Gibbsian Interfacial Excess Research Articles

Creep-induced redistribution of alloying elements in CZ1 zirconium alloys

Introducing minor alloying elements is an effective strategy to improve the corrosion and mechanical properties of zirconium alloys for nuclear applications. During in-reactor service, external environment and stress can affect the distribution of alloying elements, substantially changing the degradation process of zirconium alloys. To date, there is a lack of in-depth understanding of the interaction between creep and microchemistry changes. Here, we conducted systematic transmission electron microscopy (TEM) and atom probe tomography (APT) investigations to address creep-induced redistribution of alloying elements in CZ1 (Zr-Sn-Nb-Fe-Cr-Cu) zirconium alloy with different initial microstructures. Nb, Fe, Sn, and Cu are found to co-segregate at grain boundaries. The higher the intermediate annealing temperature, the larger the Gibbsian interfacial excesses of solute elements are. We further demonstrate that creep can reduce the excess value of Fe at grain boundaries due to the coarsening of Zr-Fe-Cr second phase particles via grain boundary and dislocation pipe diffusion. At the same time, the excess value of Sn is increased by diffusing from the matrix to grain boundaries. Moreover, Cu as a minor element in the concentration range of 0.05–0.3 wt.% is found to segregate at dislocations to form the Cottrell atmosphere and develop Cu-rich nanoclusters for suppressing dislocation motion. The new understanding of the segregation and clustering of minor alloying elements provides guidance for developing zirconium alloys with enhanced creep resistance.

On the tungsten segregation at γ/γ′ interface in a Ni-based single-crystal superalloy

Through a combination of atom probe tomography (APT) and theoretical calculations, the distributions of tungsten (W) at the γ/γ′ interfaces with exposure effects in a Ni-based single crystal superalloy are investigated. W shows the reversal partitioning behavior during long-term thermal exposure, partitioning from γ-matrix to γ′-precipitates, accompanied by varying lattice misfit. The APT results show that the γ/γ′ interface in all samples possesses a Gibbsian interfacial excess, corresponding to decrease in interfacial energy. The elemental segregation demonstrates high dependency on elastic strain energy, calculated according to lattice misfit. The driving force for the interfacial segregation is the release of elastic strain energy.

Analysis of antioxidation behavior of cryo-milled oxide-dispersion-strengthened ferritic steel incorporated with formation of Y–Ti–O(N) nano-precipitates

Oxide-dispersion-strengthened steel were produced by mechanical alloying with milled Y2O3 powder in a ball mill at room temperature (ODS-RT) and at -150 ℃ (ODS-LT). Both samples were exposed to air for 6000 h at 800 ℃ for examining their long-term oxidation resistance at a high temperature. After long-term exposure both samples showed subsurface degradation occurred concurrently such as internal precipitation, phase formation, and phase dissolution. It was associated with scale formation including a large number of short-circuit diffusion paths, which led to the rapid formation of a dense and stable protective oxide film in the initial oxidation stage. The resulting oxide scaling kinetics were more sluggish for ODS-LT compared with ODS-RT. A correlative characterization method involving transmission electron microscopy and atom probe tomography was used to determine factors affecting different oxidation kinetics. Nano-precipitate (NP) analysis showed that Y–Ti–O NPs were present at the grain boundaries of ODS-RT and that ODS-LT contained Y–Ti–O(N) NPs and a sub-micrometer-sized TiN precipitate. An assessment of segregation phenomena along with interfacial energies calculated using the Gibbsian interfacial excess showed that the behavior of Y atoms played a key role in clustering, and segregation at the interfaces. The calculated partial radial distribution function of major constituents indicated that the NPs in the two alloys had different Y content. Excess Y atoms segregated at the matrix/oxide interface in ODS-LT inhibited oxide growth. Therefore, Y atoms can be considered as a controlling factor for the oxidation rate at high temperatures.

