Partitioning Of Si Research Articles

Growth mode of austenite during reversion from martensite in Fe-2Mn-1.5Si-0.3C alloy: A transition in kinetics and morphology

The kinetics and element partitioning during austenite (γ) reversion from lath martensite in Fe-2Mn-1.5Si-0.3C (mass%) alloy have been investigated. Two different types of γ in terms of crystallography, with Kurdjumov-Sachs (K-S) and without K-S orientation relationships with respect to the surrounding tempered martensite matrix, are formed. A transition in kinetics and element partitioning is related to the differences in crystallography. At low temperature, the growth of both types of γ is accompanied with the partitioning of Mn and Si. Consequently, the growth rates and appearance of the K-S and non K-S γ are similar and the reverted γ structure consists of acicular γ dominantly. On the other hand, at high temperature, only the K-S γ grows with partitioning of alloying elements in contrast to the partitionless growth of the non K-S γ, which leads to the formation of coarse globular γ. DICTRA simulation reveals that the variations in driving force for reversion and the interface mobility induce the transition of element partitioning. At large driving force and interface mobility, partitionless growth is possible and consequently coarse globular γ is formed by the fast growth of the non K-S γ at high temperature. On the other hand, reduction of the driving force or interface mobility induces the transition from partitionless to partitioning growth, which results in the dominant formation of thin acicular γ at low temperature.

Compositional analysis on the reverted austenite and tempered martensite in a Ti-stabilized supermartensitic stainless steel: Segregation, partitioning and carbide precipitation

Controlling the amount of reverted austenite at room temperature allows for tailoring of mechanical properties in supermartensitic stainless steels. The austenite reversion and stabilization occurs during inter-critical tempering through partitioning of austenite-stabilizing elements. The degree of partitioning greatly depends on the reversion temperature, which dictates the local equilibrium conditions. Atom probe tomography and energy dispersive spectroscopy in transmission electron microscopy were used to study the austenite reversion mechanism in terms of the elemental distribution of austenite-stabilizing, ferrite-stabilizing and carbide forming elements. Synchrotron X-ray diffraction confirmed that the austenite equilibrium phase fraction was reached after 2.5h of isothermal reversion at 625°C, allowing for direct comparison with thermodynamic and kinetic calculations. However, such soaking time was not enough to produce compositional homogenization in the reverted austenite. The austenite reversion and stabilization mechanism was related mainly to strong partitioning of Ni. Negligible partitioning of Cr, Mo, Si and Ti were observed. Instead, these elements were strongly segregated at the reverted austenite/martensite interfaces. Carbon and Ti played a secondary role in the austenite stabilization through the precipitation of nano-sized Ti (C,N) with partial substitution of Ti by Mo. Virtually carbon-free austenite and martensite were observed away from the interfaces and precipitates.

Incomplete bainite transformation in Fe-Si-C alloys

Bainite isothermal transformation kinetics for Fe-(1.5% and 3%)Si-0.4%C alloys (mass%) was investigated at 400–500 °C and incomplete transformation phenomenon (ICT) of bainite transformation was observed at 450 °C for the 3Si alloy and at 400 °C for the two alloys. Unlike to the ordinary ICT reported in other alloy systems, cementite precipitation with Si partitioning took place from the beginning of ICT. Carbon enrichment in austenite during ICT was measured by three-dimensional atom probe and was found to be higher than T0 or T0′ prediction while significantly deviates from NPLE limits and PE predictions to lower carbon content. Theories for bainite transformation, such as T0 limit, solute drag and WBs limit, were examined based on the experimentally measured carbon content in austenite during ICT. T0′ limit theory is difficult to rationalize the much larger measured carbon content than T0′ prediction. In addition to solute drag effect and spike development in the NPLE mode, solute drag theory should incorporate dissipations caused by other sources in order to account for the estimated 1250–1700 J/mol deviation from PE predictions. In addition, WBs limit theory gives good descriptions on the carbon enrichment in austenite during ICT stage.

