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At least 667 records · Page 37
All sourcesNASA NTRSDOE OSTI
DOE OSTI2025

Impact of Halogen Groups on the Properties of PEA–Based 2D Pb–Sn Halide Perovskites

Tuning broad emission in 2D Pb–Sn halide perovskites (HPs) is essential for advancing optoelectronic applications, particularly for color-tunable and white-light-emitting devices. This broad emission is linked to structural factors, such as defects and phase segregation of the Pb component within the Pb–Sn system, which are strongly influenced by the molecular structure and chemical properties of spacer cations. Atomic tuning of the spacers via halogenation opens up a new way to fine-tune the molecular properties, enabling further augmentations of HP functionalities. Nevertheless, the distinct broad emission's sensitivity to spacer chemistry remains underexplored. Here, halogenation's influence is systematically investigated on 2D HP emission characteristics using a high-throughput workflow. These findings reveal that the F-containing phenethylammonium (4F-PEA) spacer narrows the broadband PL, whereas Cl broadens it. Through a correlative study, it is found that 4F-PEA reduces not only the local phase segregation but also the defect levels and microstrains in 2D HPs. This is likely attributed to the manifestation of less lattice distortion via stronger surface coordination of the dipole-augmented 4F-PEA. Furthermore, these results highlight halogenation as a key factor in modulating phase segregation and defect density in 2D Pb–Sn HPs, offering a promising pathway to tune the emission for enhanced optoelectronic performance.

2D Pb-Sn halide perovskites↗
DOE OSTI2018

Evolution of A-Type Macrosegregation in Large Size Steel Ingot After Multistep Forging and Heat Treatment

A-type macrosegregation refers to the channel chemical heterogeneities that can be formed during solidification in large size steel ingots. In this research, a combination of experiment and simulation was used to study the influence of open die forging parameters on the evolution of A-type macrosegregation patterns during a multistep forging of a 40 metric ton (MT) cast, high-strength steel ingot. Macrosegregation patterns were determined experimentally by macroetch along the longitudinal axis of the forged and heat-treated ingot. Mass spectroscopy, on more than 900 samples, was used to determine the chemical composition map of the entire longitudinal sectioned surface. FORGE NxT 1.1 finite element modeling code was used to predict the effect of forging sequences on the morphology evolution of A-type macrosegregation patterns. For this purpose, grain flow variables were defined and implemented in a large scale finite element modeling code to describe oriented grains and A-type segregation patterns. Examination of the A-type macrosegregation showed four to five parallel continuous channels located nearly symmetrical to the axis of the forged ingot. In some regions, the A-type patterns became curved or obtained a wavy form in contrast to their straight shape in the as-cast state. Mass spectrometry analysis of the main alloying elements (C, Mn, Ni, Cr, Mo, Cu, P, and S) revealed that carbon, manganese, and chromium were the most segregated alloying elements in A-type macrosegregation patterns. The observed differences were analyzed using thermodynamic calculations, which indicated that changes in the chemical composition of the liquid metal can affect the primary solidification mode and the segregation intensity of the alloying elements. Finite element modeling simulation results showed very good agreement with the experimental observations, thereby allowing for the quantification of the influence of temperature and deformation on the evolution of the shape of the macrosegregation channels during the open die forging process.

36 MATERIALS SCIENCE↗
DOE OSTI2018

On the Alternate Stirring Mode of F-EMS for Bloom Continuous Castings

Local solute transportation behaviors under different alternate stirring parameters of final electromagnetic stirring (F-EMS) and their influences on the internal quality of the as-cast bloom are compared and evaluated based on a developed coupled model of electromagnetism, heat, and solute transport. To this end, plant trials were conducted in Shaoguan Steel, China. Under the action of F-EMS, a negative segregation band in an ellipse shape is observed at the central area of strand cross section, where the minimum carbon segregation degree is decreased from 0.98 to 0.84 as the stirring duration increases from 15 to 35 seconds in the alternate stirring mode, while it is reduced to 0.805 in the continuous stirring mode. The white band and shrinkage cavity are simultaneously observed at strand center under the conditions of continuous stirring mode, and alternate stirring mode with a stirring period of 35 seconds because of the local over-sustaining melt rotation. In contrast, the V-shape porosity belt width and strand center segregation fluctuation range increase from 60 to 90 mm and from 0.12 to 0.30, respectively, as the stirring duration is reduced from 25 to 15 seconds in the alternate stirring mode because of the poor mixing of the local melt at the strand center.