Coupled segregation mechanisms of Sc, Zr and Mn at [formula omitted] interfaces enhances the strength and thermal stability of Al-Cu alloys

The refinement and thermal stability of intermediate theta-prime (θ′) precipitates are critical in the development of new high strength 2xxx series aluminium-copper (Al-Cu) alloys for high temperature applications. In this work, we use trace additions of Sc, Zr and Mn in an Al-6.5 wt.% Cu alloy to refine and stabilise the θ′ precipitates. The formation of Al3(Sc, Zr) core/shell dispersoids significantly refine the θ′ precipitates by acting as preferential nucleation sites during artificial ageing. Adding Mn results in a significant increase of hardness during ageing at 190 °C. Hardness is maintained during thermal exposure at 280 °C for up to 24 h. Transmission electron microscopy (TEM) reveals that the addition of Mn leads to a finer and denser distribution of θ′ precipitates, and greatly slows the growth and coarsening of the θ′ precipitates at elevated temperatures. Differential scanning calorimetry (DSC) shows that this can be attributed to an enhanced nucleation and improved coarsening resistance of the θ′ precipitates in the presence of Mn. Atom probe tomography (APT) reveals that the enhanced age-hardening kinetics and thermal stability arise from the independent segregation mechanisms of Mn, Sc and Zr at the semi-coherent and coherent interfaces of the θ′ precipitates. The segregation is quantified by calculating the Gibbsian interfacial excess and corresponding reduction in interfacial energy. These calculations reveal that while Sc and Zr play a significant role in the refinement of the θ′ precipitates, Mn not only refines the θ′ precipitates, but also greatly enhances their coarsening resistance and corresponding alloy's thermal stability.

Temperature increases and thermoplastic microstructural evolution in adiabatic shear bands in a high-strength and high-toughness 10 wt.% Ni steel

A 10 wt.% nickel-steel has been developed for high pressures and low-temperature applications, due to its high strength, excellent toughness, and low ductile-to-brittle transition temperature (DBTT). Under dynamic loading conditions this steel is, however, prone to shear localization that manifests as adiabatic shear bands (ASBs). The temperature increases and thermoplastic microstructural evolution in the ASB are studied in detail, from the microscopic length scale to the atomic-scale employing correlative electron-backscatter diffraction (EBSD), transmission electron microscopy (TEM), and atom-probe tomography (APT). From a calculation of the temperature increase under adiabatic conditions, based on the conversion of plastic-work to heat generation, the microstructural transitions in the ASB are discussed specifically for: (i) a b.c.c.-f.c.c. phase-transformation and elemental partitioning of alloying elements; (ii) a thermodynamic model for the compositional change of the V(Nb)-rich carbonitride precipitates during a temperature increase; (iii) grain-refinement and rotation by dynamic/mechanical recrystallization processes. Solute segregation at subgrain boundaries, measured using the Gibbsian interfacial excess methodology, reveals how solute segregation contributes to the instability of localized shear-deformation by promoting the depinning of solute elements and the migration of a grain boundary. Finally, a kinetic model for grain refinement/rotation within an ASB is described by the dynamic recrystallization behavior with: (i) sub-grain formation; (ii) rotation/refinement by deformation; and (iii) grain growth by subgrain coalescence with further rotation and a temperature increase. The temperature increase under dynamic deformation in an ASB promotes grain boundary migration and subgrain coalescence to create a large degree of equiaxed grains with a low density of imperfections.

Interfacial segregation and precipitation behavior of Cu-rich precipitates in Cu-bearing 316LN stainless steel after aging at different temperatures