Effects of Partitioning of Manganese and Silicon during Intercritical Annealing on Transformation Behavior and Mechanical Properties of Low Alloyed TRIP-assisted Steel Sheets

The effects of partitioning of Mn and Si during intercritical annealing on the transformation behavior and mechanical properties of low alloyed TRIP-assisted steel sheets were investigated by using 0.17%C-1.5%Si-1.7%Mn steel. During intercritical annealing, Mn and Si concentrated in austenite and ferrite by partitioning, respectively. This partitioning of Mn and Si suppressed the bainite transformation during austempering and decreased the volume fraction of retained austenite and its C content. This is considered to be mainly because partitioning of Mn and Si shifted the T0’ curve to the lower C concentration region and stopped the bainite transformation at a lower C concentration in residual austenite. The decrease in the volume fraction of retained austenite and its C content deteriorated the ductility of the low alloyed TRIP-assisted steel sheets. Although proper intercritical annealing is necessary in order to increase the C content in retained austenite, it should be noted that excessive intercritical annealing can deteriorate the ductility of low alloyed TRIP-assisted steel sheets by causing excessive partitioning of Mn and Si.

Speed and Accuracy Optimization for Fan-Out Die Placement

Die placement accuracy requirements for fan-out wafer level packaging vary widely based on the process flow selected. Each process flow has attributes that define the challenges and requirements for die placement accuracy. Different fan-out process flows can require die placement to be done face-up or face-down. Alignment schemes for die placement can include global alignment, local alignment or a combination of both methods. Heat may be required for the carrier or die and placement force must be carefully controlled. Predictable placement of die prior to reconstitution is a key enabling technology for advanced fan-out products requiring fine line and space redistribution layers but it has the potential to improve yield in any of the fan-out processes. Accuracy requirements are driven by the photolithography done after reconstitution. Steppers are used with the assumption die to die spacing is repeatable within a given field area, however die spacing will inevitably be somewhat irregular due normal proces...

SiP Technology Trends

System-in-Package (SiP) has become a mainstream package technology of choice by both IC suppliers and OEMs in recent years, and the adoption of package technology is evidenced by the increasing number of devices that utilize this package technology in handsets and mobile segment. However, the adoption of SiP is not limited to only the mobile market, but is also present in IoT, Wearables, as well as in high-end computing market segments. The drivers for adoption of SiP in each market segments, in addition to package technologies suitable within each segment vary. Amongst the many drivers for SiP adoption, the higher integration trend is a common theme across all market tiers as form factor reduction or miniaturization, performance increase, and faster time-to-market are all enabled by such integration schemes. These integration trends impact not only the OSAT's package technologies, but are also impacting the more traditional module assemblies that were historically done at EMS side. The differentiation between OSATs and EMSs become apparent in the level of integration that each can achieve and how their technologies can complement each other in order to collectively provide the highest level of integration possible. The technology blocks used to achieve this higher integration level in 2D packages include the conversion of package design from laminate FC to FOWLP, integration of passives on RDL, and the implementation of IPD's, or in some cases simply through utilization of improved assembly design rules and adoption of more complex process flows. These 2D SiP technologies are well suited for RF devices, and other types of control modules. For 2.5D package configurations, integration scheme is built on the Package-on-Package (PoP) technology framework, and can incorporate both laminate FCPoP as well as FOWLP (eWLB PoP). Here Interposers in combination with micro-bumps and high density interconnects are also used to enable applications in high-end computing. The value-add through such 2.5D technologies, in some cases is the departure from SoC designs with transition to Si partitioning, which yields lower system level cost in advanced nodes, and in other cases is the enhanced performance and form factor reduction that is enabled.

Partitioning of Cr and Si between cementite and ferrite derived from first-principles thermodynamics

Partitioning of Cr and Si between cementite and ferrite was investigated by first-principles thermodynamics taking into account vibrational, electronic, and magnetic Gibbs energy contributions. At finite temperatures, these contributions lower the partitioning Gibbs energy and compete with the configurational entropy, which favors impurity segregation to ferrite due to its larger volume fraction compared to cementite. Due to the large positive partitioning enthalpy contribution of Si at T = 0 K, partitioning of Si to cementite is virtually absent in agreement with experiment. The situation is drastically different for Cr impurities. Incorporation of finite-temperature effects resolves the discrepancy between experimental observations and previous T = 0 K first-principles calculations. Cr strongly segregates to cementite due to the enhanced magnetic entropy of cementite above 400 K, i.e., near the Curie temperature of cementite. The increasing magnetic fluctuations in ferrite cause a strong reduction of the partitioning coefficient in the temperature range from 773 to 973 K in qualitative agreement with experiment. Quantitative agreement with calphad data and experimental data for equilibrium Cr concentrations in a wide range of alloy compositions is achieved by renormalizing the theoretical magnetic partitioning Gibbs energy by a constant scaling factor.