36 MATERIALS SCIENCE↗
DOE OSTI2019

A Comprehensive Analysis of Macrosegregation Formation During Twin-Roll Casting

A two-phase Eulerian–Eulerian volume-averaged model is used to predict the formation of macrosegregation during the twin-roll casting of inoculated Al-4 wt pct Cu alloys. For low solid fractions, the equiaxed crystals are modeled according to the submerged object approach. However, above a given transition limit, they are assumed to behave like a viscoplastic material. This means that the solid phase behaves as a coherent structure that can influence the motion of the liquid. Such approach allows one to take into account the flow dynamics arising from the occurrence of both solidification and hot rolling simultaneously, which usually occurs in twin-roll casting. Therefore, it is possible to explain the origin of the macrosegregation patterns obtained in the simulations based on the relative motion between the phases. Compression-induced expulsion of segregated melt is observed as a result of the deformation of the solidifying shells. Such occurrence leads to a negative macrosegregation region in the outer part of the as-cast strip. Then, because the solute-enriched melt is squeezed out toward adjacent regions, two positively segregated bands can be found near the center of the domain. Furthermore, it is shown how solidification-induced feeding weakens the absolute value of the negative and positive segregation bands.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Development of Stretch Flangeable Grade Steels by Inclusion Engineering Approach

The demand for good stretch flangeability in high-strength automotive steels has necessitated due attention on steel cleanliness and processes. The presence of elongated MnS stringers and centerline segregation of nonmetallic inclusions adversely affects the transverse mechanical property. In the present investigation, stretch flangeability of industrially produced steel, with and without Ca treatment, has been compared with Ti-added steel, produced at 40 kg scale without Ca treatment. The study brings out the influence of centerline segregation and modification of nonmetallic inclusions on HER performance. The microstructural analysis of industrial steel revealed that the method of hole formation (punching/drilling) determines crack initiation mechanism. The inclusions, along with the second phase present in the centerline, drive the crack propagation. An addition of Ti in liquid steel promotes formation of Al-Ti-O-based oxide, prior to MnS precipitation, during solidification and acts as nucleation site for MnS inclusion. This obviates the elongated stringer-type MnS inclusions. The centerline segregation-free Ti-modified steel, prepared at the laboratory scale, showed consistent HER property compared with industrial steel. It is also concluded that calcium treatment can be circumvented by modifying the sulfide stringer with the addition of Ti in the steel for further improvement in HER property.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Effect of Eutectic Characteristics on Hot Tearing of Cast Superalloys

The occurrence of hot cracks in the blades of Ni-based superalloy turbocharger wheels is thought to be closely related to eutectic characteristics. Different alloys that are susceptible to hot tearing, namely K418, K419 and K424, were studied. SEM was used to obtain the microstructure of the alloys. DSC, isothermal solidification quenching process and EPMA were conducted to determine the relationship between the eutectic characteristics and solidification process, segregation behavior and dendrite coalescence. The results show that a large volume fraction of γ/γ′ eutectic indicates a wide-temperature solidification range and severe segregation. A large γ/γ′ eutectic size is a result of element segregation and poor dendrite coalescence. Ti is thought to promote the formation of a large isolated residual liquid pool and lead to a large eutectic size, leading to the poor coalescence behavior of the dendrites. The volume fraction and size of the γ/γ′ eutectic were considered, and a eutectic influence factor (E) was defined to evaluate the effect of eutectic characteristics on hot tearing in cast superalloys. It was found that a high E corresponds to high hot-tearing susceptibility.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Stability of binary nanocrystalline alloys against grain growth and phase separation