Interfacial segregation and precipitation behavior of Cu-rich precipitates in a Cu-bearing 316LN austenitic stainless steel (316LN-Cu SS) aged at 600, 700 and 800 °C were systematically investigated. The variations in morphology, composition of Cu-rich precipitates and interfacial segregation were characterized by transmission electron microscopy (TEM), atom probe tomography (APT) and Gibbsian interfacial excess. Experimental results revealed the Cu-rich precipitates led to increase in hardness and strength of steel. The 316LN-Cu SS aged at 700 °C for 6 h possessed the optimal strength and elongation due to the proper size, number density and volume fraction of the Cu-rich precipitates in the steel matrix. The average size of Cu-rich precipitates significantly increased from 0.85 ± 0.14 nm to 5.21 ± 1.51 nm with increasing the aging temperature, while the coherent relationship between Cu-rich precipitates and γ-Fe matrix remained unaltered even for large size Cu-rich precipitate. The APT analysis confirmed that the composition of Cu-rich precipitate was enriched in Cu atoms and depleted of other elements with increasing aging temperature, while their core composition became nearly pure Cu at high aging temperatures. In addition, due to strong bonding energy, Ni was significantly segregated at the γ-Fe matrix/precipitate heterophase interface, with Gibbsian interfacial excesses related to Fe and Cu of 0.6036 ± 0.52, 2.892 ± 0.55 and 2.908 ± 0.38 atoms nm-2 after aging at 600, 700 and 800 °C, respectively. This interface segregation further caused the reductions in interfacial free energies up to 7.28, 38.9 and 43.1 mJ m-2, respectively.

A Nanoscale Study of Thermally Grown Chromia on High-Cr Ferritic Steels and Associated Oxidation Mechanisms

Fe-22Cr-0.5Mn based ferritic steels are used as interconnect materials for solid oxide fuel/electrolysis cells. Four steel samples, including the commercial steel Crofer 22 H, were oxidized at 800 °C in a model Ar-4%H2−4%H2O atmosphere simulating the fuel side of the cells and investigated by atom probe tomography (APT) in conjunction with electron microscopy and thermogravimetry. All steels form an oxide scale mainly consisting of MnCr2O4 spinel on top of Cr2O3. APT revealed segregation of minor alloying constituents (Nb and Ti) to chromia grain boundaries and highlighted their effect on mass transport through the chromia scale. Relationships between segregation activity of individual elements (in terms of Gibbsian interfacial excess), oxide scale microstructure and alloy oxidation rate have been established based on the APT results. Comparison of segregation activities revealed that vacancies formation due to Wagner-Hauffe doping with aliovalent Ti and Nb impurities cannot be solely responsible for faster oxidation, assuming alteration of the grain boundary structure and associated changes of their mass transport properties. Controlled Si addition to the alloy (about 0.4 at%) suppresses the detrimental effect of Nb on the oxidation resistance but results in formation of a thin, although still discontinuous, SiO2 layer at the metal-oxide interface.

Temperature Increases and Thermoplastic Microstructural Evolution in Adiabatic Shear-Bands in a High-Strength and High-Toughness 10 wt.% Ni Steel

A 10 wt.% nickel-steel has been developed for high pressures and low-temperature applications, due to its high strength, excellent toughness, and low ductile-to-brittle transition temperature (DBTT). Under dynamic loading conditions this steel is, however, prone to shear localization that manifests as adiabatic shear bands (ASBs). The temperature increases and thermoplastic microstructural evolution in the ASB are studied in detail, from the macroscopic length scale to the atomic-scale employing correlative electron-backscatter diffraction (EBSD), transmission electron microscopy (TEM), and atom-probe tomography (APT). From a calculation of the temperature increase under adiabatic conditions, based on the conversion of plastic-work to heat generation, the microstructural transitions in the ASB are discussed specifically for: (i) a b.c.c.-f.c.c. phase-transformation and their elemental partitioning; (ii) the thermodynamic model for the compositional change of the V(Nb)-rich carbonitride precipitates during a temperature increase; and (iii) grain-refinement and rotation by dynamic/mechanical recrystallization processes. Solute segregation at subgrain boundaries, measured using the Gibbsian interfacial excess methodology, reveals how solute segregation contributes to the instability of localized shear-deformation by promoting the depinning of solute elements and the migration of a grain boundary. Finally, a kinetic model for grain refinement/rotation within an ASB is described by the dynamic recrystallization behavior with: (i) subgrain formation; (ii) rotation/refinement by deformation; and (iii) grain growth by subgrain coalescence with further rotation and a temperature increase. The temperature increase under dynamic deformation in an ASB promotes grain boundary migration and subgrain coalescence to create a large degree of equiaxed grains with a low density of imperfections.

