SEARCH · Engineering Papers

Results for “Segregation”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

SourceResearch connectionFrom yearThrough year

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 991 records · Page 55

All sources NASA NTRS DOE OSTI

DOE OSTI2018

Solidification Behavior of Calcium Ferrite Under Ultrasonic Vibration

Ultrasonic vibration was applied to the solidification process of calcium ferrite (CF) to modify the grain size and improve the homogeneity of the solidified microstructure. The effects of ultrasound on microstructure, density, compressive strength, and reducibility were studied. The acoustic pressure decreased and then increased along the axis symmetry due to the attenuation of acoustic pressure and reflection of ultrasonic waves in the melt under a fixed power. When the ultrasonic power was increased, the general acoustic pressure increased, which was beneficial to the avoidance of macropores and to improvement of the microstructure. Coarse acicular CF grains were significantly refined; the grain size decreased from 1967 to 442 μm when the ultrasonic power was increased from 0 to 300 W. The density of the samples increased from 4370 to 4680 kg/m{sup 3} due to cavitation and acoustic streaming effects. Ultrasonic vibration greatly improved the ultimate compressive strength of CF; the values were 41.8 ± 2.1 to 67.3 ± 3.4 MPa for ultrasonic powers of 100 to 300 W, respectively. Without ultrasonic treatment, it was only 37.4 ± 1.9 MPa. Ultrasonic vibration also influenced the reduction time of CF due to the composition segregation in the slag during solidification.

36 MATERIALS SCIENCE↗

DOE OSTI2018

Microsegregation Measurement: Methods and Applications

Microsegregation is chemical variation at the microstructural level in a solidified ingot, which can cause undesirable effects on material properties. Its formation is driven by different levels of solute elemental solubility between solid and liquid phases, resulting in solute buildup or depletion in the liquid phase during solidification. Characterization of microsegregation can be essential for understanding its formation and removing it through heat treatment. Measurement is frequently carried out via scanning electron microscope (SEM), though metallographic analysis, mechanical testing and optical radiography have been used. SEM analysis can consist of point measurements on single dendrites, line scans across or along dendrites, elemental maps, and extended line-based or grid-based analyses extending over many dendrites, which can generate large amounts of data. Line-based or grid-based data can be analyzed using scatterplots of element concentrations to visualize segregation behavior and sorting/ranking of data by a single element or a combination of elements, to give compositional profiles as a function of extent of solidification. For both qualitative and quantitative analyses, effects of phase transformations and secondary phases need to be considered as well as sorting of measurement errors when generating compositional profiles. Applications of analysis include determination of partition ratios, examination of solute–element interactions, determination of effects of cooling rate and back diffusion, applications to freckling studies, validations of modeling, and development of heat-treatment profiles.

36 MATERIALS SCIENCE↗

DOE OSTI2019

Unexpected Charpy Impact Toughness Spike of 1.25Cr-0.5Mo Steel After Simulated Postweld Heat Treatment

The present study elaborates on the effect of simulated postweld heat treatment (SPWHT) on microstructure and mechanical property variations of 1.25Cr-0.5Mo steel. With the increase in SPWHT holding time, lath ferrite tends to morph into blocky ferrite, and carbides aggregate along grain boundaries. Although tensile strength keeps decreasing, Charpy impact toughness undergoes a spike, followed by a sharp decrease. Such an interesting phenomenon is explained by the competition between high-angle grain boundary population and carbide segregation, and bodes well to the pressure vessel industry for optimized PWHT.

