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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 1,063 records · Page 59
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Influence of Thermal History on the Hot Ductility of a Continuously Cast Low Alloyed Cr-Mo Steel

The hot ductility of a low alloyed Cr-Mo steel has been investigated to evaluate the surface cracking sensitivity within the straightening or unbending regime during the continuous casting process. Tensile samples were subjected to various thermal treatments, including melting and solidification, and were tested at deforming temperatures ranging between 600 and 1100 °C using a strain rate of 10{sup −3} s{sup −1}. Hot ductility was evaluated based on reduction in area measurement and metallographic investigations. The investigated steel exhibits a drop in ductility at around 800 °C due to intergranular cracking. Microstructural examinations and supplementary thermokinetic computer simulations were carried out to describe the evolution of the microstructure during solidification and cooling.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Microstructural and Mechanical Characterization of Directionally Solidified Conventional and Nb-Modified Mar-M247 Superalloy

This study analyzes the effects of replacing Ta by Nb (at.% basis) in the phase transformation temperatures, Scheil solidification behavior, and microstructural mechanical features of a Nb-modified as well as conventional Mar-M247 superalloys. Both alloys were directionally solidified at a withdrawal rate of 18 cm/h under a thermal gradient of about 80 °C/cm. DSC results were compared to thermodynamic simulations of phase transformation temperatures and showed a good correlation. The replacement of Ta by Nb has not altered the solidification path of Mar-M247, keeping the sequence L → L + γ → L + γ + MC → L + γ + MC + γ/γ′ eutectic. Due to the segregation of Hf and Zr to the very last liquid to solidify, a low melting point γ/Ni{sub 5}(Hf,Zr) constituent was observed. The as-solidified microstructures of both Mar-M247(Nb) and Mar-M247 were constituted of columnar dendrites of gamma matrix (γ) phase with cuboidal gamma-prime (γ′) precipitates. In addition, γ/γ′-eutectic was observed in the interdendritic region along with MC carbides in blocky, script (blocky dendritic), needle-like and fine nodular morphologies. Both alloys contained about 1.5 vol.% of MC carbides. The MC carbides in Mar-M247(Nb) and Mar-M247 are, respectively, Nb-rich and Ta-rich, with only a limited amount of Hf being detected in the MC phase. The yield stress of Nb-modified Mar-M247 showed higher values up to 750 °C. For higher temperatures, above 800 °C, a conventional Mar-M247 presented a higher value of yield stress.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Deformation Behavior of Pure Copper Castings with As-Cast Surfaces for Electrical Parts

Herein, the tensile deformation behavior of pure copper castings with as-cast surfaces was investigated. It was found that small and sufficiently large uniform elongation types corresponded, respectively, to the presence or absence of a Cu-Cu{sub 2}O eutectic phase in the microstructure. Fissure defects were observed near the as-cast surfaces when the eutectic phase was present, probably leading to stress concentration. On the other hand, surface pinholes were observed when the eutectic phase was absent, and these were not prevented by directional solidification. Although the as-cast specimens (non-directionally solidified) exhibited a slightly reduced uniform elongation compared to the machined specimens in the absence of the eutectic phase, the directionally solidified as-cast samples exhibited a uniform elongation that was similar to that of the machined specimens. In addition, micropore arrays were formed along the grain boundaries inside the castings when the eutectic phase was absent, whereas almost no micropores were observed inside the directionally solidified as-cast specimens because the solidification process involved a faster cooling rate and/or higher temperature gradient. These results do not only suggest the existence of surface pinholes but also indicate that microporosity can reduce the cross-sectional area of the specimens, thus, affecting their uniform elongation.

36 MATERIALS SCIENCE↗
DOE OSTI2021

A stochastic scan strategy for grain structure control in complex geometries using electron beam powder bed fusion

Spatial control of microstructure within a three-dimensional component has been a dream of materials scientists for centuries. However, limitations in traditional manufacturing processes prevent detailed control over the distribution of microstructures in a single part. Here, we demonstrate the ability to control grain structure and crystallographic texture during metal additive manufacturing for arbitrary cross-sections of a practical size, with profound implications for the design and optimization of next-generation products. The key to this advance is a new geometry agnostic scan path algorithm that manipulates the spatial distribution of solidification conditions. Utilizing a fundamental understanding of solidification dynamics and a model of the heat transfer during processing, we have designed this algorithm to manipulate the natural competition between epitaxial dendrite growth and grain nucleation. With this algorithm, we successfully controlled the grain structure of Ni-based superalloy IN718 in the shape of the Mona Lisa.

