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At least 649 records · Page 36
All sourcesNASA NTRSDOE OSTI
NASA NTRS1976

Silicon ribbon growth by a capillary action shaping technique

The technique of silicon ribbon growth by the capillary action shaping is assessed for applicability to photovoltaic power device material. Ribbons 25 mm in width and up to 0.5 m in length have been grown from SiC dies, and some new characteristics of growth from such dies have been identified. Thermal modifiers have been studied, and systems were developed which reduce the frozen-in stress un silicon ribbons and improve the thickness uniformity of the ribbons. Preliminary spreading resistance measurements indicate that neither surface striations nor twin boundaries give rise to appreciable resistivity variations, but that large-angle grain boundaries cause local resistivity increases of up to 200%.

Schwuttke, G. H.↗
NASA NTRS2001

In Situ Monitoring of Crystal Growth Using Mephisto

Experiments were carried out to study the morphological stability of Bi-1atomic% Sn alloys using the MEPHISTO directional solidification apparatus aboard Space Shuttle Columbia (STS-87, launched November 19, 1997). The research program involved collaboration of the University of Florida, Centre National d'Etudes Spatiales (France), NASA/Glenn Research Center, the National Institute of Standards and Technology, and the University of New South Wales (Australia). The experiments made use of the Seebeck technique to measure the interface temperature, the resistance change across the sample for determination of the interface velocity, and Peltier current pulsing for demarcation of the interface shape. These data were obtained in real-time during various melting/solidification cycles, and the experiments were controlled via telecommands. The Seebeck signal and temperature measurements in the space experiments, as well as morphological behavior of the solid/liquid interface, were found to be significantly different than those obtained from ground-based studies. For the space-based experiments, interface breakdown was observed at growth velocities of 6.7, 27, and 40 micrometers/s, but not at 1.8 and 3.3 micrometers/s. The results further indicate that the morphological stability threshold for some grain orientations is significantly enhanced, resulting in a staggered planar to cellular transition front. This report summarizes some of these findings.

Abbaschian, Reza↗
NASA NTRS2001

Observations Regarding Small Eolian Dunes and Large Ripples on Mars

Eolian bedforms occur at the interface between a planetary surface and its atmosphere; they present a proxy record of the influence of climate, expressed in sediment transport, over that surface. High resolution images (1.5 - 12 m/pixel) from the Mars Global Surveyor (MGS) Mars Orbiter Camera provide glimpses of the most recent events shaping the martian landscape. Thousands of images exhibit small transverse dunes or large eolian ripples that have crest-to-crest spacings of 10 to 60 m, heights of a few to 10 m. Bedforms of the size and patterns seen in the Mars photographs are rarely described among Earth's eolian landforms; in terms of size and morphology, most of these fall between traditional definitions of "ripples" and "dunes". Dunes are composed chiefly of materials transported by saltation, ripples are smaller forms moved along by the impact of saltating grains (traction). The largest reported eolian ripples on Earth (granule ripples, megaripples) are typically smaller than the bedforms observed on Mars; likewise, most dunes are typically larger. The small dunes and large ripples on Mars come in a variety of relative albedos, despite an early MGS impression that they are all of high albedo. Some ripples occur on the surfaces of sand dunes; these are most likely true granule ripples. However, most of these bedforms occur in troughs, pits, craters, and on deflated plains. Despite impressions early in the MGS mission, they do not occur everywhere (e.g., they are rare on the northern plains) but they do occur at a range of elevations from the highest volcanoes to the deepest basins. Where they occur on a hard substrate among larger sand dunes, the big dunes have over-ridden the smaller bedforms, indicating that the smaller features are older and perhaps indurated or very coarse-grained. At other locales, the small bedforms have been mantled by material settled from suspension, in other cases they are being exhumed and may be lithified. Still other examples are peppered with small impact craters, implying considerable age. These bedforms present a complicated record of the geologically-recent past, one that has involved changes in climate, sediment transport capabilities, and sediment sources and sinks over time.