High pressure metal–silicate partitioning of Ni, Co, V, Cr, Si, and O

The distributions of major and minor elements in Earth’s core and mantle were primarily established by high pressure, high temperature metal–silicate partitioning during core segregation. The partitioning behaviors of moderately siderophile elements can be used to constrain the pressure–temperature conditions of core formation and the core’s composition. We performed experiments to study the partitioning of Ni, Co, V, Cr, Si, and O between silicate melt and Fe-rich metallic melt in a multianvil press and diamond anvil cell, up to 100GPa and 5700K. Combining our new results with data from 18 previous studies, we parameterized the effects of pressure, temperature, and metallic melt composition on partitioning. Ni and Co partitioning are insensitive to composition. At low pressures, these elements become less siderophile with increasing temperature, with this trend reversing above ∼45GPa. V and Cr partitioning are much more sensitive to metallic melt composition and less sensitive to pressure. Partitioning of Si and O are insensitive to pressure, but with strong and moderate temperature dependences, respectively. Our new parameterizations of Ni and Co partitioning suggest that the Earth’s distributions of these elements can be matched by single-stage core–mantle equilibration at 54±5GPa and 3300–3400K. These conditions would result in 8.5±1.4wt% Si and 1.6±0.3wt% O in the core, compatible with the core’s measured density. However, this single-stage model matches the Earth’s V and Cr distributions less well. We also incorporated our parameterizations into models of multi-stage core formation over evolving pressure–temperature-oxygen fugacity conditions, reproducing the Earth’s Ni and Co distributions while simultaneously producing a core whose light element composition is consistent with its density.

Removal of Al, Fe and Si from complex rare-earth leach solution: A three-liquid-phase partitioning approach

A new strategy was suggested to remove Fe, Al and Si impurities from rare-earth leach solution by controlling partitioning and selective enrichment of those impurities and rare earths in three-liquid-phase systems. Experimental results indicated that aqueous pH value and addition of water-soluble complexing agents or their mixtures into initial aqueous solution have significant influence on three-liquid-phase partitioning of Fe, Al, Si and rare earths. When only 1,10-phenanthroline was added, Fe and Si can be “filtered” into PEG-rich middle phase of three-liquid-phase system (Cyanex 272/PEG 2000/(NH4)2SO4–H2O) without affecting separation of heavy and light rare-earth ions in top organic and bottom salt-rich phase, respectively. Al remained in bottom phase. However, addition of EDTA and 1,10-phenanthroline promoted enrichment of Fe, Al and Si into PEG-rich middle phase, and almost all of rare earths were concentrated in salt-rich bottom phase. Those rare earths in salt-rich phase after removal of impurities can be further separated by another three-liquid-phase system of Cyanex 272/PEG 600/(NH4)2SO4–H2O. Heavy, middle and light rare earths, Yb, Eu and La, can be selectively enriched into different liquid phases. Therefore, separation of non-rare-earth impurities from rare earths, and then between light, middle and heavy rare earths can be achieved by two stages of three-liquid-phase partitioning. The present work highlights three-liquid-phase system can be a potential separation media for removal of Fe, Al, Si from complex rare-earth leach solution and group separation of different target metals into different liquid phases.