Grain boundary segregation has been established through both simulation and experiments as a successful approach to stabilize nanocrystalline materials against grain growth. However, relatively few alloy systems have been studied in this context; these vary in their efficacy, and in many cases the stabilization effect is compromised by second phase precipitation. We address the open-ended design problem of how to select alloy systems that may be stable in a nanocrystalline state. We continue the development of a general “regular nanocrystalline solution” model to identify the conditions under which binary nanocrystalline alloy systems with positive heats of mixing are stable with respect to both grain growth (segregation removes the grain boundary energy penalty) and phase separation (the free energy of the nanocrystalline system is lower than the common tangent defining the bulk miscibility gap). We calculate a “nanostructure stability map” in terms of alloy thermodynamic parameters. Three main regions are delineated in these maps: one where grain boundary segregation does not result in a stabilized nanocrystalline structure, one in which macroscopic phase separation would be preferential (despite the presence of a nanocrystalline state stable against grain growth) and one for which the nanocrystalline state is stable against both grain growth and phase separation. Additional details about the stabilized structures are also presented in the map, which can be regarded as a tool for the design of stable nanocrystalline alloys.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Effects of neutron irradiation and post-irradiation annealing on the microstructure of HT-UPS stainless steel

Microstructural changes resulted from neutron irradiation and post-irradiation annealing in a high-temperature ultra-fine precipitate strengthened (HT-UPS) stainless steel were characterized using transmission electron microscopy (TEM) and atom probe tomography (APT). Three HT-UPS samples were neutron-irradiated to 3 dpa at 500 °C, and after irradiation, two of them were annealed for 1 h at 600 °C and 700 °C, respectively. Frank dislocation loops were the dominant defect structure in both the as-irradiated and 600 °C post-irradiation-annealed (PIAed) samples, and the loop sizes and densities were similar in these two samples. Unfaulted dislocation loops were observed in the 700 °C PIAed sample, and the loop density was greatly reduced in comparison with that in the as-irradiated sample. Nano-sized MX precipitates were observed under TEM in the 700 °C PIAed sample, but not in the 600 °C PIAed or the as-irradiated samples. The titanium-rich clusters were identified in all three samples using APT. The post-irradiation annealing (PIA) caused the growth of the Ti-rich clusters with a stronger effect at 700 °C than at 600 °C. The irradiation caused elemental segregations at the grain boundary and the grain interior, and the grain boundary segregation behavior is consistent with observations in other irradiated austenitic steels. Here, APT results showed that PIA reduced the magnitude of irradiation induced segregations.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Understanding the effect of minor alloying elements on helium bubble formation in ferritic-martensitic steels

Ferritic-martensitic steels are promising structural materials for advanced nuclear reactors. To minimize long-term radioactivity, reduced-activation ferritic-martensitic steels have been developed by substituting high-activation elements like Ni and Mo with low-activation elements such as W. However, the impact of these alloying modifications on helium bubble formation, which plays a key role in material swelling, remains unclear. Here, in this study, we compared helium bubble formation in ferritic-martensitic steel T91 and reduced-activation ferritic-martensitic steel F82H. Both materials were irradiated with sequential 100 keV, 150 keV, and 200 keV helium ions to a dose of 0.5 dpa and a helium concentration of 9,000 appm at 500°C. The helium bubbles in F82H exhibited a larger average size and a lower density than those in T91, suggesting differences in minor alloying elements may influence the bubble growth. Here, to investigate the effects of these alloying elements, we characterized radiation-induced segregation near bubbles and grain boundaries. Prominent Ni-Mn-Si enriched clusters were found near bubbles in T91, while only Mn-Si enriched clusters were found near bubbles in F82H. In addition, the obvious Cr enrichment near grain boundaries was absent around bubbles in both steels. The different segregation trends among elements revealed the variations in element diffusion mechanisms and the different sink biases between bubbles and grain boundaries. Cr enrichment near grain boundaries is mostly driven by interstitial-mediated diffusion. However, since bubble growth relies on net vacancy flux, vacancy-mediated diffusion plays a dominant role in controlling element segregation near bubbles. Therefore, Cr enrichment was not found near bubbles. Because of preferential vacancy-drag diffusion for Ni, Si and Mn, these elements were enriched near bubbles. Due to the strong binding energies of vacancies with these solute atoms, the vacancy diffusivity can be reduced near these solutes. Therefore, the more prominent Ni-Si-Mn clustered near helium bubbles in T91 lead to stronger suppression of helium bubble growth compared to F82H.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Influence of yttrium content on the location of rare earth ions in LYSO:Ce crystals