Edgett, Kenneth S.↗
DOE OSTI2022

Effects of tensile loading during annealing of alnico melt spun ribbons

Conventional magnetic annealing (MA) of the permanent magnet alloy alnico involves application of an external magnetic field at temperatures within the spinodal decomposition range. This field biases the growth of the Fe-Co rich, ferromagnetic α 1 -phase in an energetically favorable 〈001〉 direction in alignment with the applied field within an Al-Ni rich, paramagnetic α 2 -phase. Utilizing a magnetic field to bias the α 1 -phase may limit alnico from reaching theoretical coercivity due to (1) the field having maximum biasing ability at temperatures near the Curie temperature where large α 1 -phase nanorods form and (2) connectivity of the α 1 -phase occurs unavoidably during MA. Both decrease the effective shape anisotropy of the α 1 -phase, thereby reducing coercivity. Herein, we explore tensile-loading as a biasing mechanism to control and optimize the final alnico nanostructure beyond that achieved by MA. Two samples of melt-spun alnico were heat-treated at 860 °C for 5 minutes: one sample was subjected to 10 MPa tensile stress for comparison with a stress-free control sample. Structural and magnetic characterization revealed that the stress-annealed ribbon sample possessed expected phase assemblages, but was distinguished by a ∼2× larger grain diameter and an elongated anisotropic α 1 -phase within grains that were oriented to a shear stress along 〈001〉 directions at an angle of ∼45° relative to the loading direction. Both types of annealing produced a similar increase in the coercivity and remanence, but a decrease in saturation magnetization.

Rinko, E. A.↗
DOE OSTI2022

Modeling and experimental validation of dynamical effects in Bragg coherent x-ray diffractive imaging of finite crystals

Bragg coherent diffractive imaging (BCDI) is a noninvasive microscopy technique that can visualize the shape and internal lattice deviations of crystals with nanoscale spatial resolution and picometer deformation sensitivity. Its strain imaging capability relies on Fourier transform–based iterative phase retrieval algorithms, which are mostly developed under the kinematical approximation. Such approximation prohibits the application of BCDI on larger crystals, which are commonly seen in most emerging functional materials. Understanding the dynamical effect in BCDI, as well as developing a validated method for modeling BCDI at the dynamical diffraction limit, is crucial for applying BCDI to hierarchical systems that contain micron-sized crystals and grains. Thus we report a comparative study on the impact of dynamical diffraction effects by comparing the reconstruction results from two measurements of the same crystal. Forward simulation is implemented to show subtle changes of interference fringes in the diffraction pattern due to the dynamical diffraction, and is compared directly with the experimental data.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Insight to the Duration of 120 °C Baking on the Performance of SRF Niobium Cavities

Low-temperature baking at 100 °C - 140 °C in ultra-high vacuum for several hours is the final cavity processing technique to achieve the highest accelerating gradient cavity by eliminating the high field Q-slope, typically on cavities made of high purity fine grain niobium and subjected to electropolishing. The temperature and duration of baking control the onset of the high field Q-slope and overall quality factor over the baseline measurements. The increase in quality factor is generally related to the reduction in the temperature-dependent surface resistance, which refers to BCS surface resistance due to the reduction in the electronic mean free path and elimination of high field Q-slope is still being actively investigated. Here, we present the results of a series of measurements on 1.3 GHz TESLA shape single-cell cavities with successive low-temperature baking at 120 °C up to 96 hours. The rf loss related to the trapping of residual magnetic field refer to flux trapping sensitivity was measured with respect to the duration of 120 °C bake.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

Mechanistic nuclear fuel performance modeling of uranium nitride

Uranium mononitride (UN) is a nuclear fuel candidate for advanced reactor designs and an alternative being considered for light water reactors due to its higher thermal conductivity and uranium density than UO 2 . As with any nuclear fuel, swelling and fission gas release are important factors for safety, while also being some of the hardest phenomena to predict with a high degree of confidence. Getting a grasp on the gas swelling behavior and release is crucial to lower the barrier for UN utilization. An accelerated swelling rate at high temperatures observed experimentally, sometimes referred to as “breakaway swelling,” further complicates the prediction of fuel performance of UN. A mechanistic model has been developed using a multiscale approach to describe the intragranular and intergranular fission gas behavior. Lower-length-scale calculations have been employed to inform models of the gas and self-diffusion behavior, resolution rate, and bubble shape. Leveraging previous work on high burnup UO 2 , two populations of intragranular bubbles are considered; small bulk bubbles and larger bubbles located along dislocations. The dislocation bubbles were found to be crucial to the overall swelling behavior, and the breakaway swelling transition was associated with the transition in the gas atom diffusion mechanism from an irradiation-induced athermal diffusion regime at lower temperatures to an intrinsic thermal equilibrium regime at higher temperatures, accelerating the growth of the dislocation bubbles. Similarly, the threshold for fission gas release was associated with the grain boundary vacancy diffusivity surpassing the gas atom diffusivity at sufficiently high temperatures, allowing the over-pressurized grain boundary bubble to grow in size and interconnect. Using thermo-mechanical models with the fission gas model, two integral fuel pin assessment cases were simulated. Finally, this work demonstrates the ability of a multiscale approach to accelerate the understanding of advanced fuel forms when experimental data is limited.

