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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.

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At least 937 records · Page 52
All sourcesNASA NTRSDOE OSTI
NASA NTRS2012

Microstructure Formations in the Two-Phase Region of the Binary Peritectic Organic System TRIS-NPG

In order to prepare for an onboard experiment on the International Space Station (ISS), systematic directional solidification experiments with transparent hypoperitectic alloys were carried out at different solidification rates around the critical velocity for morphological stability of both solid phases. The investigations were done in the peritectic region of the binary transparent organic TRIS-NPG system where the formation of layered structures is expected to occur. The transparent appearance of the liquid and solid phase enables real time observations of the dynamic of pattern formation during solidification. The investigations show that frequently occurring nucleation events govern the peritectic solidification morphology which occurs at the limit of morphological stability. As a consequence, banded structures lead to coupled growth even if the lateral growth is much faster compared to the growth in pulling direction.

Mogeritsch, Johann↗
NASA NTRS2021

Physics and Chemistry of Te and HgTe-based Ternary Melts

Historically, the theoretical treatment of the liquid phases has always been more difficult and complicated than that for solid and gas phases. A liquid has no lattice structure as crystalline solids and the atoms/molecules in the liquid can migrate through it relatively rapidly. On the other hand, it is also interacting with many other atoms/molecules so that the simplifications of the kinetic theory of gases cannot be employed. For more complicated liquids, such as the liquids of high ionicity and those containing hydrogen bonds and electric dipoles, the understanding is far from complete. At the same time, accurate information on the physics and chemistry of semiconductor melts is needed for the quantitative descriptions of the process of crystal growth from melt. The pre-crystallization phenomena in the liquid phase are critical because the properties of the grown crystals depend on the state and structure of the melt as well as the thermal history of the melt during solidification process. However, the data on the liquid phase, such as thermophysical properties of semiconductor melts are scarce, especially for the HgTe-based II-VI ternary compound semiconductors because of their high vapor pressure and extreme toxicity. Analysis of the thermophysical properties of the melt can provide information about structural transitions of the melt during the solidification process. From a broader point of view, the structure of liquids is much more complicated than the crystalline solids, especially the relaxation behavior through different thermal histories. The theory of hetero-phase fluctuations of liquids is applicable to any many-body systems including condensed-matter physics, field theory, physics of nuclear-matter, cosmology, biology and even sociology. This book summarizes the physics and chemistry from the experimental measurements and theoretical analyses of phase diagram, thermodynamic properties, density, thermal conductivity, viscosity, and electrical conductivity on the binary, pseudo-binary and ternary melts of the most advanced IR-detector material systems of HgCdTe and HgZnTe as well as the analyses of these results. The main objectives of this study are: (1) to provide the phase diagrams and thermodynamic properties of Hg-Cd-Te and Hg-Zn-Te systems through quantitatively fitting the experimental data by assuming an associated solution model for the liquid phase, (2) to experimentally measure the thermophysical properties of the Hg-Cd-Te and Hg-Zn-Te melts, including density, viscosity, electrical conductivity and thermal conductivity as functions of temperature and composition and (3) to enhance the fundamental knowledge of hetero-phase fluctuations and relaxation phenomena in the melts and extend our understanding of the solidification process in order to interpret the experimental results of crystal growth so as to improve the melt growth processes of the compound semiconductor. The physics and chemistry of Te and HgTe-based ternary melts were explored through the studies of the structural transformation during melting, the supercooling during solidification, the relaxation phenomena after rapid cooling of the melts and the metal-semiconductor transition in the melts through the analyses of electrical conductivity and Lorenz number. An in-depth study on the thermophysical properties and their time-dependent structural dynamic processes taking place in the vicinity of the solid-liquid phase transition of the narrow homogeneity range HgTe-based ternary semiconductors as well as the analysis of the homogenization process in the melt will also be presented.