Single-crystal X-ray diffraction (SCD), X-ray fluorescence (XRF), inductively coupled plasma optical emission spectroscopy (ICP-OES) and X-ray excited luminescence (XEL) measurements were performed to investigate structure details and segregation coefficients of (Lu{sub 1−x}Y{sub x}){sub 2}SiO{sub 5}:Ce (x=0 at%, 8.7 at%, 25.7 at%, 44.7 at%, 65.7 at%, 87.9 at% and 100 at%). Y{sup 3+} cations were found to have a preferential occupation for RE1 site (7-oxygen-coordinated) over RE2 site (6-oxygen-coordinated), which results in a greater increase of cell parameter c than that of a with increase in Y content due to LYSO's microstructure characteristics. Results presented here revealed that the less the difference in electronegativity and effective ionic radius between the two ions, the easier substitution of one ion by the other, and hence the higher segregation coefficients. Besides, the contribution of luminescence of Ce1 and Ce2 in the whole XEL was evaluated, and the location of Ce{sup 3+} ion was discussed. - Graphical abstract: Segregation coefficients of (Lu{sub 1−x}Y{sub x}){sub 2}SiO{sub 5}:Cce:italic> at RT/ce:italic>. Display Omitted.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Mobility of pinned twin boundaries during mechanical loading of extruded binary Mg-1Zn alloy

Highlights: • Intermediate heat treatment results in element segregation along twin boundaries. • Effect of pinned twin boundaries on their mobility was investigated. • Detwinning is realized by migration of twin boundaries or nucleation of new twins. • Pinning of twin boundaries does not completely disrupt their mobility. • Thermo-mechanical treatment compensates yield strength asymmetry. - Abstract: The influence of isothermal aging, i.e. solute segregation and precipitation, on the mobility of {10−12} twin boundaries during mechanical loading of a binary Mg-1wt.%Zn alloy was investigated. Segregation along twin boundaries leads to a pinning effect and therefore, twin growth and shrinkage based on twin boundary migration are restricted. The development of microstructure changes, especially the twinning activity, is tracked by using scanning electron microscopy. During reverse loading, a reduction of the twinned volume fraction (detwinning) is realized either by (i) nucleation of new twins inside already existing twin lamellae or by (ii) migration of pinned twin boundaries, similarly to the case, when no heat treatment is applied. No effect of the orientation of the grain with respect to the loading direction during detwinning was observed.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Effect of secondary phase particles on thermal stability of ultra-fine grained Mg-4Y-3RE alloy prepared by equal channel angular pressing

Highlights: • UFG structure with grain size of ~340 nm was achieved in WE43 by ECAP. • Fine Mg{sub 5}RE particles are responsible for microstructural stability up to 280 °C. • Segregation of Y and RE solutes in grain boundaries was observed. • Hardening at 200–280 °C was observed due to segregation and precipitation. • Effect of particles dissolution and grain growth on softening was discussed. - Abstract: As-cast magnesium alloy WE43, containing yttrium and rare earth elements, was processed by equal channel angular pressing (ECAP). The processing led to a significant grain refinement together with a massive precipitation of the secondary phase particles. Thermal stability of the ultra-fine grain (UFG) structure together with microstructural changes due to exposure to elevated temperatures were studied by several complementary techniques in the temperature range of 160–500 °C. It was found that UFG structure consisting of grains with size of ~340 nm and high density of Mg{sub 5}RE particles is stable up to 280 °C for 1 h of annealing. Moreover, only negligible change of the microstructure occurred after annealing for 16 h at 250 °C. Excellent thermal stability of UFG structure was caused by fine Mg{sub 5}RE particles, which suppressed the grain growth. Exceeding the limit of thermal stability of these particles above 280 °C resulted in material softening. Moreover, statistically significant hardening of the UFG material occurred in the temperature range of 200–280 °C. Segregation of yttrium and rare earth elements and eventually precipitation at grain boundaries was proved to be responsible for observed hardening by in-situ and ex-situ transmission electron microscope and positron annihilation spectroscopy analysis. Finally, individual effect of particles dissolution and grain growth on the material softening was investigated and discussed.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Influence of surface relaxation on solute atoms positioning within atom probe tomography reconstructions