11 - NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2022

Process-microstructure relationship of laser processed thermoelectric material Bi2Te3

Additive manufacturing allows fabrication of custom-shaped thermoelectric materials while minimizing waste, reducing processing steps, and maximizing integration compared to conventional methods. Establishing the process-structure-property relationship of laser additive manufactured thermoelectric materials facilitates enhanced process control and thermoelectric performance. This research focuses on laser processing of bismuth telluride (Bi 2 Te 3 ), a well-established thermoelectric material for low temperature applications. Single melt tracks under various parameters (laser power, scan speed and number of scans) were processed on Bi 2 Te 3 powder compacts. A detailed analysis of the transition in the melting mode, grain growth, balling formation, and elemental composition is provided. Rapid melting and solidification of Bi 2 Te 3 resulted in fine-grained microstructure with preferential grain growth along the direction of the temperature gradient. Experimental results were corroborated with simulations for melt pool dimensions as well as grain morphology transitions resulting from the relationship between temperature gradient and solidification rate. Samples processed at 25 W, 350 mm/s with 5 scans resulted in minimized balling and porosity, along with columnar grains having a high density of dislocations.

Oztan, Cagri↗
NASA NTRS2001

KT Boundary Impact Debris from DSDP Site 577

The KT boundary at DSDP Site 577 (Shatsky Rise, western North Pacific) is in a calcareous nannofossil ooze. It is bioturbated so Ir and impact debris are smeared across less than 15 cm of core. An absence of sulfides and chalcophile enrichment and the presence of Mn-micronodules indicate that this boundary did not experience anoxic conditions typical of many other calcareous KT sites. Because of its shallow burial (109 m) ejecta is extremely well preserved and has been the subject of numerous studies. Three high-temperature minerals have been identified at this site. Clinopyroxene (cpx), found only at Site 577 may be precursor to diagenetic sanidine in some European sites. Magnesioferrite spinel has the highest Fe2O3/FeO of any locality and can have and trace inclusions of Ni-magnesiowustite, which is known at only one other site (Site 596). Other debris described includes shocked quartz grains less than 200 microns, hollow spherules possibly related to impact melt glasses found near Chicxulub, and irregular Ir-rich particles that may be derived directly from meteoritic materials. The latter may be related to the fossil meteorite found at Site 576, just 500 km to the east. A large-volume (approx. 10 cc) sample was taken to systematically characterize debris. The less than 60 microns fraction of the carbonate-free sediment contained 44 mg of impact debris. 2700 particles less than l00 microns were classified based on color, shape, and morphology. About 55% are generally light colored, composed mostly of clays, and typically lack magnesioferrite spinel. Less than half of these are quite spherical; many have irregular shapes, possibly from fragmentation. Hollow spherules with smooth shiny surfaces are only 2% of the total but many fragments in the 60 - 100 microns fraction suggest that they were more abundant and that some may have been quite large; 800 microns or more. Cpx-bearing spherules, easily identified by green crystals at the surface, constitute only 2.5% of the total. Three cpx spherules are dumbbells. The other 40% of the debris is a complex assemblage of dark-colored particles, most of which have magnesioferrite spinel. We have currently separated these into 6 groups based on color and morphology, but this hardly describes the diversity of this group. They range from smooth and spherical to rough and irregular in shape. Some have a planar cleavage, similar to some broken cosmic spherules. 220 particles were irradiated for trace element analysis, as groups and as individual particles. It appears that every spherule type as at least a few ng/g Ir. Several of the spinel-bearing particles have been identified with high Ir (200-1500 ng/g), Cr (2-5 mg/g), and Ni (2-10 mg/g), consistent with results. It is hoped that some of these will yield potential fragments of the KT bolide. Only five have been sectioned to date and they are characterized by high concentrations of spinel; two contained traces of magnesiowustite. This debris constitutes a complex assemblage of ejecta materials. The light-colored, spinel-free particles likely derive mainly from target materials. A significant fraction of these are probably alteration products of cpx-spherules. The dark-colored debris is more likely to have high siderophile concentrations, and thus a high extraterrestrial component. However, the majority don't. Most of the larger spinel-bearing spherules have modest Ir concentrations (5-30 ng/g).

