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At least 199 records · Page 11
All sourcesNASA NTRSDOE OSTI
DOE OSTI2019

Effects of Fluorine on Solidification, Viscosity, Structure, and Heat Transfer of CaO-Al{sub 2}O{sub 3}-Based Mold Fluxes

To mitigate the chemical reaction between mold fluxes and high-Al steel, CaO-Al{sub 2}O{sub 3}-based mold fluxes were proposed to replace the conventional CaO-SiO{sub 2}-based mold fluxes. However, the high viscosity of CaO-Al{sub 2}O{sub 3}-based mold fluxes is harmful to the lubrication to the strand. In this article, the effects of fluorine on solidification, viscosity, structure, and heat transfer of CaO-Al{sub 2}O{sub 3}-based mold fluxes were evaluated. The addition of fluorine increased the break temperature and decreased the heat transfer rate of mold fluxes. It also reduced the flux viscosity at 1673 K (1400 °C) when fluorine content increased from 4.0 to 6.9 mass pct; but a further increase of fluorine to 12.5 mass pct did not notably change the flux viscosity. This phenomenon was correlated to the synergic effects of the dissociation of flux network and the increased potential of nucleation at high temperature with the fluorine addition.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Spurious Grain Formation at Cross-Sectional Expansion During Directional Solidification: Influence of Thermosolutal Convection

Formation of spurious grains during directional solidification (DS) of Al-7 wt.% Si and Al-19 wt.% Cu alloys through an abrupt increase in cross-sectional area has been examined by experiments and by numerical simulations. Stray grains were observed in the Al-19 wt.% Cu samples and almost none in the Al-7 wt.% Si. The locations of the stray grains correlate well where numerical solutions indicate the solute-rich melt to be flowing up the thermal gradient faster than the isotherm velocity. It is proposed that the spurious grain formation occurred by fragmentation of slender tertiary dendrite arms was enhanced by thermosolutal convection. In Al-7 wt.% Si, the dendrite fragments sink in the surrounding melt and get trapped in the dendritic array growing around them, and therefore they do not grow further. In the Al-19 wt.% Cu alloy, on the other hand, the dendrite fragments float in the surrounding melt and some find conducive thermal conditions for further growth and become stray grains.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Solidification Characteristics of Silicon-Alloyed Ductile Cast Irons

Solidification pattern of three ductile iron compositions [I-2.5%Si; II-4%Si, and III-4%Si-1.6%Mo] is studied, to evaluate the effects of high Si and Mo contents. Inoculation compensates for the negative effect of Mo on eutectic undercooling. The deviation using a sphere as reference of graphite particles is noticeably increased by silicon alloying, when a characteristic of the graphite particles appears have a larger perimeter, resulting in a large category [IV, V and VI forms, ISO 945]. The sphericity shape factor (SSF) considering the real perimeter of particle is recommended in the nodularity evaluation in high-Si ductile iron, instead of the roundness shape factor, involving maximum ferret, presently incorporated in the ISO 945 standard. The higher the minimum acceptable SSF is, the greater difference in nodularity, between conventional and Si-alloyed ductile irons. For more than 4%Si, ductile irons are characterized by a medium-quality graphite phase, with prevalent form V-ISO 945 graphite. A specific inoculant, with a powerful action on the graphite particle compactness, especially to promote VI graphite form, is necessary.

36 MATERIALS SCIENCE↗
DOE OSTI2019

On the Microstructure and Solidification Behavior of N-Bearing Ti-46Al-8Ta (at.%) Intermetallic Alloys

Ti-46Al-8Ta (at.%) intermetallic alloys exhibit acceptable strength at low and high temperatures with good oxidation resistance. Different alloying elements are usually added to TiAl-based alloys to develop required properties. In this work, effects of N addition up to 2 at.% on microstructural features and solidification behavior of the Ti-46Al-8Ta alloy is investigated. The achieved results showed that alloying with N causes dendrite refinement and reduction of the retained metastable B2 phase in the as-cast microstructure. The dendrite morphology is also changed from fourfold to fivefold and then sixfold symmetry by increasing N content. The effect of N on interlamellar spacing, amount of constituent phases, and lattice parameters was not uniform. Increasing N content resulted in an increase in both small punch maximum load and hardness up to 1.05 at.%, whereas further N alloying, despite increasing of hardness, led to a drop in the maximum load.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Evolution of local atomic structure during solidification of Al2Au liquid: An ab initio study