Misorientation-dependent solute enrichment at interfaces and its contribution to defect formation mechanisms during laser additive manufacturing of superalloys

A vital issue during selective laser melting of nonweldable polycrystalline nickel-base superalloys is the formation of microcracks. These are cracks occurring during the last stage of solidification and only at high angle grain boundaries (HAGBs). Solute enrichment to the remaining interdendritic liquid and its partial back-diffusion into the solid contributes to the crack nucleation mechanism. Here we use atom probe tomography coupled with transmission Kikuchi diffraction to determine the misorientation and chemical composition profiles across HAGBs (with and without cracks) and across crack-free low angle grain boundaries (LAGBs). The Gibbsian interfacial excess of solutes (mainly B, C, Si, and Zr) is at least two times higher at the HAGB compared to the LAGB. The chemical profiles show the opposite behavior to established model predictions of the last stage of solidification. Our diffusion calculations elucidate that the chemical profiles are influenced by both microsegregation (of Ti, Nb, and Si) during solidification and solid-state segregation (of B, C, and Zr) during cooling. The chemical profiles in the topmost layer indicate a negligible effect of remelting and reheating. Except for Ti-rich carbides, no secondary phases are found. Additionally, we study an alloy with a reduced content of Zr and Si (by at least 60 wt. %), relative to the standard IN738LC composition. We achieved a 99% reduction in crack length per unit area. However, the grain boundary enrichment of Zr and Si in the modified alloy was similar to the standard alloy. Based on these findings, we critically discuss the contribution of various mechanisms proposed for solidification cracking.

The Element Segregation Between γ/γʹ Phases in a Ni-Based Single Crystal Superalloy Studied by 3D-APT and Its Potential Impact on Local Interfacial Misfit Strain

Three-dimensional atom-probe tomography was used to characterize the γ/γʹ interface structure in a third-generation Ni-based single crystal superalloy with Re addition. It was found that an element-segregation layer with Re, Co and Cr was formed in the γ phase close to the γ/γ′ interface, resulting in a more negative local interface misfit (− 0.29%) compared to the measured result (− 0.16%) from high-resolution X-ray diffraction. Furthermore, the total reduction of interfacial free energy due to the solute atom segregation based on the Gibbsian interfacial excess was calculated to indicate that Re element was the most beneficial element in producing this more negative local misfit with the largest interfacial free energy reduction (13.67 ± 0.21 mJ/m2). Simultaneously, because of the co-segregation of Re, Co and Cr, and the depletion of Ni in the γ phase close to the γ/γ′ interface, it was also deduced that some harmful topologically close-packed phases might be easier to nucleate and grow in the γ phase close to the γ/γ′ interface in service. The element-segregation layer across γ/γ′ interface resulted in a more negative local interface misfit (− 0.29%), compared to the measured result (− 0.16%) from highresolution X-ray diffraction, and the total reduction of interfacial free energy due to the solute atom segregation was calculated to indicate that Re element was the most beneficial element in producing this more negative local misfit with the largest interfacial free energy reduction (13.67 ± 0.21 mJ/m2).

Complex study of grain boundary segregation in long-term irradiated reactor pressure vessel steels

Grain boundary (GB) embrittlement of reactor pressure vessel steel occurs at the long-term operation through the radiation-enhanced segregation of impurities. All the known techniques of studying the GB segregation have their advantages and disadvantages, and complete information can be obtained only through a comprehensive analysis by different methods.In this paper the level of GB segregation in VVER-1000 RPV steels with high nickel content was assessed by combined methods of Auger electron spectroscopy (by the P monolayer coverage and atomic concentration of other elements); atom probe tomography (by the Gibbsian interfacial excess) and fractographic analysis (by the maximum fraction of brittle intergranular fracture).The results obtained by different methods are in good agreement and show that phosphorus GB segregation increases with the increase of fast neutron fluence, GB concentration of Ni and Mn, the bulk concentration of Ni and Mn, and correlates with the fraction of brittle intergranular fracture.