Kyte, Frank T.↗
NASA NTRS2006

Evolution and Control of 2219 Aluminum Microstructural Features through Electron Beam Freeform Fabrication

Electron beam freeform fabrication (EBF3) is a new layer-additive process that has been developed for near-net shape fabrication of complex structures. EBF3 uses an electron beam to create a molten pool on the surface of a substrate. Wire is fed into the molten pool and the part translated with respect to the beam to build up a 3-dimensional structure one layer at a time. Unlike many other freeform fabrication processes, the energy coupling of the electron beam is extremely well suited to processing of aluminum alloys. The layer-additive nature of the EBF3 process results in a tortuous thermal path producing complex microstructures including: small homogeneous equiaxed grains; dendritic growth contained within larger grains; and/or pervasive dendritic formation in the interpass regions of the deposits. Several process control variables contribute to the formation of these different microstructures, including translation speed, wire feed rate, beam current and accelerating voltage. In electron beam processing, higher accelerating voltages embed the energy deeper below the surface of the substrate. Two EBF3 systems have been established at NASA Langley, one with a low-voltage (10-30kV) and the other a high-voltage (30-60 kV) electron beam gun. Aluminum alloy 2219 was processed over a range of different variables to explore the design space and correlate the resultant microstructures with the processing parameters. This report is specifically exploring the impact of accelerating voltage. Of particular interest is correlating energy to the resultant material characteristics to determine the potential of achieving microstructural control through precise management of the heat flux and cooling rates during deposition.

Taminger, Karen M.↗
DOE OSTI2021

BraggNN : fast X-ray Bragg peak analysis using deep learning

X-ray diffraction based microscopy techniques such as high-energy diffraction microscopy (HEDM) rely on knowledge of the position of diffraction peaks with high precision. These positions are typically computed by fitting the observed intensities in detector data to a theoretical peak shape such as pseudo-Voigt. As experiments become more complex and detector technologies evolve, the computational cost of such peak-shape fitting becomes the biggest hurdle to the rapid analysis required for real-time feedback in experiments. To this end, we propose BraggNN, a deep-learning based method that can determine peak positions much more rapidly than conventional pseudo-Voigt peak fitting. When applied to a test dataset, peak center-of-mass positions obtained from BraggNN deviate less than 0.29 and 0.57 pixels for 75 and 95% of the peaks, respectively, from positions obtained using conventional pseudo-Voigt fitting (Euclidean distance). When applied to a real experimental dataset and using grain positions from near-field HEDM reconstruction as ground-truth, grain positions using BraggNN result in 15% smaller errors compared with those calculated using pseudo-Voigt. Recent advances in deep-learning method implementations and special-purpose model inference accelerators allow BraggNN to deliver enormous performance improvements relative to the conventional method, running, for example, more than 200 times faster on a consumer-class GPU card with out-of-the-box software.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Performance Study of Virtual Frisch Grid CdZnTeSe Detectors

Nuclear detectors for x-ray and gamma-ray spectroscopy and imaging are a vital tool in many homeland security, medical imaging, astrophysics and other applications. Most of these applications require room-temperature operation due to the operational constraints imposed by a cryogenic cooling system. CdZnTe (CZT) has been the main material with the desired detection properties, and CZT crystals have been used commercially for three decades. However, CdZnTe still suffers from long-standing issues of high densities of performance-limiting intrinsic defects such as Te inclusions and networks of dislocation walls (sub-grain boundaries). A recently invented new quaternary material CdZnTeSe showed excellent material properties for radiation detection. The material was found to be free from dislocation networks, possess reduced Te inclusions, and have better compositional homogeneity. Virtual Frisch grid detectors were fabricated from crystals taken from a CdZnTeSe ingot that was grown by the traveling heater method. The detectors were fabricated from an as-grown ingot, bypassing the post-growth annealing process commonly practiced for industrial-grade CZT. The performances of the detectors were studied with different Frisch grid lengths using an amplifier shaping time ranging from 1–6 µs. The detectors showed high-quality spectroscopic performance with an as-measured energy resolution of ~1.1% at 662 keV for an optimum Frisch grid length of 3 mm. The charge collection was observed to enhance for longer Frisch grids.