36 MATERIALS SCIENCE↗

DOE OSTI2019

Assessing the Influence of NaCl on the Reduction of a Siliceous Laterite Nickel Ore Under Caron Process Conditions

The reduction by hydrogen and the thermal decomposition of a siliceous nickel laterite ore in the presence of NaCl were studied using thermogravimetric analysis (TGA). Reduction tests on H{sub 2} atmosphere in rotary kiln were performed in the presence of NaCl and the product of the reaction was leached in ammoniacal solution. The presence of only 1 pct of NaCl in the furnace increased the nickel extraction values from ≈ 3 to ≈ 90 pct when the experiment was carried out at 850 °C. According to the results of X-ray diffractometry and TGA, the presence of NaCl in the system and the reducing atmosphere of H{sub 2} promote the segregation of nickel according to the following steps: (1) Nickel oxide reduction by hydrogen: NiO + H{sub 2(g)} → Ni + H{sub 2}O{sub (g)}. (2) Formation of HCl: Al{sub 2}SiO{sub 5} + 2NaCl + SiO{sub 2} + H{sub 2}O → 2HCl(g) + 2NaAlSiO{sub 4}. (3) Chlorination of metallic nickel by HCl: Ni + 2HCl{sub (g)} → NiCl{sub 2} + H{sub 2(g)}. (4) Reduction of nickel chloride by hydrogen: NiCl{sub 2} + H{sub 2(g)} → Ni + 2HCl{sub (g)}.

36 MATERIALS SCIENCE↗

DOE OSTI2019

Porosity Prediction in A356 Wheel Casting

A FLUENT™-based thermal-fluid-compositional model has been developed and applied to a low-pressure, die-cast, A356 aluminum alloy wheel to explore the effects of macro-segregation of hydrogen and late-stage liquid encapsulation on pore formation during solidification. Based on the evolution of temperature, pressure, and hydrogen concentration in the liquid output from FLUENT, the porosity size distribution has been predicted at a number of locations in a commercially produced wheel using an in-house micro-porosity model. The porosity model incorporates pore nucleation kinetics based on hydrogen supersaturation, and two modes of pore growth: the first, occurring at low solid fractions is based on hydrogen diffusion; and the second, occurring at high solid fractions is based on conservation of volume. Samples from a commercially cast wheel have been analyzed using X-ray micro-tomography to provide basic validation of the micro-porosity model. The predicted results showed that the evolution in pressure has the dominant effect on pore growth, but only under conditions where pores have nucleated prior to late-stage liquid encapsulation. Otherwise, the cooling rate appears to have the dominant effect. The accuracy of the model is discussed in terms of the predicted pore size distribution and number density of pores. Areas for continued development are presented.

36 MATERIALS SCIENCE↗

DOE OSTI2019

Structure and Reactivity of Low-Rank Coal Chars Prepared from Fluidized Bed and Moving Bed Pyrolyzers and the Potential for Using Them in Pulverized Coal Injection (PCI)

Utilizing pyrolysis chars as the injection fuel in a blast furnace can reduce the dependence on high-quality pulverized coal injection (PCI) coal. In this study, the physical properties and reaction characteristics of chars prepared from two different industrial-scale pyrolyzers (fluidized bed and moving bed) are investigated. The results reveal remarkable component segregation in the char particles with different particle sizes. The properties of large-size char particles are found to better satisfy the PCI fuel requirement in a blast furnace. Compared with PCI coal, the char particles exhibit a rough surface, well-developed pore structure, and less-ordered chemical structure of carbon. Results from isothermal combustion, isothermal gasification, and drop tube furnace experiments show that both types of chars exhibit a better reactivity than PCI coal. The excellent reactivity of chars is closely related to its high Brunauer–Emmett–Teller surface area and relatively poor chemical ordering during conversion. Based on these characteristics, both fluidized bed char (FBC) and moving bed char (MBC) have a great potential to be used as PCI fuels. The application method of chars for PCI in a blast furnace is proposed finally.