Partitioning of Si and platinum group elements between liquid and solid Fe–Si alloys

Crystallization of the Earth’s inner core fractionates major and minor elements between the solid and liquid metal, leaving physical and geochemical imprints on the Earth’s core. For example, the density jump observed at the Inner Core Boundary (ICB) is related to the preferential partitioning of lighter elements in the liquid outer core. The fractionation of Os, Re and Pt between liquid and solid during inner core crystallization has been invoked as a process that explains the observed Os isotopic signature of mantle plume-derived lavas (Brandon et al., 1998; Brandon and Walker, 2005) in terms of core–mantle interaction. In this article we measured partitioning of Si, Os, Re and Pt between liquid and solid metal. Isobaric (2 GPa) experiments were conducted in a piston-cylinder press at temperatures between 1250°C and 1600°C in which an imposed thermal gradient through the sample provided solid–liquid coexistence in the Fe–Si system. We determined the narrow melting loop in the Fe–Si system using Si partitioning values and showed that order–disorder transition in the Fe–Si solid phases can have a large effect on Si partitioning. We also found constant partition coefficients (DOs, DPt, DRe) between liquid and solid metal, for Si concentrations ranging from 2 to 12 wt%. The compact structure of Fe–Si liquid alloys is compatible with incorporation of Si and platinum group elements (PGEs) elements precluding solid–liquid fractionation. Such phase diagram properties are relevant for other light elements such as S and C at high pressure and is not consistent with inter-elemental fractionation of PGEs during metal crystallization at Earth’s inner core conditions. We therefore propose that the peculiar Os isotopic signature observed in plume-derived lavas is more likely explained by mantle source heterogeneity (Meibom et al., 2002; Baker and Krogh Jensen, 2004; Luguet et al., 2008).

Evidence of sulfur-bound reduced copper in bamboo exposed to high silicon and copper concentrations

We examined copper (Cu) absorption, distribution and toxicity and the role of a silicon (Si) supplementation in the bamboo Phyllostachys fastuosa. Bamboos were maintained in hydroponics for 4 months and submitted to two different Cu (1.5 and 100 μm Cu2+) and Si (0 and 1.1 mM) concentrations. Cu and Si partitioning and Cu speciation were investigated by chemical analysis, microscopic and spectroscopic techniques. Copper was present as Cu(I) and Cu(II) depending on plant parts. Bamboo mainly coped with high Cu exposure by: (i) high Cu sequestration in the root (ii) Cu(II) binding to amino and carboxyl ligands in roots, and (iii) Cu(I) complexation with both organic and inorganic sulfur ligands in stems and leaves. Silicon supplementation decreased the visible damage induced by high Cu exposure and modified Cu speciation in the leaves where a higher proportion of Cu was present as inorganic Cu(I)S compounds, which may be less toxic.

純亜鉛融液／鋼界面におけるFe-Zn系金属間化合物形成に及ぼす鋼中Siの影響

The effect of Si additions to the steel on the microstructure and growth kinetics of the Fe-Zn intermetallic compound (IMC) in the Fe/Zn diffusion couple with limited Zn supply was experimentally investigated. Three different steels with Si contents of less than 0.01 wt%, 0.44 wt%, and 1.85 wt% were machined in cylindrical shape of 8 mmφ×6 mmh, and their bottom surfaces were polished chemically and mechanically to remove oxide and Si depletion layer. Pure Zn (99.99%) film was inserted between two steel cylinders, and enclosed in the steels in vacuum. The samples were heated at 450 °C for various holding time. Without Si, an IMC layer composed of δp, δk, and Γ phases were formed first, and it was later replaced by an IMC layer composed only of Γ phase by the growth of δk and Γ phases. The addition of Si in steel suppresses the formation of Γ phase as well as the growth rate of δk phase, and IMC layers composed mainly of either δp or δk phases were successfully fabricated. Si was found to accumulate in α-Fe at the α-Fe/δk interface, which indicates that Si partitioning at the α-Fe/δk interface is responsible for the instability of Γ phase and slow supply of Fe from steel substrates. Finally, the fracture toughness and adhesion of IMC layer composed mainly of δk phase were demonstrated inferior to those of δp phase.