Thermophysical properties↗
NASA NTRS2023

Hot Cracking Behavior and Beam-Induced Grain Refinement in Electron Beam Freeform Fabricated Al 7075

Electron beam freeform fabrication (EBF 3 ) is a high deposition rate, wire-based directed energy deposition (DED) additive manufacturing (AM) process used to print metallic parts in a vacuum environment. While high specific strength 7xxx-series Al alloys are of interest in the aerospace and automotive industries, these alloys suffer from hot cracking issues during fusion welding and AM. In this work, linear deposits of Al 7075 were fabricated with EBF 3 to study the effects of baseplate thickness, preheat temperature, and beam focus on hot cracking behavior. Solidification cracks were observed in the first deposition layer, and by the third layer coalesced into large “macrocracks” running vertically through the build height. Fine, intergranular liquation “microcracks” appeared between macrocracks below the root of the final deposit layer fusion zone. Increasing the substrate preheat temperature from 165°C to 320°C deepened the partially melted zone and resulted in a more than twofold increase in the liquation microcrack density. Analysis of Scheil solidification diagrams revealed that fugitive losses of Zn and Mg during deposition increased the susceptibility to solidification cracking and promoted substrate liquation cracking. Focused electron beam (EB) conditions combined with a high aspect ratio elliptical raster pattern resulted in bands of refined, equiaxed grains. The refined microstructure suppressed solidification macrocracking and caused a nearly fivefold reduction in liquation crack density, demonstrating that beam-induced grain refinement may be a promising low-cost method for reducing hot cracking during AM.

aluminum↗
NASA NTRSDate not supplied

Uncertainty Quantification of Classical Theories of Dendritic Growth Kinetics Applied to Nickel-Based Alloys

The solidification velocity in a model nickel-alloy single crystal during laser spot melting was recently characterized using synchrotron X-ray imaging. The measured solidification velocity was found to exceed the absolute stability threshold predicted by the Kurz-Giovanola-Trivedi (KGT) model. The discrepancies between the model and experiments motivate the further assessment of accurate material properties. This work quantifies the impact material property uncertainty has on model predictions of the absolute stability threshold velocities. Properties from the literature are reviewed and compared to those calculated using computational thermodynamics to provide uncertainty estimates on input properties to the KGT model. Global sensitivity analysis is used to quantify the influence of each uncertain input property on the predicted threshold velocity. This work supports the understanding of the nickel-alloy solidification during powder bed fusion additive manufacturing and identifies the solidification material properties that are the most important to assess from first-principles computations and experiments.

Computational thermodynamics↗
DOE OSTI2020

Columnar-to-equiaxed transition in a laser scan for metal additive manufacturing

In laser powder bed fusion additive manufacturing (LPBFAM), different solidification conditions, e.g., thermal gradient and cooling rate, can be achieved by controlling the process parameters, such as laser power and laser speed. Tailoring the behaviour of the columnar to equiaxed transition (CET) of the printed alloy during fabrication can facilitate the production of highly customized microstructures. In this study, effective analytical solutions for both thermal conduction and solidification are employed to model solidifying melt pools. Microstructure textures and solidification conditions are evaluated for numerous combinations of laser power and laser speed under bead-on-plate conditions. This analytical-based high-throughput tool was demonstrated to select specific process parameters that lead to desired microstructures. Two selected process conditions were examined in detail by a highly parallelized microstructural solidification model to reveal both nucleation and grain growth. Both numerical solutions agree well with experiments that are performed based on bead-on-plate conditions, indicating that these numerical models aid evaluation of the nucleation parameters, providing insights for controlling CET during the LPBFAM processing.

Lang, Yuan↗
DOE OSTI2020

Viscosity of the Mercurian Magma Ocean: Implications of Sulfur-Free and Sulfur-Bearing Magma Oceans for Differentiation and Crustal Petrogenesis

Mercury is believed to have had a magma ocean in its early evolution whose solidification dynamics contributed to the planet’s present day surface composition. The surface is characterized by two main geological provinces; the Northern Smooth Plains (NSP),and the Heavily Cratered Terrain (HCT) and Intercratered Terrian (IcT). These provinces are defined by the differences in morphology, composition, and age. The NSP are located in the northern hemisphere and have low Mg/Si, Ca/Si, and S/Si ratios and high Al/Si ratios, and are compositionally analogous to a magnesian alkali basalt. The HCT/IcT cover the majority of the planet and are older than the NSP, and are characterized by high Mg/Si, Ca/Si, and S/Si ratios and low Al/Si ratios. The dichotomy in surface composition motivates the study into the structure of the mantle as a product of magma ocean solidification. Two important and interrelated parameters that determine the efficiency of crystal fractionation during magma ocean solidification are convection vigor and viscosity of the magma ocean. Crystal suspension in a cooling magma ocean could be efficient or inefficient which suggests two end member possibilities for magma ocean solidification. The first is the formation of a compositionally stratified mantle reflecting a cumulate sequence produced along a fractional liquid line of descent. The second is the formation of a well-mixed, batch crystallized mantle by high viscosity and/or vigorously convecting magma ocean. Determining which of these end members, or where on the spectrum between them is most relevant to Mercury’s magma ocean can be experimentally constrained through high P-T falling-sphere viscometry experiments.