01 COAL, LIGNITE, AND PEAT↗

DOE OSTI2019

Effect of Diffusion Length in Modeling of Equiaxed Dendritic Solidification under Buoyancy Flow in a Configuration of Hebditch–Hunt Experiment

Modeling of equiaxed solidification is vital for understanding the solidification process of metallic alloys. In this work, an extended literature review is given for the models currently used for equiaxed solidification simulations. Based on this analysis, we present a three-phase multiscale equiaxed solidification model in which some approximations regarding solute transport at microscopic scale are put together in a new way and incorporated into macroscopic transport equations. For the latter, a term relating to the momentum exchange between the two phases is revised, and a modification for the grain packing algorithm is proposed. A modernized model for equiaxed dendrite growth is tested using a case of solidification of Sn-5 wt pct Pb alloy in a parallelepiped cavity that mimics the Hebditch–Hunt experiment. The results obtained using two approaches to calculate diffusion length are presented and compared both with each other and with numerical results from elsewhere. It is demonstrated that diffusion length has a crucial effect on the final segregation pattern.

36 MATERIALS SCIENCE↗

DOE OSTI2017

Precipitation Kinetics of M{sub 23}C{sub 6} Carbides in the Super304H Austenitic Heat-Resistant Steel

The precipitation kinetics of M{sub 23}C{sub 6} carbides in Super304H and TP304H steels were investigated using the selective-etching method, SEM backscattered electron images and Image-Pro-Plus 6.0 software. Precipitation–temperature–time (PTT) diagrams of M{sub 23}C{sub 6} carbides in the as-received Super304H (fine grains), coarsened Super304H (coarse grains) and TP304H (coarse grains) steels all show the typical C-shaped character with nose temperature range from 800 to 850 °C. Compared with the TP304H steel, the same trend is found of the PTT curve of M{sub 23}C{sub 6} carbides for both kinds of Super304H steels, but their start lines move to the right and finish lines to the left. The preferential formation of Nb(C,N) phase at grain boundaries in the Super304H steels inhibited the nucleation of M{sub 23}C{sub 6} carbides in the early stage of precipitation, causing the right shift of the start line of PTT curve. The main reason for the left shift of the finish line of the two Super304H steels was the quicker growing and coarsening rate of M{sub 23}C{sub 6} in the later precipitation stage due to their higher C content than in TP304H. For the difference in PPT curves between the two grain sizes of the Super304H steel, the lower diffusion rate of atoms in the coarse-grained Super304H steel may explain its righter finish line than the fine-grained counterpart, while the reason for its lefter start line is due to the higher solute segregation along coarse-grained boundaries.

36 MATERIALS SCIENCE↗

DOE OSTI2017

Effect of Mo-rich Fillers in Pulsed Current Gas Tungsten Arc Welding of Inconel 718 for Improved Strength and Hot Corrosion Resistance

The formation of Laves phase in the fusion zone of Inconel 718 welds is considered to be detrimental and requires major attention, as this phase deteriorates the mechanical properties and high-temperature corrosion resistance. This present study addresses the use of current pulsing technique on the gas tungsten arc welding of 5-mm-thick Inconel 718 plates. Nb-free fillers, namely ERNiCrMo-10 and ERNiCrMo-14, are chosen for joining these plates. Microstructure studies revealed the segregation of Mo-rich phases at the inter-dendritic regions of the fusion zone of both the weldments. Further, the occurrence of detrimental Laves phase was minimized while employing Mo-rich fillers. It is inferred from the tensile studies that the fractures occurred in the parent metal for ERNiCrMo-10 filler and at the fusion zone while employing ERNiCrMo-14. Room-temperature Charpy impact studies also affirm that the impact toughness of ERNiCrMo-10 welds is significantly greater than that of ERNiCrMo-14. Hot corrosion studies were performed on the coupons by exposing them to a synthetic molten salt environment comprising Na{sub 2}SO{sub 4} + 60%V{sub 2}O{sub 5} at 800 °C for 50 h. Owing to the presence of higher amounts of Cr{sub 2}O{sub 3} and NiO, both fusion zones exhibited better hot corrosion resistance than the parent metal. The novel outcome of the study reiterates that with the use of current pulsing and Mo-rich fillers, the detrimental phase shall be controlled.