Effect of carbon, sulfur and silicon on iron melting at high pressure: Implications for composition and evolution of the planetary terrestrial cores

High-pressure melting experiments in the Fe–S–C ternary and Fe–S–Si–C quaternary systems have been conducted in the range of 3.5–20GPa and 920–1700°C in the multi-anvil press. The mutual solubility, melting relations, and crystallization sequences were systematically investigated with changes of pressure, temperature and bulk composition. Five starting materials of Fe(84.69wt%)–C(4.35wt%)–S(7.85wt%), Fe(84.87wt%)–C(2.08wt%)–S(11.41wt%), Fe(86.36wt%)–C(0.96wt%)–S(10.31wt%), Fe(85.71wt%)–C(0.33wt%)–S(11.86wt%) and Fe(82.95wt%)–C(0.66wt%)–S(13.7wt%)–Si(2.89wt%) were employed. For Fe(84.69wt%)–C(4.35wt%)–S(7.85wt%), the first crystallized phase is Fe3C at 5GPa and Fe7C3 at 10–20GPa. For Fe(84.87wt%)–C(2.08wt%)–S(11.41wt%), Fe3C is the stable carbide at subsolidus temperature at 5–15GPa. For Fe(86.36wt%)–C(0.96wt%)–S(10.31wt%) and Fe(85.71wt%)–C(0.33wt%)–S(11.86wt%), the first crystallized phase is metallic Fe instead of iron carbide at 5–10GPa. The cotectic curves in Fe–S–C ternary system indicate only a small amount of C is needed to form an iron carbide solid inner core with the presence of S. Experiments on Fe(82.95wt%)–C(0.66wt%)–S(13.7wt%)–Si(2.89wt%) showed that a small amount of C does not significantly change the closure pressure of miscibility gap compared with that in Fe–S–Si system. It is observed that S preferentially partitions into molten iron while a significant amount of Si enters the solid phase with temperature decrease. Meanwhile, the C concentration in the liquid and solid iron metal changes little with temperature variations. If S, C and Si partitioning behavior between molten iron and solid iron metal with temperature remains the same under Earth’s present core pressure conditions, the solid inner core should be iron dominated with dissolved Si. On the other hand, the liquid outer core will be S rich and Si poor. Moderate carbon will be evenly present in both solid and liquid cores. Based on our melting data in a multi-component system, no layered liquid core should exist in the Earth, Mars and Mercury.

Simultaneous partitioning of silicon and oxygen into the Earth's core during early Earth differentiation

AbstractSilicon and oxygen are potential light elements in the Earth's core and may be involved in metal‐silicate reactions at the present day core‐mantle boundary. We have performed multianvil experiments at 25 GPa and 2770–3080 K to understand the simultaneous partitioning of these elements between liquid iron–rich metal and silicate melt. The presence of O in liquid Fe at high temperatures influences the partitioning of Si, causing more Si to partition into the metal than would be expected based on lower temperature measurements. Although Si and O are mutually exclusive in Fe metal at <3000 K, the level at which both element concentrations are similar in the liquid metal rises above 1 wt % at >3000 K. We have developed a thermodynamic model based on these experiments that accounts for the interaction between O and Si in the liquid metal. Comparison between this model and the previous results of diamond‐anvil cell experiments up to 71 GPa indicates very little pressure dependence but a strong temperature dependence for O and Si partitioning. Our model predicts that subequal concentrations of Si and O, sufficient to explain the outer core density deficit, would have partitioned into core‐forming metal if equilibration occurred between the metal and a magma ocean with a bulk silicate Earth composition at an average depth of ~1200 km (~50 GPa and ~3300 K). An O‐ and Si‐enriched buoyant layer may have developed at the top of the outer core as a result of subsequent equilibration with the overlying mantle.

Effect of silicon additions on the high temperature oxidation behaviour of Cr–Cr2Ta alloys

The effect of Si additions between 0 and 15 at.% on the microstructure and oxidation resistance of Cr–Ta alloys has been studied. After annealing at 1300 °C for 500 h, the microstructures of the alloys were observed to consist predominantly of a Cr-rich solid solution and a Cr2Ta Laves phase, with the Si partitioning preferentially to the Laves phase. Si was found to be beneficial to high temperature oxidation resistance, with the ternary alloys showing a lower weight gain than binary Cr–Ta alloys after isothermal exposure at 1100 °C for 100 h. However, no further improvement in oxidation behaviour under these conditions was achieved with increasing the Si-content beyond 7 at.%. The oxide scales formed on the alloys were found to comprise typically of Cr2O3 and mixed (Cr,Ta)-based oxides. The scales formed at 1100 °C and 1300 °C spalled readily, whilst those formed at 1000 °C showed better adherence to the substrate. Importantly, the presence of Si was also found to eliminate the internal nitridation otherwise encountered during the oxidation of binary Cr–Ta alloys. The formation of SiO2 in the alloys was observed only at 1300 °C, where the alloys displayed a massive weight gain with rapid oxidation of the Cr-rich solid solution.