The local atomic structure evolution in Al2Au alloy during solidification from 2000 K to 400 K was studied by ab initio molecular dynamics simulations and analyzed using the structure factor, pair correlation functions, bond angle distributions, the Honeycutt-Anderson (HA) index and Voronoi tessellation methods. It was found that the icosahedral-like clusters are negligible in the Al2Au stable liquid and supercooled liquid states, and the most abundant clusters are those having HA indices of 131 and 120 or Voronoi indices of < 0,4,4,0 >, < 0,3, 6,0 > and < 0,4,4,2 > with coordination numbers of 8, 9 and 10, respectively. These clusters are similar to the local atomic structures in the CaF2-type Al2Au crystal, revealing the existence of structure heredity between liquid and crystalline phase in Al2Au alloy. (C) 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Grain growth competition during thin-sample directional solidification of dendritic microstructures: A phase-field study

Here we present the results of a comprehensive phase-field study of columnar grain growth competition in bi-crystalline samples in two dimensions (2D) and in three dimensions (3D) for small sample thicknesses allowing a single row of dendrites to form. We focus on the selection of grain boundary (GB) orientation during directional solidification in the steady-state dendritic regime, and study its dependence upon the orientation of two competing grains. In 2D, we map the entire orientation range for both grains, performing several simulations for each configuration to account for the stochasticity of GB orientation selection and to assess the average GB behavior. We find that GB orientation selection depends strongly on whether the primary dendrite growth directions have lateral components (i.e. components perpendicular to the axis of the temperature gradient) that point in the same or opposite directions in the two grains. Here, we identify a range of grain orientations in which grain selection follows the classical description of Walton and Chalmers. We also identify conditions that favor unusual overgrowth of favorably-oriented dendrites at a converging GB. We propose a simple analytical description that reproduces the average GB orientation selection from 2D simulations within statistical fluctuations of a few degrees. In 3D, we find a similar GB orientation selection as in 2D when secondary branches grow in planes parallel and perpendicular to the sample walls. Remarkably, quasi-2D behavior is also observed even when those perpendicular sidebranching planes are rotated by a finite azimuthal angle about the primary dendrite growth axis as long as the absolute values of those azimuthal angles are equal in both grains. In contrast, when the absolute values of those azimuthal angles differ markedly, we find that unusual overgrowth events at a converging GB are promoted by a high azimuthal angle in the least-favorably-oriented grain. We also find that diverging GBs can be strongly affected by those azimuthal angles, while converging GBs exhibit a weak dependence on those angles. Finally, for diverging GBs, GB orientation is also strongly affected by the relative signs of the lateral components of the primary dendrite growth directions in both grains.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Interfacial free energy and stiffness of aluminum during rapid solidification

Using molecular dynamics simulations and the capillary fluctuation method, we have calculated the anisotropic crystal-melt interfacial free energy and stiffness of aluminum in a rapid solidification system where a temperature gradient is applied to enforce thermal non-equilibrium. To calculate these material properties, the standard capillary fluctuation method typically used for systems in equilibrium has been modified to incorporate a second-order Taylor expansion of the interfacial free energy term. The result is a robust method for calculating interfacial energy, stiffness and anisotropy as a function of temperature gradient using the fluctuations in the defined interface height. This work includes the calculation of interface characteristics for temperature gradients ranging from 11 to 34 K/nm. The captured results are compared to a thermal equilibrium case using the same model and simulation technique with a zero gradient definition. We define the temperature gradient as the change in temperature over height perpendicular to the crystal-melt interface. The gradients are applied in MD simulations using defined thermostat regions on a stable solid-liquid interface initially in thermal equilibrium. The results of this work show that the interfacial stiffness and free energy for aluminum are dependent on the magnitude of the temperature gradient, however the anisotropic parameters remain independent of the non-equilibrium conditions applied in this analysis. As a result, the relationships of the interfacial free energy/stiffness are determined to be linearly related to the thermal gradient, and can be interpolated to find material characteristics at additional temperature gradients.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Benchmark microgravity experiments and computations for 3D dendritic-array stability in directional solidification