36 MATERIALS SCIENCE↗

DOE OSTI2018

Microstructure and Mechanical Properties of Three-Layer TIG-Welded 2219 Aluminum Alloys with Dissimilar Heat Treatments

2219-C10S and 2219-CYS aluminum alloys are 2219 aluminum alloys with different heat treatment processes, and they have been widely used in the aerospace industry. In the present study, 2219-C10S and 2219-CYS aluminum alloys were butt-welded by three-layer tungsten inert gas arc welding (with the welding center of the third layer shifted toward the CYS side), and the microstructure characteristics and mechanical properties of the welded joint were investigated. The lamellar θ′ phases, the bulk or rod θ phases, and the coarse rod-shaped or pancake-shaped Al-Cu-Fe-Mn phases coexisted in the two aluminum alloys. The Cu content of the α-Al matrix and the distribution of eutectic structures of different welding layers in the weld zone (WZ) were varied, implying that the segregation degrees of the Cu element were different due to the different welding thermal cycles in different welding layers. The microhardness values of the CYS side were much higher than those of the C10S side in each region on both sides of the joint. The tensile test deformation was concentrated mainly in the regions of WZ and the over aged zone (OAZ), where the microhardness values were relatively low. The main deformation concentrated region was transferred from the CYS side to the C10S side with the increase in the tensile load during the tensile test. The fracture behavior of the tensile test showed that the macroscopic crack initiated near the front weld toe had gone through the crack blunt region, the shear fracture region of the partially melted zone (PMZ), and the shear fracture region of OAZ. Meanwhile, the fracture characteristics gradually evolved from brittle to ductile. The concentrated stress and the dense eutectic structure in the region near the front weld toe of the C10S side contributed to the fracture of the joint. The shift of the welding center of the third layer to the CYS side resulted in two effects: (i) the microhardness values from the middle layer to the top layer in the PMZ of the CYS side were the most significantly increased and (ii) the distance between the front weld toe and the fusion line of the CYS side was significantly larger.

36 MATERIALS SCIENCE↗

DOE OSTI2018

Microstructural Characterization, Mechanical Properties, and Corrosion Resistance of Dental Co-Cr-Mo-W Alloys Manufactured by Selective Laser Melting

In this paper, the microstructure, mechanical properties, and corrosion resistance of the dental Co-Cr-Mo-W alloys fabricated by selective laser melting (SLM) were systematically investigated and correlated. The microstructure of the SLM group (SG) specimens is composed of extremely fine columnar crystals, exhibiting supersaturated and homogeneous character. The matrix mainly consists of face-centered-cubic (FCC) cobalt solid solutions, and minimal hexagonal-close-packed (HCP) cobalt solid solutions. Mechanical test results showed that the 0.2% yield strength of SG was 775 ± 7 MPa, and the ultimate tensile strength was 1118 ± 25 MPa. The elongation of SG was 8.28 ± 0.4%, and the microhardness of SG was 476 ± 10 HV. All of these values largely exceeded the cast group (CG) specimens. The high strength is attributed to their fine columnar crystals and supersaturated matrices. The fewer content from the HCP phase and a lack of intermetallic compounds or eutectic contribute to the high ductility. Furthermore, a higher E{sub corr}, lower I{sub corr} were acquired for better corrosion resistance compared to cast specimens, due to even less composition segregation and more microstructural homogeneity.

36 MATERIALS SCIENCE↗

DOE OSTI2018

Mechanical Properties of the Mg-Gd-Y-Zn-Zr Alloys with Different Morphologies of Long-Period Stacking Ordered Phases

Long-period stacking ordered (LPSO) phases with different morphologies and distributions were obtained in the direct chill cast Mg-6.9Gd-3.2Y-1.5Zn-0.5Zr (wt.%) alloy by controlling the homogenization time or cooling rate. Microstructural evolution and the effect of various LPSO phases on the mechanical properties were investigated. There is no obvious α-Mg grain growth during homogenization at 510 °C. Intragranular thin-platelet and coagulated LPSO phases hardly improve the strength but help to develop the ductility. Numerous lamellar phases, which contain “ABCA”-type building blocks with RE and Zn segregation, form during the furnace-cooling process. Lamellar phases, which exhibit lower thermostability compared with the thin-platelet LPSO, slightly improve the tensile yield strength and induce numerous cracks which result in the premature fracture.

