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At least 1,027 records · Page 57
All sourcesNASA NTRSDOE OSTI
DOE OSTI2017

Probing the growth and melting pathways of a decagonal quasicrystal in real-time

How does a quasicrystal grow? Despite the decades of research that have been dedicated to this area of study, it remains one of the fundamental puzzles in the field of crystal growth. Although there has been no lack of theoretical studies on quasicrystal growth, there have been very few experimental investigations with which to test their various hypotheses. In particular, evidence of the in situ and three-dimensional (3D) growth of a quasicrystal from a parent liquid phase is lacking. To fill-in-the-gaps in our understanding of the solidification and melting pathways of quasicrystals, we performed synchrotron-based X-ray imaging experiments on a decagonal phase with composition of Al-15at%Ni-15at%Co. High-flux X-ray tomography enabled us to observe both growth and melting morphologies of the 3D quasicrystal at temperature. We determined that there is no time-reversal symmetry upon growth and melting of the decagonal quasicrystal. While quasicrystal growth is predominantly dominated by the attachment kinetics of atomic clusters in the liquid phase, melting is instead barrier-less and limited by buoyancy-driven convection. These experimental results provide the much-needed benchmark data that can be used to validate simulations of phase transformations involving this unique phase of matter.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Hierarchical Eutectoid Nano-lamellar Decomposition in an Al 0.3 CoFeNi Complex Concentrated Alloy

This paper reports a novel eutectoid nano-lamellar (FCC + L1 2 )/(BCC + B2) microstructure that has been discovered in a relatively simple Al 0.3 CoFeNi high entropy alloy (HEA) or complex concentrated alloy (CCA). This novel eutectoid nano-lamellar microstructure presumably results from the complex interplay between Al-mediated lattice distortion (due to its larger atomic radius) in a face-centered cubic (FCC) CoFeNi solid solution, and a chemical ordering tendency leading to precipitation of ordered phases such as L1 2 and B2. This eutectoid microstructure is a result of solid-state decomposition of the FCC matrix and therefore distinct from the commonly reported eutectic microstructure in HEAs which results from solidification. This novel nano-lamellar microstructure exhibits a tensile yield strength of 1074 MPa with a reasonable ductility of 8%. The same alloy can be tuned to form a more damage-tolerant FCC + B2 microstructure, retaining high tensile yield stress (~900 MPa) with appreciable tensile ductility (>20%), via annealing at 700 °C. Such tunability of microstructures with dramatically different mechanical properties can be effectively engineered in the same CCA, by exploiting the complex interplay between ordering tendencies and lattice distortion.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Thermal perturbation of NMR properties in small polar and non-polar molecules

Water is an important constituent in an abundant number of chemical systems; however, its presence complicates the analysis of in situ 1 H MAS NMR investigations due to water’s ease of solidification and vaporization, the large changes in mobility, affinity for hydrogen bonding interactions, etc., that are reflected by dramatic changes in temperature-dependent chemical shielding. To understand the signatures of water and other small molecules in complex environments, in particular how water interacts with the other molecules, in this work the thermally-perturbed NMR properties of water are investigated in detail by in situ MAS NMR. Our results substantially extend the previously published temperature-dependent 1 H and 17 O chemical shifts, linewidths, and spin-lattice relaxation times over a much wider range of temperatures and with significantly enhanced thermal resolution. The following major results are obtained: Hydrogen bonding is clearly shown to weaken at elevated temperatures in both 1 H and 17 O spectra, showing an increase in chemical shielding. At low temperatures, tetrahedral domains of H-bonding networks are evidenced and the observation of solid ice is made under in situ MAS conditions. The 1 H chemical shift properties in other small polar and non-polar molecules have also been described over a range of temperatures, showing the dramatic effect hydrogen bonding perturbation. Gas phase species are described where chemical exchange between gas and liquid phases plays an important role on the observed NMR shifts. The results disclosed herein lay the foundation for a clearer interpretation of complex systems during the increasingly popular in situ NMR characterization at elevated temperatures and pressures for studying chemical systems containing water molecules.