In this study, we present a comprehensive quantitative analysis of stability bands for dendritic arrays during directional solidification of a transparent succinonitrile-0.46 wt % camphor alloy, spanning a broad range of pulling velocities. Taking advantage of the microgravity environment aboard the International Space Station where most convection effects are suppressed, we obtain unique measurements that quantify the stable primary spacing range of spatially extended three-dimensional dendritic array structures under purely diffusive growth conditions. Through carefully designed velocity jump experiments and detailed examination of sub-grain boundary dynamics, we characterize key instabilities, including elimination and tertiary branching, shedding new light on the mechanisms governing dynamic dendritic spacing selection in extended 3D arrays. Phase field simulations are performed to characterize the stability limits of dendritic array structures for quantitative comparison with the flight experiments. Although the simulations capture general trends, significant deviations are noted at the upper stability boundary, indicating the influence of additional, unexplored factors. These findings contribute to a deeper understanding of dendritic growth dynamics and offer valuable benchmark data that could aid in refining predictive models and improving control of dendritic microstructures in metallurgical applications.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Role of scan strategies on thermal gradient and solidification rate in electron beam powder bed fusion

Local microstructure control in electron beam powder bed fusion (EB-PBF) is of great interest to the additive manufacturing community to realize complex part geometry with targeted performance. The local microstructure control relies on having a detailed understanding of local melt pool physics (e.g., 3-D melt pool shape as well as spatial and temporal variations of thermal gradient (G) and solidification rate (R)). In this research, a new scan strategy referred to as ghost beam is numerically evaluated as a candidate to achieve the targeted G and R of IN718 alloy. The boundary conditions for simulations, including the speed (490 mm/s) and spatial locations of the beam within a given layer, are obtained by using series of snapshot images, recorded at 12,000 frames per second, using a high-speed camera. The heat transfer simulations were performed using TRUCHAS an open-source software deployed within a high-performance computational infrastructure. The simulation results showed that reheating at short beam on-time and time delay decreases both G and R. Local variation of R at the center of the melt pool trailing edge showed periodic temporal fluctuations. Finally, the ghost beam scan strategy was compared to other existing raster and spot scan strategies.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Effect of gadolinium addition on microstructural evolution and solidification characteristics of Al-15%Mg2Si in-situ composite

Highlights: • The Al{sub 3}Gd is the heterogeneous nucleation of primary Mg{sub 2}Si in Al-15%Mg{sub 2}Si composite. • Addition of Gd to Al-15%Mg{sub 2}Si resulted in the formation of Al{sub 2}Si{sub 2}Gd and AlSiGd intermetallic compounds. • Absorption of Gd atoms preferentially on {100} facets of Mg{sub 2}Si crystals changed its growth manners. • Primary Mg{sub 2}Si transformed from coarse-dendritic to perfect truncated octahedral shape after treating with 1.0 wt% Gd. • Gd addition increased the nucleation and growth temperatures of primary Mg{sub 2}Si. - Abstract: The current study investigated the effects of gadolinium (Gd) addition (0, 0.3, 0.5, 1.0, and 2.0 wt%) on microstructural development and solidification characteristics of hypereutectic Al-15%Mg{sub 2}Si in-situ composite. Microstructural examinations were carried out via optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffraction (XRD), transmission electron microscopy (TEM), and electron backscattered diffraction (EBSD). The results showed that an addition of 1.0 wt% Gd, changed the morphology of primary Mg{sub 2}Si from coarse dendritic to fine truncated octahedral shape. Furthermore, the average size of primary Mg{sub 2}Si particles decreased from 40 μm to 20 μm and density increased from 495 to 1167 particles/mm{sup 2}. Besides that, cooling curve thermal analysis (CCTA) technique revealed that Gd addition increased nucleation and growth temperatures of primary Mg{sub 2}Si phase. It is proposed that modification and refinement mechanism of Gd addition are mainly attributed to formation of tiny Al{sub 3}Gd particles as the heterogeneous nucleation substrates for primary Mg{sub 2}Si, which results in a refined distribution of these particles and poisoning effect by adsorbing Gd atoms, preferentially on {100} facets of Mg{sub 2}Si crystals. Moreover, it is believed that formation of Gd intermetallic compounds is responsible for refinement of primary Mg{sub 2}Si particles.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Three-dimensional morphology of an ultrafine Al-Si eutectic produced via laser rapid solidification