36 MATERIALS SCIENCE↗
DOE OSTI2023

3D Printed eutectic aluminum alloy has facility for site-specific properties

Additive manufacturing (AM) has the ability to print structures with site-specific properties. Existing AM approaches for site-specific properties are, however, based on complex processing-microstructure relationships in conventional alloys that were not designed for this purpose. Here, in this work, we report a straightforward approach for achieving site-specific properties that takes advantage of eutectic solidification characteristics. We demonstrate that the yield strength of a eutectic Al-Cu-Ce-Zr alloy can be tuned by varying the laser scan speed in concert with hatch spacing in laser powder bed fusion AM. A faster speed increases solidification rate resulting in finer eutectic spacing and higher strength. The hatch spacing is reduced at faster speeds to ensure overlap between melt pools which become smaller with increasing scan speed. The yield strength and its anisotropy relative to build direction are further tunable with a heat treatment. The scan speed-eutectic spacing-strength relationship is successfully applied to print a complex pattern of site-specific hardness in the alloy. The generalized principle of using AM for a eutectic alloy to create site-specific properties and anisotropy in properties is demonstrated.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Segmentation of tomography datasets using 3D convolutional neural networks

Dendritic microstructures are ubiquitous in nature and are the primary solidification morphologies in metallic materials. Techniques such as X-ray computed tomography (XCT) have provided new insights into dendritic phase transformation phenomena. However, manual identification of dendritic morphologies in microscopy data can be both labor intensive and potentially ambiguous. The analysis of 3D datasets is particularly challenging due to their large sizes (terabytes) and the presence of artifacts scattered within the imaged volumes. Here, in this study, we trained 3D convolutional neural networks (CNNs) to segment 3D datasets. Three CNN architectures were investigated, including a new version of FCDenseNet which we extended to 3D. We show that using hyperparameter optimization (HPO) and fine-tuning techniques, both 2D and 3D CNN architectures outperform the previous state of the art. The 3D U-Net architecture trained in this study produced the best segmentations according to quantitative metrics (intersection-over-union of 95.56% and a boundary displacement error of 0.58 pixels), while 3D FCDense produced the smoothest boundaries and best segmentations according to visual inspection. The trained 3D CNNs are able to segment entire 852 × 852 × 250 voxel 3D volumes in only ~60 s, thus hastening the progress towards a deeper understanding of phase transformation phenomena such as dendritic solidification.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Massively parallel phase-field simulations targeting exascale

The interface thickness in the phase-field (PF) method limits its simulation scales. Consequently, large-scale PF simulations become prohibitively expensive for resolving the extremely fine microstructures that typically form during rapid solidification processing. This challenge is significant in predicting microstructure evolution in metal additive manufacturing and has been identified by the United States Department of Energy’s Exascale Computing Project. Here, to address this, we develop a multi-GPU and MPI-based massively parallel simulation code, utilizing state-of-the-art algorithms, software, and libraries, for large-scale three-dimensional (3D) PF simulations. We report the first GPU-parallel PF simulations on Frontier (currently the second TOP500 exascale cluster) and Summit machines, taking dendritic growth as an example problem. We evaluate the parallel performance of our implementation using scaling studies with more than 24 000 GPUs (among the largest known computations to date) and the acceleration performance using large-scale simulations of dendritic growth in 3D. Finally, massively parallel GPUs in these supercomputers enabled the first coupled multiscale simulations of laser melting and subsequent dendritic solidification on the scale of a full melt-pool, demonstrating the feasibility of performing PF simulations with a point total over 2 billion grid points within an acceptable time.

Exascale↗
DOE OSTI2019

Effect of Sb substitution on crystal structure, texture and hard magnetic properties of melt-spun MnBi alloys