17O NMR↗
DOE OSTI2022

Texture evolution during processing and post-processing of maraging steel fabricated by laser powder bed fusion

Abstract In this study, the evolution of solidification texture during LPBF of Ti-free grade 300 maraging steel, and its effect on texture development during subsequent post-fabrication heat treatments was characterized using Electron Backscatter Diffraction (EBSD). It was found that in the as-fabricated state, no texture was observed in the room temperature martensitic phase. However, the reconstructed parent austenite phase displayed a Cube texture with a minor fraction of Rotated Goss texture. During subsequent aging treatments involving two different routes, namely direct aging of the as-fabricated samples, and conventional solution treatment + aging of the as-fabricated samples, significant changes in the texture components of parent austenite were observed, whereas no changes in texture were observed in the room temperature martensitic phase. During direct aging, it was found that with an increase in the aging temperature up to 520 °C, the texture components of the parent austenite changed from Cube/Rotated Goss to Brass, whereas during the conventional solution treatment and aging cycle, interestingly a change in texture component to rotated copper was observed. The transitions in texture components have been discussed using the concepts of recrystallization and twinning in austenite during annealing and/or aging, and strain energy release maximization (SERM) theory. Furthermore, the importance of these preferred orientations on the mechanical properties was quantified using transformation potential diagrams.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Real-time laser ultrasonic monitoring of laser-induced thermal processes

Abstract Intra- and inter-layer integrity of components fabricated with advanced manufacturing techniques, such as laser powder bed fusion, is dependent upon rapid heating, melting, and solidification processes. There is a need for new techniques to provide in situ feedback of these processes. Here a laser-based ultrasonic technique to probe thermal effects induced by a high-power continuous wave laser in titanium samples is described. Numerical simulations were performed to show that, for a spatially uniform heating beam, laser-induced surface acoustic waves are strongly influenced by surface heating conditions, are dispersive in the case of rapid heating, and that an abrupt velocity reduction happens upon the onset of surface melting. Furthermore, laser-based ultrasound experimental results which monitor the transient change of surface wave travel time associated with high power laser surface heating are provided. A pulsed laser is used to generate high frequency surface acoustic waves that propagate through the laser-heated region and are detected using a photorefractive crystal-based interferometer. Qualitative agreement is observed between theory and experiment with both showing a rapid reduction in the surface wave velocity at the onset of illumination and further decrease in surface wave velocity associated with melting. It is demonstrated that changes in the surface wave velocity can be used to track local heating and detect the onset of surface melting in real time.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Three-dimensional characterization of modifications in sapphire exposed to laser-induced damage using multimodal spectral microimaging

Sapphire (Al 2 O 3 ) is a commonly used dielectric material with many applications in lasers and optical systems. Owing to its high resistivity to laser induced damage, it is particularly suitable for use in high power laser systems. This work focuses on developing techniques to characterize material modifications in sapphire. These techniques were applied following localized laser induced ablation, commonly referred to as laser-damage, resulting from exposure to single 100-ps and 6-ns pulses. Measurements of fluorescence-based piezospectroscopy and confocal Raman microscopy were performed with spatial resolution on the order of 1 μ m. Raman microscopy reveals that the relaxation of material exposed to the rapid laser heating, elastic and viscoplastic deformation, melting, and solidification leads to the formation of a polycrystalline material phase. In addition, narrowband fluorescence lines, referred to as R 1 and R 2 , exhibit pressure-sensitive changes to their spectral profiles, allowing 3D internal stresses to be recorded with spatial resolution of the order of a few micrometers.

Laser-damage↗
DOE OSTI2023

High frequency beam oscillation keyhole dynamics in laser melting revealed by in-situ x-ray imaging

The metal additive manufacturing industry is actively developing instruments and strategies to enable higher productivity, optimal build quality, and controllable as-built microstructure. A beam controlling technique, laser oscillation has shown potential in all these aspects in laser welding; however, few attempts have been made to understand the underlying physics of the oscillating keyholes/melt pools which are the prerequisites for these strategies to become a useful tool for laser-based additive manufacturing processes. Here, to address this gap, we utilized a synchrotron-based X-ray operando technique to image the dynamic keyhole oscillation in Ti-6Al-4V using a miniature powder bed fusion setup. We found good agreement between the experimental observations and simulations performed with a validated Lattice Boltzmann multiphysics model. The study revealed the continuous and periodic fluctuations in the characteristic keyhole parameters that are unique to the oscillating laser beam processing and responsible for the chevron pattern formation at solidification. In particular, despite the intrinsic longer-range fluctuation, the oscillating technique displayed potential for reducing keyhole instability, mitigating porosity formation, and altering surface topology. These insights on the oscillating keyhole dynamics can be useful for the future development and application of this technique.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Biomimetic composite scaffold from an in situ hydroxyapatite coating on cellulose nanocrystals