Al-Si alloys processed by laser rapid solidification yield eutectic microstructures with ultrafine and interconnected fibers. Such fibrous structures have long been thought to bear resemblance to those formed in impurity-doped alloys upon conventional casting. Here, we show that any similarity is purely superficial. By harnessing high-throughput characterization and computer vision techniques, we perform a three-dimensional analysis of the branching behavior of the ultrafine eutectic and compare it against an impurity-modified eutectic as well as a random fractal (as a benchmark). Differences in the branching statistics point to different microstructural origins of the impurity- and quench-modified eutectic. Finally, our quantitative approach is not limited to the data presented here but can be used to extract abstract information from other volumetric datasets, without customization.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Note: Strain sensitivity comparison between fiber Bragg gratings inscribed on 125 and 80 micron cladding diameter fibers, case study on the solidification monitoring of a photo-curable resin

The influence of fiber Bragg grating diameter when measuring strain is investigated and quantified. Two fiber Bragg gratings with bare cladding diameter of 125 μm and 80 μm are produced by excimer laser irradiation through a phase mask, and are used to simultaneously monitor the Bragg wavelength shift due to the strain produced by the solidification of a photo-curable resin during light exposure. It is found that the ratio of the measured strains in the two fiber Bragg gratings is close to the inverse ratio of the fiber’s cladding diameter. These results represent a direct simultaneous comparison between 125 μm and 80 μm diameter fiber Bragg grating strain sensors, and demonstrate the feasibility of strain measurements in photo-curable resins using bare 80 μm cladding diameter fiber Bragg gratings with an increased sensitivity and spatial resolution compared with standard 125 μm diameter fiber Bragg gratings.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2015

Solidification of radioactive waste resins using cement mixed with organic material

Solidification of radioactive waste resins using cement mixed with organic material i.e. biochar is described in this paper. Different percentage of biochar (0%, 5%, 8%, 11%, 14% and 18%) was investigated in this study. The characteristics such as compressive strength and leaching behavior were examined in order to evaluate the performance of solidified radioactive waste resins. The results showed that the amount of biochar affect the compressive strength of the solidified resins. Based on the data obtained for the leaching experiments performed, only one formulation showed the leached of Cs-134 from the solidified radioactive waste resins.

12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE W↗
DOE OSTI2015

Numerical modeling of heat transfer in molten silicon during directional solidification process

Numerical investigation is performed for some of the thermal and fluid flow properties of silicon melt during directional solidification by numerical modeling. Dimensionless numbers are extremely useful to understand the heat and mass transfer of fluid flow on Si melt and control the flow patterns during crystal growth processes. The average grain size of whole crystal would increase when the melt flow is laminar. In the silicon growth process, the melt flow is mainly driven by the buoyancy force resulting from the horizontal temperature gradient. The thermal and flow pattern influences the quality of the crystal through the convective heat and mass transport. The computations are carried out in a 2D axisymmetric model using the finite-element technique. The buoyancy effect is observed in the melt domain for a constant Rayleigh number and for different Prandtl numbers. The convective heat flux and Reynolds numbers are studied in the five parallel horizontal cross section of melt silicon region. And also, velocity field is simulated for whole melt domain with limited thermal boundaries. The results indicate that buoyancy forces have a dramatic effect on the most of melt region except central part.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Near-isothermal furnace for in situ and real time X-ray radiography solidification experiments