Role of silicon in accelerating the nucleation of Al3(Sc,Zr) precipitates in dilute Al–Sc–Zr alloys

The effects of adding 0.02 or 0.06at.% Si to Al–0.06Sc–0.06Zr (at.%) are studied to determine the impact of Si on accelerating Al3(Sc,Zr) precipitation kinetics in dilute Al–Sc-based alloys. Precipitation in the 0.06at.% Si alloy, measured by microhardness and atom-probe tomography (APT), is accelerated for aging times <4h at 275 and 300°C, compared with the 0.02at.% Si alloy. Experimental partial radial distribution functions of the α-Al matrix of the high-Si alloy reveal considerable Si–Sc clustering, which is attributed to attractive Si–Sc binding energies at the first and second nearest-neighbor distances, as confirmed by first-principles calculations. Calculations also indicate that Si–Sc binding decreases both the vacancy formation energy near Sc and the Sc migration energy in Al. APT further demonstrates that Si partitions preferentially to the Sc-enriched core rather than the Zr-enriched shell in the core/shell Al3(Sc,Zr) (L12) precipitates in the high-Si alloy subjected to double aging (8h/300°C for Sc precipitation and 32days/400°C for Zr precipitation). Calculations of the driving force for Si partitioning confirm that: (i) Si partitions preferentially to the Al3(Sc,Zr) (L12) precipitates, occupying the Al sublattice site; (ii) Si increases the driving force for the precipitation of Al3Sc; and (iii) Si partitions preferentially to Al3Sc (L12) rather than Al3Zr (L12).

SILICON NUTRITION AND PHYTOPHTHORA DRECHSLERI INFECTION EFFECTS ON GROWTH AND MINERAL NUTRIENTS CONCENTRATION, UPTAKE, AND RELATIVE TRANSLOCATION IN HYDROPONIC-GROWN CUCUMBER

A hydroponic experiment was performed to investigate silicon (Si) and Phytophthora drechsleri root rot effects on growth and tissue partitioning of Si, zinc (Zn), iron (Fe), and manganese (Mn) in two cucumber cultivars (Cucumis sativus L. cvs. ‘Dominus’ and ‘Super Dominus’). Root length, plant height, and root fresh weight were significantly decreased by P. drechsleri, which were all significantly alleviated by 1.0 mM Si. Increasing Si level in the nutrient solution was accompanied with its enhanced uptake by cucumbers. Plants infected with P. drechsleri transported lower Mn to shoot than non-infected plants. Influence of P. drechsleri infection on root Fe concentration was dependent on cucumber cultivar while a decrease in root Zn concentration was found in infected cucumbers. Silicon nutrition increased Zn and Fe uptake in both cucumber cultivars. It is suggested that Si nutrition improved the crop growth, particularly under biotic stress, and hereby, increased micronutrients uptake by cucumber.

Calculation of Microstructure in Vacuum Carburizing Incorporating Kinetics Modeling of Grain-boundary Cementite

In the vacuum carburization of steels, short-time carburization is usually followed by a diffusion period to eliminate the filmy cementite (θ) grown on austenite (γ) grain boundaries. The surface C content in γ was found to increase past the solubility of θ due to radical reaction with decomposed hydro-carbon gas. The thermodynamic condition was recognized as the metastable equilibrium of supersaturated γ with graphite. Based on these results, a calculation model for not only C concentration profiles but also quantitative evaluation of grain-boundary θ was developed.The existing model estimates the amount of θ with the equilibrium fractions for local C contents in a framework of the finite difference method (FDM). However, with the corresponding surface condition of equilibrium γ plus θ with graphite, it overrates the amount of θ observed after several minutes of carburization. In the developed model, a parabolic law was assumed for the thickening of θ and a rate controlling process was considered for partitioning of Si. The local Si concentrations at the γ/θ interface were determined from the C isoactivity condition with supersaturated γ. The change in the rate coefficient (αSi) was also validated using multicomponent diffusion simulations. Coupled with the CALPHAD software, a one-dimensional FDM program calculates the increment of θ using the updated local equilibrium. Predictions of the C profile and θ volume fraction corresponded much better with the experimental results than existing models.