47 OTHER INSTRUMENTATION↗
DOE OSTI2020

Disk-integrated Thermal Properties of Ceres Measured at Millimeter Wavelengths

We observed Ceres at three epochs in 2015 November and 2017 September and October with Atacama Large Millimeter/submillimeter Array (ALMA) 12 m array and in 2017 October with the ALMA Compact Array (ACA), all at ∼265 GHz continuum (wavelengths of ∼1.1 mm) to map the temperatures of Ceres over a full rotation at each epoch. We also used 2017 October ACA observations to search for HCN. The disk-averaged brightness temperature of Ceres is measured to be between 170 and 180 K during our 2017 observations. The rotational light curve of Ceres shows a double-peaked shape with an amplitude of about 4%. Our HCN search returns a negative result with an upper limit production rate of ∼2 × 10{sup 24} molecules s{sup −1}, assuming globally uniform production and a Haser model. A thermophysical model suggests that Ceres’s top layer has higher dielectric absorption than lunar-like materials at a wavelength of 1 mm. However, previous observations showed that the dielectric absorption of Ceres decreases toward longer wavelengths. Such distinct dielectric properties might be related to the hydrated phyllosilicate composition of Ceres and possibly abundant micrometer-sized grains on its surface. The thermal inertia of Ceres is constrained by our modeling as likely being between 40 and 160 thermal inertia units, much higher than previous measurements at infrared wavelengths. Modeling also suggests that Ceres’s light curve is likely dominated by spatial variations in its physical or compositional properties that cause changes in Ceres’s observed thermal properties and dielectric absorption as it rotates.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1984

Heat transfer distributions around nominal ice accretion shapes formed on a cylinder in the NASA Lewis icing research tunnel

Local heat transfer coefficients were obtained on irregular cylindrical shapes which typify the accretion of ice on circular cylinders in cross flow. The ice shapes were grown on a 5.1 cm (2.0 in.) diameter cylinder in the NASA Lewis Icing Research Tunnel. The shapes were 2, 5, and 15 min accumulations of glaze ice and 15 min accumulation of rime ice. Heat transfer coefficients were also measured around the cylinder with no ice accretion. These icing shapes were averaged axially to obtain a nominal shape of constant cross section for the heat transfer tests. Heat transfer coefficients around the perimeter of each shape were measured with electrically heated copper strips embedded in the surface of the model which was cast from polyurethane foam. Each strip contained a thermocouple to measure the local surface temperature. The models were run in a 15.2 x 68.6 cm (6 x 27 in.) wind tunnel at several velocities. Background turbulence in the wind tunnel was less than 0.5 percent. The models were also run with a turbulence producing grid which gave about 3.5 percent turbulence at the model location with the model removed. The effect of roughness was also simulated with sand grains glued to the surface. Results are presented as Nusselt number versus angle from the stagnation line for the smooth and rough models for both high and low levels of free stream turblence. Roughness of the surface in the region prior to flow separation plays a major role in determining the heat transfer distribution.

Vanfossen, G. J.↗
NASA NTRS2021

Far-infrared Polarization Spectrum of the OMC-1 Star-forming Region

We analyze the wavelength dependence of the far-infrared polarization fraction toward the OMC-1 star-formingregion using observations from HAWC+/SOFIA at 53, 89, 154, and 214μm. Wefind that the shape of the far-infrared polarization spectrum is variable across the cloud and that there is evidence of a correlation between theslope of the polarization spectrum and the average line-of-sight temperature. The slope of the polarization spectrumtends to be negative(falling toward longer wavelengths)in cooler regions and positive orflat in warmer regions.This is very similar to what was discovered inρOph A via SOFIA polarimetry at 89 and 154μm. Like the authorsof this earlier work, we argue that the most natural explanation for our falling spectra is line-of-sight superpositionof differing grain populations, with polarized emission from the warmer regions and less-polarized emission fromthe cooler ones. In contrast with the earlier work onρOph A, we do notfind a clear correlation of polarizationspectrum slope with column density. This suggests that falling spectra are attributable to variations in grainalignment efficiency in a heterogeneous cloud consistent with radiative torques theory. Alternative explanations inwhich variations in grain alignment efficiency are caused by varying gas density rather than by varying radiationintensity are disfavored.