Melt-spun Mn 50 Bi 50-x Sb x alloys with x ≤ 5 were prepared at different solidification rates and characterized both in the as-spun state and after annealing. In the as-spun alloys, the Sb substitution leads to the formation of a metastable phase, similar to the binary "quenched high-temperature phase" but without the superstructure of the latter and exhibiting a different – increasing – temperature dependence of coercivity. The new nanocrystalline metastable phase is not only itself characterized by a high room-temperature coercivity (in excess of 20 kOe), but upon annealing it transforms into a high-coercivity (up to 13.5 kOe) "low-temperature" α phase. However, the advantage of obtaining a high coercivity without the otherwise required milling made possible by the Sb substitution is undermined by the absence of a local crystallographic texture in the melt-spun alloys. The best combination of the isotropic hard magnetic properties realized for the Mn 55 Bi 43.5 Sb 1.5 composition is a remanence of 35.5 emu/g and a coercivity of 8.1 kOe. The annealed Sb-free Mn 50 Bi 50 alloys did possess a local crystallographic alignment and, for high solidification rates, a moderate coercivity up to 5.6 kOe; however, they had an inadequate "rectangularity" in the demagnetization curve. The orthorhombic compound known as MnBi 0.9 Sb 0.1 and described earlier as antiferromagnetic was obtained in the annealed ribbons with x ≈ 5. The compound was found to order ferromagnetically between 200 K and ≈250 K and to exhibit a significant coercivity, 14.3 kOe at 50 K. Here, it is suggested that the MnBi 0.9 Sb 0.1 may actually be a Sb-stabilized "new phase" observed earlier as metastable in the Bi–MnBi eutectic alloys.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Formation of intermetallic structures at the interface of steel-to-aluminium explosive welds

Highlights: • The interface of Fe-Al explosive welds was characterised by SEM/EDS and EBSD. • The intermetallics formed at the weld interface are rich in Al (Fe{sub 4}Al{sub 13}, Fe{sub 2}Al{sub 5}). • The presence of intermetallic phases on the interface deeply affects the weldability. • Al-rich intermetallics increase the solidification time of the weld interface. - Abstract: The formation of intermetallic structures at the interface of carbon steel to 6082 aluminium alloy explosive welds and their influence on the weldability of these two materials were studied. The morphology, the microstructure, the chemical and phase compositions of the welds were characterised by several types of microscopy techniques. The interface characterisation proved that explosive mixtures with a lower detonation velocity were revealed as being more suitable for achieving consistent welds since jet entrapment was prevented and continuous molten layers were not formed at the weld interface. It was also found that the physical properties of the intermetallic phases generated at the weld interface have a strong influence on the weldability of steel-to-aluminium explosive welds. Specifically, it was shown that the formation of aluminium-rich intermetallic phases at the weld interface increases the solidification time of the interfacial molten material, decreasing the weldability of these two materials. The formation of these intermetallic compounds should be avoided by reducing the interaction between the flyer and the baseplate as well as by avoiding excessive molten layers.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Microstructural evolution of rapidly solidified hypoeutectic Al 10Cu alloy during non-isothermal annealing transients induced by nano-second laser pulses

The evolution of characteristic nonequilibrium features presenting in morphologically distinct regions of rapid solidification (RS) microstructures in a hypoeutectic Al—10Cu (atomic %) in response to non-isothermal annealing transients has been studied by transmission electron microscopy (TEM). The capabilities of the Movie-Mode Dynamic TEM (MM-DTEM) instrument were used to expose select regions of the RS microstructure to sequences of rapid heating and cooling transients induced by nanosecond laser pulses while permitting in-situ observation. Partial melting, microstructural scale coarsening, morphological changes of the nonequilibrium features in the multi-phase RS microstructure, and solid-state phase transformation were observed. Heterogeneous nucleation of nanoscale θ-Al 2 Cu phase involved metastable supersaturated α-Al and the θ'-Al 2 Cu phases, establishing different sets of orientation relationships for the stable θ-Al 2 Cu and α-Al phases. Replacement of banded morphology grains that formed under conditions driven farthest from equilibrium by an equiaxed nanocrystalline structure comprised of α-Al phase, the primary solidification product, and an intergranular network of Al 2 Cu crystals has been attributed to local remelting. Here the experimental approach explored, permitted discovery of mechanistic details of location-specific transformation pathways activated in the multi-phase RS microstructure of hypoeutectic Al—Cu during subsequent nonisothermal transients.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Microstructure and mechanical behavior of laser remelted amorphous Al-Ni-La welds

To explore the feasibility of using laser powder bed fusion systems to print bulk amorphous Al alloys, a single autogenous weld produced by laser remelting on a cast Al-5La-9Ni (at.%) alloy was studied for its microstructure and mechanical behavior. The solidification rates experienced by material within the welds were high enough to produce entirely amorphous regions within the welds. Welds were characterized using SEM, STEM, and APT and were found to contain Ni-rich amorphous clusters. Micropillar compression tests were used to assess mechanical properties of the welds and found that all of the amorphous regions exhibited yield strengths around 1 GPa. In conclusion, these results are discussed in the context of diffusivity of Ni and rare earth elements in liquid, glass forming ability, thermodynamic driving force for phase formation during solidification, free volume concentrations in bulk metallic glasses, and cluster-related softening.