36 MATERIALS SCIENCE↗

DOE OSTI2021

Powder spreading, densification, and part deformation in binder jetting additive manufacturing

Binder jetting additive manufacturing (AM) can print complex structures in economical and scalable manner. Binder jetting AM comprises of deposition and weak binding of particles, known as green part, at room temperature and subsequent binder removal and sintering densification at high temperatures. However, during the densification (i.e., sintering), the part significantly deforms due to volume shrinkage. The deformation during sintering is difficult to predict, which prevents the widespread application of this technology. In this research, powder spreading simulation using discrete element method (DEM) was performed first to capture local variations in powder bed configuration. Second, finite element method (FEM) with a phenomenological continuum constitutive model was used to predict part shrinkage during the sintering process. DEM simulation showed variations in packing density, particle segregation, formation of uneven powder bed surface, and shift in particle size distribution (PSD). The sintering simulation modeled part deformation with a reasonable accuracy of 3% for solid-state sintering and intermediate liquid phase sintering. A demonstration case with non-uniform initial packing density showed that inhomogeneous green part density and PSD should be accounted for prediction of part deformation in binder jetting AM.

36 MATERIALS SCIENCE↗

DOE OSTI2021

Atomic structures of interfacial solute gateways to θ' precipitates in Al-Cu alloys

Many materials employed in critical structural applications depend upon metastable strengthening precipitates that transform or dissolve at elevated temperatures. Herein, aberration-corrected scanning transmission electron microscopy and first-principles calculations are used to accurately determine the atomic structure of the highly mobile, semi-coherent precipitate interfaces that control this process in the classic θ' (Al 2 Cu) precipitate in the Al-Cu system. Semi-coherent {110} interfaces are found to be composed of an array of unexpected misfit dislocations that are arranged in two different structural units. Dislocations accommodate nearly all of the misfit between the Al matrix and strengthening phase. Cu is observed to segregate to the compressed edge of the dislocation cores at specific sites in this interface. First-principles calculations revealed the energetic landscape that facilitates these sites to become entry and exit gateways of Cu atoms in this semi-coherent interface. In conclusion, this investigation reveals critical features within semi-coherent interfaces that determine the thermal stability of precipitation-hardened alloys.

36 MATERIALS SCIENCE↗

DOE OSTI2025

Development of an interatomic potential for the Ta–Li system

A new interatomic potential for the Ta–Li system is introduced to facilitate the study of phase stability, mechanical properties and non-equilibrium dynamics after Li implantation in Ta. Here, this potential is based on a generalization of the embedded atom method (GEAM) and includes contributions from embedding energy, explicit two- and three-body interactions, and nonlocal many-body interaction terms. The parameters of the potential are optimized using energies and atomic forces for a wide range of configurations obtained from ab initio density functional theory (DFT) calculations. The potential is rigorously validated across a range of physical properties, including elastic constants, equations of state, phonon dispersion curves, point defect properties, and melting temperatures for different compositions. Although our potential is trained on a small dataset, its accuracy is comparable to that of available machine learning potentials for Li and Ta. Our simulations show that at temperatures below 500 K, Li atoms in Ta–Li alloys form clusters separated by Ta-rich domains, and we find no evidence of ordered phase formation. For Li concentrations below a few percent, Li atoms preferentially segregate to surfaces and grain boundaries. However, in alloys containing more than ~10% Li, the accumulation of Li in symmetric-tilt grain boundaries can lead to one of the following effects: formation of amorphous-like regions, changes in grain boundary structural units, or lateral movement of the grain boundary.