A novel nanocomposite scaffold was developed by homogeneous deposition of hydroxyapatite (HAP) on a cellulose nanocrystals (CNCs) matrix suspended in a simulated body fluid (SBF). By adjusting the pH of the SBF, the HAP content in the nanocomposite could be controlled between 15 wt% and 47 wt%. Physical and chemical characteristics of the nanocomposites were analyzed by SEM, FTIR, XRD, SAED, and TEM, which confirmed the successful incorporation of HAP onto the CNCs. The nanocomposites were then freeze-casted into porous scaffolds by different solidification technologies ( i.e. , directional freezing (DF), plunging in liquid N 2 (PL) or in a -20 °C freezer (FZ)) followed by lyophilization. Compression testing of the HAP/CNCs foams indicated that DF caused significant improvement in mechanical properties due to the specific orientation and anisotropic porous structure compared to conventional freezing methods such as PL and FZ. Moreover, the scaffold with high HAP content exhibited improved mechanical and thermal properties, which holds potential for application in bone tissue engineering.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Orientation-controlled crystallization of γ-glycine films with enhanced piezoelectricity

Glycine, the simplest amino acid, is considered a promising functional biomaterial owing to its excellent biocompatibility and strong out-of-plane piezoelectricity. Practical applications require glycine films to be manufactured with their strong piezoelectric polar $\langle$001$\rangle$ direction aligned with the film thickness. Based on the recently-developed solidification approach of a polyvinyl alcohol (PVA) and glycine aqueous solution, in this work, we demonstrate that the crystal orientation of the as-synthesized film is determined by the orientation of glycine crystal nuclei. By controlling the local nucleation kinetics via surface curvature tuning, here we shifted the nucleation site from the edge to the middle of the liquid film, and thereby aligned the $\langle$001$\rangle$ direction vertically. As a result, the PVA–glycine–PVA sandwich film exhibits the highest average piezoelectric coefficient d 33 of 6.13 ± 1.13 pC N –1 . This work demonstrates a promising kinetic approach to achieve crystallization and property control in a scalable biocrystal manufacturing process.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Toward precision cosmochronology

The continuous cooling of a white dwarf is punctuated by events that affect its cooling rate. The most significant of these events is the crystallization of its core, a phase transition that occurs once the C/O interior has cooled down below a critical temperature. This transition releases latent heat, as well as gravitational energy due to the redistribution of the C and O ions during solidification, thereby slowing down the evolution of the white dwarf. The unambiguous observational signature of core crystallization–a pile-up of objects in the cooling sequence–was recently reported. However, existing evolution models struggle to quantitatively reproduce this signature, casting doubt on their accuracy when used to measure the ages of stellar populations. The timing and amount of the energy released during crystallization depend on the exact form of the C/O phase diagram. Using the advanced Gibbs–Duhem integration method and state-of-the-art Monte Carlo simulations of the solid and liquid phases, we obtained a very accurate version of this phase diagram that allows a precise modeling of the phase transition. Despite this improvement, the magnitude of the crystallization pile-up remains underestimated by current evolution models. We conclude that latent heat release and O sedimentation alone are not sufficient to explain the observations, and that other unaccounted physical mechanisms, possibly 22 Ne phase separation, play an important role.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

On the limits to Ti incorporation into Si using pulsed laser melting

Fabrication of p-Si(111) layers with Ti levels well above the solid solubility limit was achieved via ion implantation of 15 keV {sup 48}Ti{sup +} at doses of 10{sup 12} to 10{sup 16} cm{sup −2} followed by pulsed laser melting using a Nd:YAG laser (FWHM = 6 ns) operating at 355 nm. All implanted layers were examined using cross-sectional transmission electron microscopy, and only the 10{sup 16} cm{sup −2} Ti implant dose showed evidence of Ti clustering in a microstructure with a pattern of Ti-rich zones. The liquid phase diffusivity and diffusive velocity of Ti in Si were estimated to be 9 × 10{sup −4} cm{sup 2}/s and (2 ± 0.5) × 10{sup 4} m/s, respectively. Using these results the morphological stability limit for planar resolidification of Si:Ti was evaluated, and the results indicate that attaining sufficient concentrations of Ti in Si to reach the nominal Mott transition in morphologically stable plane-front solidification should occur only for velocities so high as to exceed the speed limits for crystalline regrowth in Si(111)