Mouser, M. D.↗
DOE OSTI2022

Experimental and numerical study on the impacts of various long carbon fiber inserts on the thermal performance of phase change materials

In thermal energy storage systems, phase change materials (PCMs) are widely used to provide efficient storing and extraction of thermal energy. However, most PCMs have low thermal conductivities, which limit the heat transfer of the latent energy inward/outward. In this study, the impacts of various fiber inserts on the solidification and melting processes with PCM were investigated by experiments and simulations. It was found that long anisotropic carbon fibers with high thermal conductivities can not only increase the heat transfer within PCM by enlarging hot-cold interface areas, but also act as crystallization nuclei and increase the solidification of PCM during the charging process. Recalescence of PCM during solidification was observed. COMSOL models were used to understand the thermal transfer mechanisms. COMSOL Multiphysics models of a single carbon fiber filament in PCM showed that the anisotropy in thermal conductivities controlled the radial and axial heat transfer ratios along its fiber axis and can enhance both the solidification and melting processes.

Zhou, Xinzhang↗
DOE OSTI2019

Accounting for Melt Flow Pattern and Solid Fraction Evolution in DC Casting of Al-Cu Alloy Using v{sup 2}−f Turbulence Model

Experimental techniques remain reliable approaches for investigating the effects of parameters of the direct chill (DC) casting process on macrosegregation, grain structure formation, and surface quality. However, experimental analysis methods of these effects are expensive and can often prove difficult to implement. Understanding of the complex physics of the solidification process remains limited, and this makes it indispensable to use accurate numerical modeling tools as a complement to experiments. Numerical modeling without consideration of turbulent flow is unsuitable for application to solidification occurring during DC casting, since significant turbulent flow exists in the upper section of the cast, where the liquid pool exists. The present work employed a low-Re v{sup 2}−f turbulence model and a dual-zone solidification flow model to predict the melt flow pattern, turbulence level, sump depth, transition zone thickness, and mushy zone thickness in the DC casting of Al-Cu alloy. The obtained results provided evidence of significant damping of the flow in the slurry zone. Furthermore, results of different simulation cases revealed a narrower transition zone and shallower sump.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Numerical Simulation of Bulging Deformation for Wide-Thick Slab Under Uneven Cooling Conditions

In the present work, the bulging deformation of a wide-thick slab under uneven cooling conditions was studied using finite element method. The non-uniform solidification was first calculated using a 2D heat transfer model. The thermal material properties were derived based on a microsegregation model, and the water flux distribution was measured and applied to calculate the cooling boundary conditions. Based on the solidification results, a 3D bulging model was established. The 2D heat transfer model was verified by the measured shell thickness and the slab surface temperature, and the 3D bulging model was verified by the calculated maximum bulging deflections using formulas. The bulging deformation behavior of the wide-thick slab under uneven cooling condition was then determined, and the effect of uneven solidification, casting speed, and roll misalignment were investigated.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Thermomechanical Processing of Metal Feedstock for Semisolid Forming: A Review

Thermomechanical processing of cast structures is an effective solid working route of generating thixotropic morphologies after subsequent partial melting and is in use for decades to manufacture the bulk billet feedstock for thixoforming from a variety of alloys. The solid-state deformation is also critical for coarse particulate feedstock, utilized for semisolid forming either directly or after compaction into billets. Although the original concept, called strain-induced melt activation (SIMA), defined the specific procedure, the term became generally recognized synonym for a variety of thermomechanical treatments offering an opportunity of controlling the solidification microstructure. This review covers transformations during solid-state deformation, reheating to semisolid state and isothermal holding within solidus–liquidus range, and solidification of thixotropic slurries. Essentials of semisolid metal processing, necessary to understand the subject, are supported by details related to specific implementation techniques and alloys. Application examples at laboratory and commercial levels and properties achieved with conventional and severe plastic deformation techniques, for different alloys along with present limitations, are described. The link between solid-state deformation-enhanced melting and liquid metal engineering is emphasized throughout the paper in terms of the common goal of controlling the solidification outcome in order to develop technology for mass-scale production of net-shape components having performance characteristics superior to conventional castings.