GEAM potential↗

DOE OSTI2022

Application of the core shell model for strengthening polymer filament interfaces

While surface segregation has been intensively studied in thin films, relatively little has been investigated in filament geometries. In contrast to thin films, the filament surface forms a continuum, with interactions both at the free air and internal filament interface, which makes it difficult to achieve an equilibrium configuration. This asymmetry has become particularly apparent with the increasing use of additive manufacturing where entire structures are constructed via layered deposition of filamentous layers from a moving thermal nozzle. In addition, this process is highly non-equilibrated with poor thermal retention and is responsible for insufficient filament–filament interfacial fusion and internal pore formation along the printing direction. These features all contribute to poor mechanical and structural properties, which are exacerbated in semi-crystalline polymers, such as Polylactic Acid (PLA), where the melting point, T m , is much higher than the glass transition, T g . Even though system is liquid the molecules become ordered within the crystal structure and this ordering interferes with interdiffusion, which in turn prevents proper interfacial fusion. Capitalizing on the differential in surface tension, we show that addition of less than 1%, or trace amounts, of longer chain polymers, an oriented core shell structure, will be formed with a self-limiting thickness of three time the radius of gyration, R g , of the tracer chains. The lower T g of the tracers and their orientation makes them poised for interdiffusion when subsequent layers are deposited producing structures with significant improvement of the mechanical properties and internal structural integrity.

36 MATERIALS SCIENCE↗

DOE OSTI2021

Heavy ion irradiation response of an additively manufactured 316LN stainless steel

Additive manufacturing has become an appealing technique to fabricate three-dimensional metallic materials and components for nuclear reactors. However, response of additively manufactured alloys to high-dose heavy ion irradiations at elevated temperatures is still not well understood. Here, an additively manufactured 316LN austenitic stainless steel with high-density solidification cells was irradiated using 3.5 MeV Fe ion to a peak dose of 220 dpa at 450 °C. Microscopy studies show a lower Frank loop density and smaller size in the additively manufactured sample compared with its cold worked counterpart, and the cellular structures may largely suppress the formation of perfect loops and dislocation networks and reduce the magnitude of solute segregations comparing with high angle grain boundaries. In conclusion, the present work advances the understanding on the high-temperature irradiation response of additively manufactured steels for nuclear reactor applications.

36 MATERIALS SCIENCE↗

DOE OSTI2018

Characterization of thin films prepared by co-sputtering iron and titanium precursors and thermal oxidation under air atmosphere

Highlights: • Thin films of mixed iron and titanium oxide were prepared by Ti-Fe annealing. • Segregation and phase separation of iron and titanium oxides are observed. • Formation of iron rich (Fe{sub x}O) nanoparticles at the surface region is revealed. • Formation of thin films with high refractive index can be obtained. - Abstract: Nanocomposites of metal oxides are useful materials for operation in many energy conversion systems. In this study, such nanocomposites were prepared by oxidation of mixtures of iron and titanium precursor metallic thin films at 520 °C under air atmosphere. The metallic films with different iron percentages were obtained by radio frequency (RF) magnetron sputtering on glass and silicon substrates. The films were characterized by means of X-ray Diffraction (XRD), Raman spectroscopy, UV–Vis spectroscopy, High Resolution Scanning Transmission Electron Microscopy (HRSTEM), Ellipsometry and X-ray Photo-electron Spectroscopy (XPS). The results show that these films present mainly two nanometric phases, namely Fe{sub 2}O{sub 3} and TiO{sub 2}. A phase separation was observed; the film surface was found to be iron rich oxide or clusters of iron rich oxide (Fe{sub x}O{sub y}, x > y), whereas titanium accumulated deeply in the bulk, forming TiO{sub 2} far below the film surface. A red shift of optical absorption as well as a relatively stable high refractive index (varying between 3.1 and 3.5) over a broad band of optical frequencies was observed for the films containing higher iron initial concentrations.

36 MATERIALS SCIENCE↗