An Automated Computational Approach for Complete In-Plane Compositional Interface Analysis by Atom Probe Tomography.

We introduce an efficient, automated computational approach for analyzing interfaces within atom probe tomography datasets, enabling quantitative mapping of their thickness, composition, as well as the Gibbsian interfacial excess of each solute. Detailed evaluation of an experimental dataset indicates that compared with the composition map, the interfacial excess map is more robust and exhibits a relatively higher resolution to reveal compositional variations. By field evaporation simulations with a predefined emitter mimicking the experimental dataset, the impact of trajectory aberrations on the measurement of the thickness, composition, and interfacial excess of the decorated interface are systematically analyzed and discussed.

On the effect of Re addition on microstructural evolution of a CoNi-based superalloy

In this study, the effect of rhenium (Re) addition on microstructural evolution of a new low-density Co-Ni-Al-Mo-Nb based superalloy is presented. Addition of Re significantly influences the γ′ precipitate morphology, the γ/γ′ lattice misfit and the γ/γ′ microstructural stability during long term aging. An addition of 2 at.% Re to a Co-30Ni-10Al-5Mo-2Nb (all in at.%) alloy, aged at 900 °C for 50 h, reduces the γ/γ′ lattice misfit by ∼ 40% (from +0.32% to +0.19%, measured at room temperature) and hence alters the γ′ morphology from cuboidal to round-cornered cuboidal precipitates. The composition profiles across the γ/γ′ interface by atom probe tomography (APT) reveal Re partitions to the γ phase (KRe=0.34) and also results in the partitioning reversal of Mo to the γ phase (KMo=0.90) from the γ′ precipitate. An inhomogeneous distribution of Gibbsian interfacial excess for the solute Re (ΓRe, ranging from 0.8 to 9.6 atom.nm−2) has been observed at the γ/γ′ interface. A coarsening study at 900 °C (up to 1000 h) suggests that the coarsening of γ′ precipitates occurs solely by evaporation–condensation (EC) mechanism. This is contrary to that observed in the Co-30Ni-10Al-5Mo-2Nb alloy as well as in some of the Ni-Al based and high mass density Co-Al-W based superalloys, where γ′ precipitates coarsen by coagulation/coalescence mechanism with an extensive alignment of γ′ along <100> directions as a sign of microstructural instability. The γ′ coarsening rate constant (Kr) and γ/γ′ interfacial energy are estimated to be 1.13 × 10−27 m3/s and 8.4 mJ/m2, which are comparable and lower than Co-Al-W based superalloys.

On the Effect of Re Addition on Microstructural Evolution of a CoNi-Based Superalloy

In this study, the effect of rhenium (Re) addition on microstructural evolution of a new low-density Co-Ni-Al-Mo-Nb based superalloy is presented. Addition of Re significantly influences the γ' precipitate morphology, the γ/γ' lattice misfit and the γ/γ' microstructural stability during long term aging. An addition of 2 at.% Re to a Co-30Ni-10Al-5Mo-2Nb (all in at.%) alloy, aged at 900°C for 50 hours, reduces the γ/γ' lattice misfit by ~ 40% (from 0.32% to 0.19%, measured at room temperature) and hence alters the γ' morphology from cuboidal to round-cornered cuboidal precipitates. The composition profiles across the γ/γ' interface by atom probe tomography (APT) reveals Re partitions to the γ' phase (KRe=0.34) and also results in the partitioning reversal of Mo to the γ' phase (KMo=0.90) from the γ' precipitate. An inhomogeneous distribution of Gibbsian interfacial excess for the solute Re (ΓRe, ranging from 0.8 to 9.6 atom.nm-2) has been observed at the γ/γ' interface. Coarsening study at 900°C (up to 1000 hours) suggests that the coarsening of γ' precipitates occurs solely by an evaporation-condensation (EC) mechanism. This is contrary to that observed in the Co-30Ni-10Al-5Mo-2Nb alloy as well as in some of the Ni-Al based and high mass density Co-Al-W based superalloys, where γ' precipitates coarsens by coagulation/coalescence mechanism with extensive alignment of γ' along directions as a sign of microstructural instability. The γ' coarsening rate exponent (Kr) and γ/γ' interfacial energy are estimated to be 1.41 x 10-27 (m3/s) and 8.4 mJ/m2, that are comparable and lower than Co-Al-W based superalloys respectively.