Joseph M. Michail↗
DOE OSTI2021

Effect of the scanning strategy on the formation of residual stresses in additively manufactured Ti-6Al-4V

During the laser-powder bed fusion (L-PBF) process, high laser intensities, short interaction times and highly localized heat input drive large thermal gradients that result in a state of high residual stresses. Generally, the residual stresses that develop during the L-PBF process can compromise the performance of the component. Up to now, the literature has indicated that the magnitude of the residual stresses can be affected by various process parameters. In this study, all process parameters such as laser power and speed are held fixed and the focus is solely on the effect of the laser scan strategy on the three-dimensional residual stress state of L-PBF metallic components. Four Ti-6Al-4V bridge shaped components were built using island and continuous scanning patterns parallel and offset 45° from the sample axes. High-energy X-ray diffraction was used to determine the residual stress field in each of the components. Two of them were re-measured after being partially removed from the build plate. The assumptions implicit in diffraction measurements of stress are reviewed and discussed in depth because the unique microstructure associated with L-PBF Ti-6Al-4V renders the validity of those assumptions uncertain. Specifically, additional data was collected and analyzed to evaluate the relationship between grain scale and macroscopic scale stresses. The observed residual stresses were large, ½ to ¾ of the yield strength, particularly the build direction stresses near the lateral edges of the bridges. Here in this work, the higher stresses were observed in the bridges built via the island scan strategies, chiefly near the edges of the parts.

36 MATERIALS SCIENCE↗
NASA NTRS1985

The infrared characteristics of circumstellar silicate grains

A theoretical study of the infrared emission from circumstellar shells around late-type giants is made, with the aim of deriving the infrared characteristics of the silicate grains condensing in these shells. A large grid of models has been compared with observations of optically visible Miras, IRC sources and OH/IR stars. From fitting the observed relation between the color temperature and the strength of the 10-micron feature, it is concluded that the ratio of the 3.5 to 10-micron absorption efficiencies of the dust is about 0.25, a factor of 2 less than a previous determination. Detailed modeling of the 2 to 13-micron spectrum of OH 26.5 + 0.6, IRC + 10011 and R Cas yielded a similar ratio. These detailed models also show that the shape of the 10-micron feature, particularly around 8 and 13 microns, varies from source to source. The derived 10-micron feature is narrower for larger dust column densities. These observed differences in the intrinsic shape of the 10-micron feature are not due to differences in size of the condensing particles. Probably they are related to structural or compositional differences in the condensing silicates.

Schutte, W.↗
DOE OSTI2022

ADDITIVE MANUFACTURING OF REFRACTORY ALLOYS

Refractory alloys are known for their high temperature capabilities and are intended for use at ultra-high temperatures above even 1200°C, which exceeds the capabilities of conventional superalloys. Additive manufacturing (AM) is an attractive alternative processing route because refractory alloys are difficult to fabricate through traditional methods; AM can form a near net-shape part with a tailored microstructure. This work seeks to evaluate the solidification behavior of refractory alloys under AM conditions and establish corresponding solidification models. Two binary alloys, Mo30Nb and Nb7.5Ta, as well as the commercial Nb alloy, C103, and one refractory high entropy alloy (RHEA), MoNbTaTi, were subjected to single track melts in a laser powder bed fusion (LPBF) machine. Each melt track was evaluated in the scanning electron microscope (SEM), which determined that some AM conditions used led to non-ideal behavior. Electron backscatter diffraction (EBSD) revealed that the Nb7.5Ta, C103, and MoNbTaTi all exhibited new grain nucleation to various extents while the Mo30Nb exhibited primarily epitaxial growth. This was found to be inconsistent with solidification models that were developed for each alloy and each set of AM conditions to predict the columnar to equiaxed transition behavior (CET) of the experimental microstructures. This disparity was largely attributed to the input parameters for the solidification model, which were developed using Thermo-Calc and SYSWELD. These input parameters include the alloy specific Gibbs-Thomson coefficient and thermophysical properties, the solute specific liquidus slopes and partitioning coefficients, as well as the process specific thermal gradients. Each input parameter was evaluated to determine the likely changes required for the modeled values to generate solidification models that better correspond to what was observed experimentally.

Miklas, Abigail↗