36 MATERIALS SCIENCE↗
DOE OSTI2019

CoCrFeNi Multi-principal Element Alloy Prepared Via Self-propagating High-Temperature Synthesis Plus Investment Casting Method

A CoCrFeNi multi-principal element alloy was successfully prepared via the combination of self-propagating high-temperature synthesis and investment casting. The phase identification, the as-cast microstructure, and the solidification phenomena were investigated. Unlike the dendrite morphology obtained by arc melting, cellular growth in the as-cast alloy was observed as a consequence of the rapid solidification. The solidification process is discussed with regard to the rate of advance of the liquid–solid interface, the equilibrium melting range, and the interface temperature gradient.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Engineering Nucleation Kinetics of Graphite Nodules in Inoculated Cast Iron for Reducing Porosity

Inoculation of cast iron with spherical graphite (SGI) controls nucleation kinetics of graphite nodules and is used for improving casting performance and eliminating solidification micro-porosity. In this study, thermodynamic simulations were used to design inoculants. Thermal and chemical stability of different potential heterogeneous nuclei formed in the melt above the liquidus temperature and in the mushy zone during solidification was predicted. Several inoculation treatments of SGI were performed in laboratory heats. An automated SEM/EDX analysis was applied to examine the graphite nodule size distribution and the family of the non-metallic inclusions in the experimental castings. The data were used to reconstruct a relative graphite nodule nucleation rate in the castings. It was shown that the graphite nodule nucleation kinetics in the castings were significantly different from those predicted by classical nucleation models. In inoculated SGI, the observed bi-modal distribution of graphite nodules was related to a continuous nucleation that occurred towards the end of solidification. The origin of the continuous nucleation and the possibility of engineering graphite nodules nucleation kinetics to control SGI casting soundness by inoculation are discussed.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Thermal Analysis of Rare Earth Additions to HY100

This paper discusses the results of a series of experiments on rare earth additions to HY100. Previous work has found little or no refinement to occur with rare earth additions to this alloy. Thermal analysis has been used to provide a better understanding of this and assist in the development of a suitable grain refiner. Heats of HY100 were melted in an induction furnace and tapped into a small hand ladle. Thermal analysis cups were poured from the ladle, and the resulting cooling data recorded. Heats with no additions and 0.3 wt.% rare earth silicide were poured. Commercial software analyzed the cooling curves to determine the liquidus and solidus temperatures. A manual analysis was employed for identifying the peritectic and other reactions. Examination of the cooling curves detected the liquidus, solidus and even peritectic reactions. It was observed that the solidification changed with the rare earth silicide or engineered grain refiner additions. Additional phase reactions appeared before the solidus were not present in the baseline heat. Phase identification was done with thermodynamic calculations. The solidus, peritectic and liquidus were found. Rare earth inclusions were observed in the microstructure of the treated steel. Interestingly, macrostructure examination indicated no reduction in overall grain size. Thermodynamic predictions also indicated that the inclusions formed late during solidification. It was postulated that the additions caused a change in solidification but those occurred late enough during freezing that the resulting microstructure was not affected.

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 OSTI2019

Multiscale Modeling of Microstructural Evolution in Fused-Coating Additive Manufacturing

A quantitative phase field model with adaptive-computational-domain-expansion (ACDE) method was developed to simulate the microstructural evolution in fused-coating additive manufacturing process. A heat transfer and fluid flow model was established to investigate the solidification behavior of the molten pool in a single deposited layer. The solidification parameters such as temperature gradient and solidification rate were calculated from the macroscopic model and then fed into the phase field model to get the microstructure at different positions within a single deposited layer. Columnar dendrites were formed at the bottom of the deposited layer, and equiaxed dendrites appeared in the upper part of the layer. The ACDE method is used to improve the simulation efficiency. The metallographic experiments and energy-dispersive spectrometer analysis for composition were conducted on the fused-coating specimen, and the results were in good accordance with the simulations.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Controlling solute channel formation using magnetic fields

