), the major mineral of the Earth's Dʺ layer, is not fully known. In this paper, large-scale ab initio simulations based on density functional theory (DFT) within the generalized gradient approximation (GGA) have been carried out in order to investigate the substitution mechanism of Al 3+ into PPv at high pressures. We have examined three types of Al substitution in PPv: 6.25 mol% Al substitution via a charge-coupled mechanism (CCM), 6.25 mol% Al substitution via oxygen-vacancy mechanism (OVM), and an oxygen-vacancy Si-free end member Mg 2Al 2O 5. For both the CCM and OVM, five models, where the Al atoms were put in different positions, were simulated at various pressures in the range 10–150 GPa. Our calculations show that the most favorable mechanism is a charge-coupled substitution where Al 3+ replaces the next-nearest-neighbor cation pairs in the PPv structure. The calculated zero-pressure bulk modulus of Al-bearing PPv is 3.15% lower than that of the Al-free PPv. In agreement with previous works, we find that the incorporation of Al 2O 3 slightly increases the post-perovskite phase transition pressure, with the Al partition coefficient K = 2.67 at 120 GPa and 3000 K.

Partitioning of Ca and Al between forsterite and silicate melt in dynamic systems with implications for the origin of Ca, Al‐rich forsterites in primitive meteorites

Abstract— We report the results of dynamic crystallization experiments that were specifically designed to study the dependence of Ca and Al partitioning between forsterite and melt in rapidly cooling Caand Al‐rich melts. The partitioning of Ca between olivine and silicate melt is found to be independent of the cooling rate within the range of 1.5 to 1000°C/hr and at CaO contents of up to 25 wt%. Within analytical uncertainty, our data plot on the equilibrium partitioning curve obtained by Libourel (1999). The partitioning behavior of Al at high cooling rates is more complex. Aluminum is much more heterogenously distributed in the olivine and the co‐existing melt than Ca. But, no systematic trend of Al partition coefficient with cooling rate is observed. We apply the results of the experiments to the formation of meteoritic forsterites with relatively high contents of Ca and Al. Although these forsterites are found frequently inside chondrules, the Ca contents of their host chondrules are far too low to crystallize these high Ca‐forsterites. This is also true for very rapid cooling of chondrule melts. The parental melt of these forsterites requires CaO contents above 20 wt%.

Formation of CaS on Al2O3-CaO inclusions during solidification of steels

In this article, the effect of CaS formation on the evolution of Al2O3-CaO inclusions in low-carbon Al-killed and Ca-treated steel during the solidification process is investigated through high-temperature confocal scanning laser microscopy (CSLM). The inclusions started as mostly liquid-globular inclusions that did not agglomerate with each other on the melt surface but during solidification were seen to change shape into an irregular morphology. The shape change was found to be due to the reaction between the Al2O3-CaO inclusions with the dissolved S and Al in the melt, resulting in the formation of dense CaS shells around the inclusions. The melt composition during solidification, estimated from the observed solid δ-front advance rate, was compared to the thermodynamic limit for CaS precipitation. The observed growth rate of the CaS shell was found to initially increase with decreasing temperature because of the higher, solid δ-front advance rates at lower temperatures, which results in higher rates of S and Al partitioning. Once CaS had precipitated, the inclusions were found to form agglomerates on the melt surface because of fluid flow, initially, and later, the capillary depression.