In this paper, we present a newly developed near-isothermal X-ray transparent furnace for in situ imaging of solidification processes in thin metallic samples. We show that the furnace is ideally suited to study equiaxed microstructure evolution and grain interaction. To observe the growth dynamics of equiaxed dendritic structures, a minimal temperature gradient across the sample is required. A uniform thermal profile inside a circular sample is achieved by positioning the sample in the center of a cylindrical furnace body surrounded by a circular heater arrangement. Performance tests with the hypo-eutectic Al-15wt.%Cu and the near-eutectic Al-33wt.%Cu alloys validate the near-isothermal character of the sample environment. Controlled cooling rates of less than 0.5 K min{sup −1} up to 10 K min{sup −1} can be achieved in a temperature range of 720 K–1220 K. Integrated in our rotatable laboratory X-ray facility, X-RISE, the furnace provides a large field of view of 10.5 mm in diameter and a high spatial resolution of ∼4 μm. With the here presented furnace, equiaxed dendrite growth models can be rigorously tested against experiments on metal alloys by, e.g., enabling dendrite growth velocities to be determined as a function of undercooling or solutal fields in front of the growing dendrite to be measured.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Solidification and loss of hydrostaticity in liquid media used for pressure measurements

We carried out a study of the pressure dependence of the solidification temperature in nine pressure transmitting media that are liquid at ambient temperature, under pressures up to 2.3 GPa. These fluids are 1:1 isopentane/n-pentane, 4:6 light mineral oil/n-pentane, 1:1 isoamyl alcohol/n-pentane, 4:1 methanol/ethanol, 1:1 FC72/FC84 (Fluorinert), Daphne 7373, isopentane, and Dow Corning PMX silicone oils 200 and 60,000 cS. We relied on the high sensitivity of the electrical resistivity of Ba(Fe 1–x Ru x ) 2 As 2 single crystals to the freezing of the pressure media and cross-checked with corresponding anomalies observed in the resistance of the manganin coil that served as the ambient temperature resistive manometer. In addition to establishing the temperature-pressure line separating the liquid (hydrostatic) and frozen (non-hydrostatic) phases, these data permit rough estimates of the freezing pressure of these media at ambient temperature. As a result, this pressure establishes the extreme limit for the medium to be considered hydrostatic. For higher applied pressures, the medium has to be treated as non-hydrostatic.

47 OTHER INSTRUMENTATION↗
DOE OSTI2020

Coupling solidification kinetics with phase-behavior computations in hydrodynamic simulations of high-pressure, dynamic-compression processes

In this study, we report a numerical scheme to integrate models for the kinetics of solidification processes together with phase-behavior computations in the context of continuum-scale hydrodynamic simulations. The objective of the phase-behavior computations is to determine the pressure and temperature, given the following three sets of inputs: (1) an appropriate equation of state to describe our system, (2) the phase fraction(s) produced by the kinetic models, (3) and the volume and internal energy obtained by solving the conservation equations that govern the hydrodynamic behavior. The kinetics are assumed to be governed by the Kolmogorov–Johnson–Mehl–Avrami equation, and the nucleation and growth rates that enter into that equation are functions of the pressure and temperature produced by the phase-behavior computations. Our formulation allows for the fluid and solid phases to be at different temperatures (thermal nonequilibrium) and pressures (arising from surface-tension-induced Laplace contributions). The formulation is presented in a fairly general setting that is independent of any particular material, although we demonstrate it in some examples related to high-energy-density science applications where materials are rapidly compressed to pressures exceeding several gigapascals in less than a microsecond. We conclude with a critical evaluation of our approach and provide suggestions for future work to improve the predictive capabilities and generality of the models.

42 ENGINEERING↗
DOE OSTI2022

Multi-physics modeling of grain growth during solidification in electron beam additive manufacturing of Inconel 718

While experimental work has shown promising results regarding control of additive manufacturing metal grain structure, the effects of processing parameters on the grain structure is difficult to understand and predict from experiment alone. To this end, a modeling framework is developed which sequentially couples a macro-scale, semi-analytic thermal model, and a meso-scale, cellular automata-based microstructure model. This framework is applied to electron beam additive manufacturing of Inconel 718 using a complex spot scan pattern. The model shows that, with the same scan pattern, variations in the spot time and electron-beam current produce thermal histories with significant spatial and temporal differences, which then produce complex solidification conditions from the interplay between molten pools in the same layer and subsequent layers, resulting in vastly different grain structures. Additionally, it is noted that the framework can significantly reduce the computational expenses for coupled thermal-metallurgical problems, and has the potential to be used for component level problems.

36 MATERIALS SCIENCE↗