Highlights: • APT measurement of Al/Cu alloy is simulated with a combined electrostatics and molecular dynamics (MD) approach. • Results show segregation of dilute Cu at [111] poles in correspondence to experimental measurements. • No temperature is involved. The effect originates in both disparate evaporation field and bond-energies in the alloy. While thermal or field-induced surface migration has been invoked often in past literature to rationalize the experimental observations of solute and impurity atoms segregation to specific poles and zones lines in atom probe tomography (APT) reconstructions, it does not satisfactorily explain all experimental results. Using a combined finite-element and molecular dynamics simulation approach that models the field evaporation process from mechanically relaxed emitter structures, we show that an additional mechanism, athermal relaxation, can also drive significant solute migration. The mechanism is illustrated for Cu in solid solution in Al, which exhibits segregation of Cu to 111 poles and along [110] zone lines in reconstructed APT data. The higher bonding energy of Cu atoms on the surface, confirmed using density-functional theory calculations, causes longer retention on the surface and preferential evaporation of neighboring Al atoms with subsequent athermal relaxation of Cu into higher-neighbor-count sites left vacant by evaporating Al neighbors. This in turn results in barrier-free long-range migration of the Cu atoms by a series of relaxation events that follow the receding terrace ledges towards high-neighbor count poles and zone lines. This mechanism fundamentally limits spatial resolution in the limit of zero temperature.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Anticipating impacts of introducing aluminum-cerium alloys into the United States automotive market

Increasing demand of abundant co-product rare earths such as cerium and lanthanum has become a focus for improving mine economics. The recent development of an aluminum-cerium (Al-Ce) alloy targeted toward broad market adoption in the automotive industry is one recent example. This alloy has shown exceptional mechanical properties and cost-saving potential. However, since market adoption is unknown, impacts of this alloy on Ce demand remains uncertain. We estimate Al-Ce impacts by simulating adoption in different automotive sectors using a dynamic material flow analysis model. We then project future Ce content in the recycled Al stream given current recycling practices. We also compute the required segregation given different Ce impurity tolerances of existing automotive Al alloys. Finally, once secondary supply is derived from segregation, we calculate primary Ce demand. Results showed that segregation of Al-Ce components from non-AlCe automotive Al components is needed for most adoption scenarios and impurity thresholds. Ce metal demand was found to be lowest at 1000 mt/yr if the target market is light duty trucks. If Al-Ce could penetrate the High Efficiency Spark Ignition market (i.e., turbocharger market), Ce demand could reach 23,200 mt Ce/year in 2045, without accounting for Chinese and Australian Ce primary production capacity. Furthermore, when accounting for primary Ce production capacity, demand could reach up to 16,200 mt/yr.

99 GENERAL AND MISCELLANEOUS↗
DOE OSTI2025

The physical origin of heterogeneous solute clustering and nanoprecipitation at grain boundaries in ultrafine-grained immiscible alloys

Here, grain-boundary segregation effects on heterogeneous solute clustering and nanoprecipitation at low solute concentrations were investigated in sputter-deposited ultrafine-grained Ag-Cu alloy films. X-ray diffraction and scanning transmission electron microscopy revealed extended solubility of Cu in Ag matrix, accompanied by the formation of Cu-rich nanoprecipitates and solute clusters at grain boundaries and their junctions. Atomistic simulations further demonstrated that Cu solutes heterogeneously segregate to Ag GBs and form small clusters that grow into nuclei for Cu nanoprecipitates. These findings provide critical insights into the role of heterogeneous grain-boundary segregation in governing the phase separation pathways of immiscible nanocrystalline and ultrafine-grained alloys.