APT↗
DOE OSTI2020

Cooling dynamics of two titanium alloys during laser powder bed fusion probed with in situ X-ray imaging and diffraction

Metal parts produced by laser powder bed fusion (LPBF) additive manufacturing exhibit characteristic microstructures comparable to those observed in laser welding. The primary cause of this characteristic microstructure is rapid, localized heating and cooling cycles, which result in extreme thermal gradients where material solidification is followed by fast cooling in the solid state. The final microstructure and mechanical performance are also influenced by pore formation caused by melt pool fluid dynamics. Here, we use high speed, in situ X-ray diffraction to probe the kinetics of cooling and solid-solid phase transitions after laser melting in two aerospace titanium alloys: Ti-6Al-4V, an α + β alloy; and Ti-5Al-5V-5Mo-3Cr, a near-β alloy. We complement these diffraction studies with in situ X-ray imaging to probe melt pool dynamics and pore formation. From these two complementary probes, we quantify pore formation during melting and the subsequent microstructural evolution as the material rapidly cools after solidification. These results are critical for understanding defect formation and residual stress development in different titanium alloys under LPBF conditions and can help inform process models to predict final part performance.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Disorder trapping and formation of antiphase nanodomains in Ni 3 Sn: in situ observation and high resolution characterization

Rapid solidification of the intermetallic Ni 3 Sn compound from an undercooled liquid was investigated using time-resolved synchrotron X-ray diffraction. Primary growth of an ordered and a disordered cubic solid was observed in situ at a low and a high undercooling, respectively. The disordered cubic solid was reordered after rapid solidification and experienced a solid-state transformation in cooling. A HRTEM study revealed high density of crystal defects and nanodomains in the transformed matrix. A HAADFSTEM study determined that the nanodomains are antiphase domains with chemical disorder. These observations indicated a possibility of online control of mechanical properties in additively manufactured intermetallics.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Molecular dynamics simulation of phase competition in terbium

The competition among multiple solid phases determines the final microstructures of a material. Such competition can originate at the very beginning of the solidification process. We report the results of molecular dynamics simulation of the phase competition between the hexagonal close-packed (hcp), face-centered cubic (fcc), and body-centered cubic (bcc) phases during the solidification of pure Tb. We found that the liquid supercooled below the hcp melting temperature has both bcc and hcp/fcc nuclei, but only the bcc nuclei grow such that the liquid always solidifies into the bcc phase, even at temperatures where the hcp phase is more stable. The hcp phase can only form in the last liquid droplet or at the bcc grain boundaries. Depending on the bcc grain orientations, the hcp phase jammed between the bcc grains either completely disappears or slowly grows via a solid-state massive transformation mechanism. Once the hcp phase becomes large enough, the stresses associated with its appearance can trigger a martensitic transformation. Yet, not the entire bcc phase is consumed by the martensitic transformation and the remaining bcc phase is transformed into the hcp phase via the solid-state massive transformation mechanism. If the supercooling is too large, the nucleation becomes almost barrier free and the liquid solidifies into a structure consisting of ultra-fine hcp and bcc grains after which the bcc phase quickly disappears.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Interpreting dynamic-compression experiments to uncover the time dependence of freezing: Application to gallium

Using pulsed-power magnetic field sources to compress gallium to gigapascal pressures on nanosecond timescales, we report here experiments on shockless dynamic compression of a liquid metal. Time-resolved velocimetry data reveal signatures of rapid freezing from a metastable liquid state, and we demonstrate that the kinetics of this nonequilibrium solidification can be accurately simulated with a computational modeling framework we have developed in previous studies, where classical nucleation theory is coupled with hydrodynamics. Notably, velocity traces in some of our experiments show evidence of a phase transition, while others do not, even though other types of evidence suggest that solidification may be occurring in all of them. We explain how predictions made by our models regarding the presence or absence of these phase-transition signatures motivated additional experiments that later confirmed the theoretical predictions. Our analysis shows that due to the rapid, quasi-isentropic nature of the loading path, our experiments were able to compress liquid gallium to metastable states that are undercooled below the equilibrium melt temperature by more than 300 K and exhibit pressures that approach five times the equilibrium melt pressure. The understanding gained in this study should form the basis for future dynamic-compression experiments aimed at interrogating melt curves at high pressures.

gallium↗
DOE OSTI2020

Sparse thermal data for cellular automata modeling of grain structure in additive manufacturing