Solute channel formation introduces compositional and microstructural variations in a range of processes, from metallic alloy solidification, to salt fingers in ocean and water reservoir flows. Applying an external magnetic field interacts with thermoelectric currents at solid/liquid interfaces generating additional flow fields. This thermoelectric (TE) magnetohydrodynamic (TEMHD) effect can impact on solute channel formation, via a mechanism recently drawing increasing attention. To investigate this phenomenon, we combined in situ synchrotron X-ray imaging and Parallel-Cellular-Automata-Lattice-Boltzmann based numerical simulations to study the characteristics of flow and solute transport under TEMHD. Observations suggest the macroscopic TEMHD flow appearing ahead of the solidification front, coupled with the microscopic TEMHD flow arising within the mushy zone are the primary mechanisms controlling plume migration and channel bias. Two TE regimes were revealed, each with distinctive mechanisms that dominate the flow. Further, we show that grain orientation modifies solute flow through anisotropic permeability. These insights led to a proposed strategy for producing solute channel-free solidification using a time-modulated magnetic field.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Design of heterogeneous structured Al alloys with wide processing window for laser-powder bed fusion additive manufacturing

Required microstructural attributes of an alloy vary with structural applications. The microstructural fine-tuning capability of laser-powder bed fusion (L-PBF) additive manufacturing (AM) enables application specific manufacture of the components. Such manufacture with L-PBF AM requires alloys that exhibit wide processing window and are amenable to multiple deformation mechanisms. However, high hot cracking susceptibility of Al alloys poses a barrier to such printability-performance synergy. In this work we show that an integration of, a) grain refinement through heterogeneous nucleation, and b) eutectic solidification, leads to crack-free parts at wide range of process parameters, microstructural heterogeneity, and hierarchy in the Al-Ni-Ti-Zr alloy. Such an integration targets hot cracking at multiple stages of solidification in L-PBF as opposed to the contemporary alloy design strategies that target hot-cracking at only specific stages of solidification. The Al-Ni-Ti-Zr alloy exhibits excellent printability and a high as-built tensile performance. Due to the wide processing window and amenability to multiple deformation mechanisms, the alloy microstructure and subsequently the performance, can be fine-tuned. Here, such strategy opens the gateway for application-specific manufacture of Al alloys with L-PBF AM and establishes a fundamental shift in current methodologies for design of these alloys for L-PBF AM.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Schmid factor crack propagation and tracking crystallographic texture markers of microstructural condition in direct energy deposition additive manufacturing of Ti-6Al-4V

Metallic additive manufacturing (AM) provides a customizable and tailorable manufacturing process for new engineering designs and technologies. The greatest challenge currently facing metallic AM is maintaining control of microstructural evolution during solidification and any solid state phase transformations during the build process. Ti-6Al-4V has been extensively surveyed in this regard, with the potential solid state and solidification microstructures explored at length. In this study, we evaluate the applicability of previously determined crystallo- graphic markers of microstructural condition observed in electron beam melting powder bed fusion (PBF-EB) builds of Ti-6Al-4V in a directed energy deposition (DED) build process. The aim of this effort is to elucidate whether or not these specific crystallographic textures are useful tools for indicating microstructural conditions in AM variants beyond PBF-EB. Parent β-Ti grain size was determined to be directly related to α-Ti textures in the DED build process, and the solid state microstructural condition could be inferred from the intensity of specific α-Ti orientations. Qualitative trends on the as-solidified β-Ti grain size were also determined to be related to the presence of a fiber texture, and proposed as a marker for as-solidified grain size in any cubic metal melted by AM. Analysis of the DED Ti-6Al-4V build also demonstrated a near complete fracture of the build volume, suspected to originate from accumulated thermal stresses in the solid state. Crack propagation was found to only appreciably occur in regions of slow cooling with large α +β colonies. Schmid factors for the basal and prismatic α-Ti systems explained the observed crack pathway, including slower bifurcation in colonies with lower Schmid factors of both slip systems. Colony morphologies and localized equiaxed β-Ti solidification were also found to originate from build pauses during production and uneven heating of the build edges during deposition. Tailoring of DED Ti-6Al-4V microstructures with the insight gained here is proposed, along with cautionary insight on preventing unplanned build pauses to maintain an informed and controlled thermal environment for microstructural control.

36 MATERIALS SCIENCE↗