Heterogeneous segregation↗
DOE OSTI2025

Czochralski Growth and Characterization of a Compositionally Complex Rare Earth Aluminum Garnet Scintillator: (Gd 1/4 Y 1/4 Tb 1/4 Lu 1/4 ) 3 Al 5 O 12 :Ce

Compositionally complex oxides have garnered increasing interest for their enhanced phase stability and tunable functional properties, yet their development as bulk single crystal scintillators remains limited. Herein, we report the Czochralski growth and characterization of (Gd 1/4 Y 1/4 Tb 1/4 Lu 1/4 ) 3 Al 5 O 12 :Ce (GYTLAG), a compositionally complex garnet incorporating four dodecahedrally coordinated principal rare earth elements. The garnet phase was confirmed by powder and single crystal X-ray diffraction, and macroscopic defects are described. X-ray absorption near-edge structure measurements confirm the 3+ oxidation state of all rare earths and support their occupation of the same crystallographic site; white line intensity variations correlate with the anticipated segregation behavior. Elemental segregation is quantified by SEM/EDS and ICP-OES, and a linear trend was established between the segregation coefficient and the difference between each rare earth’s ionic radius (r) and the average ionic radius (AIR) of the dodecahedral site. This trend offers a predictive framework for compositional control in future REAG crystals grown by the Czochralski method. Photoluminescence and radioluminescence measurements reveal both Ce 3+ and Tb 3+ emission. Scintillation pulses exhibit four-component decay with dominant ~230 µs and ~1.2 ms components, and the light yield is estimated to be up to 43,000 ph/MeV under 137Cs γ-ray excitation. GYTLAG also demonstrates a strong radioluminescence efficiency and 50% lower afterglow at 20 ms compared to a LuAG:Ce reference, underscoring its promise for scintillator applications.

Compositionally complex oxide, high entropy oxide,↗
DOE OSTI2025

Cu–Ni Oxidation Mechanism Unveiled: A Machine Learning-Accelerated First-Principles and in Situ TEM Study

Here, the development of accurate methods for determining how alloy surfaces spontaneously restructure under reactive and corrosive environments is a key, long-standing, grand challenge in materials science. Using machine learning-accelerated density functional theory and rare-event methods, in conjunction with in situ environmental transmission electron microscopy (ETEM), we examine the interplay between surface reconstructions and preferential segregation tendencies of CuNi(100) surfaces under oxidation conditions. Our modeling approach predicts that oxygen-induced Ni segregation in CuNi alloys favors Cu(100)-O c(2 × 2) reconstruction and destabilizes the Cu(100)-O (2√2 × √2)R45° missing row reconstruction (MRR). In situ ETEM experiments validate these predictions and show Ni segregation followed by NiO nucleation and growth in regions without MRR, with secondary nucleation and growth of Cu 2 O in MRR regions. Our approach based on combining disparate computational components and in situ ETEM provides a holistic description of the oxidation mechanism in CuNi, which applies to other alloy systems.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Exotic Long-Range Surface Reconstruction on La 0.7 Sr 0.3 MnO 3 Thin Films

Due to an extremely diverse phase space, La 1–x Sr x MnO 3 , as with other manganites, offers a wide range of tunability and applications including colossal magnetoresistance and use as spin-polarized electrodes. Here, we study an unprecedented, exotic surface reconstruction (6 × 6) in La 1–x Sr x MnO 3 (x = 0.3) observed via low-energy electron diffraction (LEED). Scanning tunneling microscopy (STM) shows the surface is relatively flat, with unit-cell step heights, and X-ray photoelectron spectroscopy (XPS) reveals a strong degree of Sr segregation at the surface. By combining electron diffraction and first-principles computations, we propose that the long-range surface reconstruction consists of a Sr-segregated surface with La (6 × 6) ordering. This study expands our understanding of manganite systems and underscores their ability to form interesting surface reconstructions, driven largely by cation segregation that can potentially be controlled for tuning surface ordering.

36 MATERIALS SCIENCE↗