Microstructural and creep properties of boron- and zirconium-containing cobalt-based superalloys

The effects of micro-additions of boron and zirconium on grain-boundary (GB) structure and strength in polycrystalline γ(f.c.c.) plus γ′(L12) strengthened Co-9.5Al-7.5W-X at% alloys (X=0-Ternary, 0.05B, 0.01B, 0.05Zr, and 0.005B-0.05Zr at%) are studied. Creep tests performed at 850°C demonstrate that GB strength and cohesion limit the creep resistance and ductility of the ternary B- and Zr-free alloy due to intergranular fracture. Alloys with 0.05B and 0.005B-0.05Zr both exhibit improved creep strength due to enhanced GB cohesion, compared to the baseline ternary Co-9.5Al-7.5W alloy, but alloys containing 0.01B or 0.05Zr additions display no benefit. Atom-probe tomography (APT) is utilized to measure GB segregation, where B and Zr are demonstrated to segregate at GBs. A Gibbsian interfacial excess of 5.57±1.04 atoms nm−2 was found for B at a GB in the 0.01B alloy and 2.88±0.81 and 2.40±0.84 atoms nm−2 for B and Zr, respectively, for the 0.005B-0.05Zr alloy. The GBs in the highest B-containing (0.05B) alloy exhibit micrometer-sized boride precipitates with adjacent precipitate denuded-zones (PDZs), whereas secondary precipitation at the GBs is absent in the other four alloys. The 0.05B alloy has the smallest room temperature yield strength, by 6%, which is attributed to the PDZs, but it exhibits the largest increase in creep strength (with an ~2.5 order of magnitude decrease in the minimum strain rate for a given stress at 850°C) over the baseline Co-9.5Al-7.5W alloy.

Influence of Ni Solute segregation on the intrinsic growth stresses in Cu(Ni) thin films

Using intrinsic solute segregation in alloys, the compressive stress in a series of Cu(Ni) thin films has been studied. The highest compressive stress was noted in the 5at.% Ni alloy, with increasing Ni concentration resulting in a subsequent reduction of stress. Atom probe tomography quantified Ni's Gibbsian interfacial excess in the grain boundaries and confirmed that once grain boundary saturation is achieved, the compressive stress was reduced. This letter provides experimental support in elucidating how interfacial segregation of excess adatoms contributes to the post-coalescence compressive stress generation mechanism in thin films.

New atom probe approaches to studying segregation in nanocrystalline materials

Atom probe is a technique that is highly suited to the study of nanocrystalline materials. It can provide accurate atomic-scale information about the composition of grain boundaries in three dimensions. In this paper we have analysed the microstructure of a nanocrystalline super-duplex stainless steel prepared by high pressure torsion (HPT). Not all of the grain boundaries in this alloy display obvious segregation, making visualisation of the microstructure challenging. In addition, the grain boundaries present in the atom probe data acquired from this alloy have complex shapes that are curved at the scale of the dataset and the interfacial excess varies considerably over the boundaries, making the accurate characterisation of the distribution of solute challenging using existing analysis techniques. In this paper we present two new data treatment methods that allow the visualisation of boundaries with little or no segregation, the delineation of boundaries for further analysis and the quantitative analysis of Gibbsian interfacial excess at boundaries, including the capability of excess mapping.