Silicon isotopes in lunar rocks: Implications for the Moon’s formation and the early history of the Earth

Silicon isotopic data from a range of lunar samples are presented to assess the degree of heterogeneity of the lunar mantle and its similarity to bulk silicate Earth (BSE). Multi-collector inductively-coupled-plasma mass spectrometry (MC-ICPMS) was used to analyse 24 samples, including both high and low-Ti basalts, as well as Highland anorthosites and picritic glasses, covering all the Apollo sample return missions. No systematic δ30Si differences are found between any of the bulk sample lithologies (±2σSD) (δ30SiLow-Ti basalt=−0.29±0.06, δ30SiHigh-Ti basalt=−0.32±0.09, δ30Silunar glass=−0.29±0.05 and δ30SiHighland rocks=−0.27±0.10). The average of the lunar samples is δ30Si=−0.29±0.08 (2σSD), which is identical to the composition of BSE, δ30Si=−0.29±0.08 (2σSD), from Savage et al. (2010). The BSE Si isotope composition is thought to be the result of Si partitioning between metal and silicate, and consequent isotopic fractionation during core formation. The Moon-forming impactor would not be expected to share that composition, because it is thought to have been relatively small (∼0.1 Earth masses) like Mars and formed under relatively low temperatures and pressures that are insufficient for Si to partition into the core. Therefore, the identical lunar and BSE Si isotope data show that Si isotopes, like those of oxygen, must have homogenised in the aftermath of the Moon-forming impact, if smooth particle hydrodynamic simulations of the Giant Impact are correct in showing that most lunar material should have originated from the impactor rather than the Earth. The data presented here are in agreement with other isotope systems and experimental studies that indicate that the majority of core formation happened early and before the Giant Impact. It has been predicted (Pahlevan et al., 2011) that the Moon and the BSE should show a ∼0.14‰ offset in δ30Si for the Moon to have an Fe/(Fe+Mg) ratio twice that of BSE. The current resolution and sample population size of the Si data for the Moon and Earth would allow such an offset to be detected. The fact that is it not observed can put constraints on the element ratios and lunar budgets as modelled by Pahlevan et al. (2011); in particular, it constrains the Fe/(Fe+Mg) ratio of the Moon to be only 1–1.3 BSE.

Precipitates in Al–Cu alloys revisited: Atom-probe tomographic experiments and first-principles calculations of compositional evolution and interfacial segregation

Atom-probe tomography, transmission electron microscopy, X-ray diffraction and first-principles calculations are employed to study: (i) compositional evolution of GPII zones and θ′ precipitates; and (ii) solute segregation at α-Al/ θ′ interfaces in Al–1.7 at.% Cu (Al–4 wt.% Cu) alloys. GPII zones are observed after aging at 438 K for 8 h, whereas higher aging temperatures, 463 K for 8 h and 533 K for 4 h, reveal only θ′ precipitates. Most GPII zones and θ′ precipitates are demonstrated to be Cu-deficient at the lower two aging temperatures; only the 533 K treatment resulted in θ′ stoichiometries consistent with the expected Al 2Cu equilibrium composition. For alloys containing ∼200 at. ppm Si we find evidence of Si partitioning to GPII zones and θ′ precipitates. Significant Si segregation is observed at the coherent α-Al/ θ′ interface for aging at 533 K, resulting in an interfacial Si concentration more than 11 times greater than in the α-Al matrix. Importantly, the Si interfacial concentration undergoes a transition from a non-equilibrium delocalized profile to an equilibrium localized profile as the aging temperature is increased from 463 to 533 K. Consistent with these measurements, first-principles calculations predict a strong thermodynamic driving force favoring Si partitioning to Cu sites in θ′. Silicon segregation at, and partitioning to, θ′ precipitates results in a decrease in interfacial free energy, and concomitantly an increase in the nucleation current. Our results suggest that Si catalyzes the early stages of precipitation in these alloys, consistent with the higher precipitate number densities observed in commercial Al–Cu–Si alloys.