Grain growth in the wake of the melt pool formed during alloy-based additive manufacturing (AM) is complex and multifaceted, depending on parameters governing heat transport, fluid flow, and solidification itself. Cellular automata (CA) models have proven effective in providing computationally efficient and physically sound predictions of grain structure for several AM problems, but their efficiency is tied to the performance of heat transport models. CA models use only a small portion of the problem's temperature data (near the moving melt pool boundary), and much of the CA calculations do not affect the final result due to re-melting of material. Coupling of and communication between heat transport and solidification models, and eliminating operations irrelevant towards final grain structure prediction, will be necessary for using these methods for efficient simulation of large parts. In this work, we introduce a procedure of decoupling the CA from temperature field simulation, using files of relevant temperature data written by the heat transport model. This approach is validated against the standard coupling approach using data obtained through the computational fluid dynamics software OpenFOAM. Negligible differences are seen in grain size, volume, and texture distributions for multilayer simulation of test problems, while the quantity of temperature data for these test problems was reduced by four orders of magnitude (from 100s of GB to 10s of MB) and the code performance sped up by a factor of around 50. Furthermore, variability in microstructure as a function of cell size, substrate, time step, and nucleation parameters is studied, and it is found that cell sizes less than or equal to 1.67 μm and sufficiently small time steps yield Αstatistically equivalent microstructures. Finally, a potential use case for this CA approach—the layer-wise convergence in grain structure starting from extremes in initial grain size—is examined. This approach's ability to simulate expected trends in nucleation and epitaxial grain growth for large regions of microstructure, simulated independently of heat transport models themselves, should prove useful for investigation of various microstructure uncertainties and prediction of part-scale experimental results.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Crack-Free 30% Chromium-Nickel Alloy Welding Products for Nuclear Service

We report prior research in the development of 30% chromium-nickel alloy nuclear welding wires has resulted in the resolution of primary water stress corrosion cracking (PWSCC) and ductility dip cracking (DDC) as well as improvement in solidification cracking (SC) resistance. The resolution of DDC exhibits some Laves phase, which has a negative effect on SC resistance. In this study, the use of an alternate carbide former, tantalum (Ta), in combination with niobium (Nb) was researched. Three heats of recently designed Filler Metal 52MSS-Ta (i.e., HV1648, HV1673A, and VX131WXW) were melted, fabricated, and systematically studied. DDC and SC were evaluated with thermodynamic modeling using the Scheil solidification simulation model, two types of varestraint tests, and strain-to-fracture (STF) testing. The varestraint and STF test results showed an improved SC resistance with reduced Laves phase and concurrent excellent DDC resistance. Optimized compositions with low Laves phase also exhibited high threshold strain values (TSVs) in the STF test. VX131WXW — which contains 2.81 wt-% Ta, 0.6 wt-% Nb, and 6 wt-% iron (Fe) - exhibited a TSV of 24%. Thermo-Calc computed the Laves phase to be 0.24% for VX131WXW compared to 0.06% in HV1673A. This difference in Laves phase resulted in the lower SC resistance of VX131WXW compared to HV1673A when measured with longitudinal varestraint testing. The maximum crack distance for HV1673A was about 0.6 mm while that of heat VX131WXW was about 1.0 mm. The typical diluted weld deposit made with VX131WXW was also resistant to PWSCC due to the chromium content exceeding 24%. These simultaneous results mark progress toward crack-free welds and provide direction for further optimization of Ta-containing filler metals.

30% chromium-nickel alloy↗
DOE OSTI2019

Study on Application of Geopolymer to the Retrieval of Fuel Debris in Fukushima Dai-ichi NPP

In order to retrieve fuel debris in Fukushima Dai-ichi NPP, many retrieval methods are proposed. Geopolymer is the candidate material applied for an alternative retrieval method. In this concept, Geopolymer can be used to stabilize fuel debris and suppress the diffusion of radioactive materials. Objective: Evaluation of geopolymer applied for retrieving fuel debris. Slump Flow Test: Slump flow values decreased as the test temperature increased. In air condition, water molecules evaporated from geopolymer and the viscosity of geopolymer increased. In water condition, the same trends as in air condition were observed. Slump flow values of geopolymer also depended on the elapsed time after mixing. By adding water, the slump flow values of geopolymer became higher (Even at 80 deg. C). It is possible to adjust the solidification time (Setting time) of geopolymer. Solidification Process of Geopolymer: After the first layer solidified, the second layer was injected. At 300 deg. C, porous structure was observed in the first layer, while the second layer structure was intact. In the actual application, multilayered geopolymer may be used to stabilize fuel debris. Conclusion: Fluidity of geopolymer decreased with raising temperature. Setting time can be adjusted by adding water. Geopolymer may be applied by injecting multiple times on high temperature materials above 100 deg. C.

12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE W↗