Silicide-phase evolution and platinum redistribution during silicidation of Ni0.95Pt0.05/Si(100) specimens

We investigated the temporal evolution of nickel-silicide phase-formation and the simultaneous redistribution of platinum during silicidation of a 10 nm thick Ni0.95Pt0.05 film on a Si(100) substrate. Grazing incidence x-ray diffraction (GIXRD) and atom-probe tomography (APT) measurements were performed on as-deposited films and after rapid thermal annealing (RTA) at 320 or 420 °C for different times. Observation of the Ni2Si phase in as-deposited films, both with and without platinum alloying, is attributed to surface preparation. RTA at 320 °C for 5 s results in the formation of the low-resistivity NiSi intermetallic phase and nickel-rich phases, Ni2Si and Ni3Si2, as demonstrated by GIXRD measurements. At 420 °C for 5 s, the NiSi phase grows outward from the silicide/Si(100) interface by consuming the nickel-rich silicide phases. On increasing the annealing time at 420 °C to 30 min, this reaction is driven towards completion. The nickel-silicide/silicon interface is reconstructed in three-dimensions employing APT and its chemical root-mean-square roughness, based on a silicon isoconcentration surface, decreases to 0.6 nm with the formation of the NiSi phase during silicidation. Pt redistribution is affected by the simultaneous reaction between Ni and Si during silicidation, and it influences the resulting microstructure and thermal stability of the NiSi phase. Short-circuit diffusion of Pt via grain boundaries in NiSi is observed, which affects the resultant grain size, morphology, and possibly the preferred orientation of the NiSi grains. Pt segregates at the NiSi/Si(100) heterophase interface and may be responsible for the morphological stabilization of NiSi against agglomeration to temperatures greater than 650 °C. The Gibbsian interfacial excess of Pt at the NiSi/Si(100) interface after RTA at 420 °C for 5 s is 1.2 ± 0.01 atoms nm−2 and then increases to 2.1 ± 0.02 atoms nm−2 after 30 min at 420 °C, corresponding to a decrease in the interfacial free energy of 7.1 mJ m−2.

Effects of Li additions on precipitation-strengthened Al–Sc and Al–Sc–Yb alloys

Two Al–Sc-based alloys (Al–0.12Sc and Al–0.042Sc–0.009Yb, at.%) and their counterparts with Li additions (Al–2.9Li–0.11Sc and Al–5.53Li–0.048Sc–0.009Yb, at.%) are aged at 325°C. For both base alloys, the addition of Li results in greater peak hardness from incorporation of Li in the L12-structured α′-Al3(Sc,Li) and α′-Al3(Sc,Li,Yb) precipitates, and a concomitant increase in number density and volume fraction of the precipitates and a reduction in their mean radius. These changes result from a combination of: (i) an increase in the driving force for precipitate nucleation due to Li; (ii) a decrease in the elastic energy of the coherent misfitting precipitates from a decrease in their lattice parameter mismatch due to their Li content; and (iii) a decrease in the interfacial free energy, as determined from measurements of the relative Gibbsian interfacial excess of Li. In Al–2.9Li–0.11Sc (at.%), the Li concentration of the precipitates decreases from 9.1at.% in the peak-aged state (8h) to 5.7at.% in the over-aged state (1536h). As a result, the precipitate volume fraction decreases from 0.56% at peak aging time to 0.45% at 1536h. In Al–5.53Li–0.048Sc–0.009Yb (at.%), the relatively limited Li concentration produces only a small increase in Vickers microhardness from precipitation of metastable δ′-Al3Li upon a second aging at 170°C following the primary aging at 325°C.

Segregation of tungsten at γ′(L12)/γ(fcc) interfaces in a Ni-based superalloy: An atom-probe tomographic and first-principles study

γ ( fcc ) / γ ′ ( L 1 2 ) heterophase interfaces in a Ni-based superalloy are investigated using atom-probe tomography and first-principles calculations. Flat {100} interfaces exhibit a confined (nonmonotonic) Gibbsian interfacial excess of tungsten, ΓW=1.2±0.2 nm−2, corresponding to a 5 mJ m−2 decrease in interfacial free energy. Conversely, no measurable segregation of W is detected at curved interfaces. First-principles calculations for a Ni–Al–W system having a {100} interface indicate a decrease in the interfacial energy of 5 mJ m−2 due to W segregation. Similar calculations for {110} and {111} interfaces predict an increase of 1 and 9 mJ m−2 in their energies, respectively, and therefore no heterophase segregation.