36 MATERIALS SCIENCE↗
DOE OSTI2017

Structural evolution and atomic dynamics in Ni–Nb metallic glasses: A molecular dynamics study

In this work, the composition and temperature dependence of static and dynamic structures in Ni x Nb 1– x ( x = 50–70 at. %) were systematically studied using molecular dynamics with a new-released semi-empirical embedded atom method potential by Mendelev. The calculated pair correlation functions and the structure factor match well with the experimental data, demonstrating the reliability of the potential within relatively wide composition and temperature ranges. The local atomic structures were then characterized by bond angle distributions and Voronoi tessellation methods, demonstrating that the icosahedral $\langle$0,0,12,0$\rangle$ is only a small fraction in the liquid state but increases significantly during cooling and becomes dominant at 300 K. The most abundant clusters are identified as $\langle$0,0,12,0$\rangle$ and distorted icosahedron$\langle$0,2,8,2$\rangle$. The large fraction of these two clusters hints that the relatively good glass forming ability is near the eutectic point. Unlike Cu–Zr alloys, both the self-diffusion coefficient and shear viscosity are insensitive to compositions upon cooling in Ni–Nb alloys. The breakdown of the Stokes-Einstein relation happens at around 1.6 T g ( T g : glass transition temperature). In the amorphous state, the solid and liquid-like atoms can be distinguished based on the Debye-Waller factor $\langle$ u 2 $\rangle$. The insensitivity of the dynamic properties of Ni–Nb alloys to compositions may result from the relatively simple solidification process in the phase diagram, in which only one eutectic point exists in the studied composition range.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

The thermodynamics of a liquid-solid interface at extreme conditions: A model close-packed system up to 100 GPa

The first experimental insight into the nature of the liquid-solid interface occurred with the pioneering experiments of Turnbull, which simultaneously demonstrated both that metals could be deeply undercooled (and therefore had relatively large barriers to nucleation) and that the inferred interfacial free energy γ was linearly proportional to the enthalpy of fusion [D. Turnbull, J. Appl. Phys. 21, 1022 (1950)]. By an atomistic simulation of a model face-centered cubic system via adiabatic free energy dynamics, we extend Turnbull’s result to the realm of high pressure and demonstrate that the interfacial free energy, evaluated along the melting curve, remains linear with the bulk enthalpy of fusion, even up to 100 GPa. This linear dependence of γ on pressure is shown to be a consequence of the entropy dominating the free energy of the interface in conjunction with the fact that the entropy of fusion does not vary greatly along the melting curve for simple monoatomic metals. Here, based on this observation, it appears that large undercoolings in liquid metals can be achieved even at very high pressure. Therefore, nucleation rates at high pressure are expected to be non-negligible, resulting in observable solidification kinetics.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Acoustic properties and density of polyurea at pressure up to 13.5 GPa through Brillouin scattering spectroscopy

Brillouin scattering was performed on an elastomeric polyurea, a significant technological polymer. Being widely used for impact modification, of particular interest is its response to extreme pressure conditions. We applied pressures up to 13.5 GPa utilizing a diamond anvil cell and measured the longitudinal and transverse sound velocities via Brillouin light scattering. From these data, the equation of state, the elastic moduli, and Poisson's ratio were obtained. By comparison with previous dilatometry measurements up to 1 GPa, we show how viscoelastic effects can be accounted for in order to obtain an accurate equation of state. Due to the extreme strain-rate hardening of vitrifying polyurea, the property changes associated with its solidification are more subtle in the high frequency Brillouin data than observed in conventional mechanical testing and dilatometry.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Measuring femtometer lattice displacements driven by free carrier diffusion in a polycrystalline semiconductor using time-resolved x-ray scattering

We show that time-resolved x-ray scattering can be applied to polycrystalline materials for the measurement of carrier diffusion. A polycrystalline indium antimonide sample is prepared by high-intensity ultrafast laser surface melting and re-solidification under vacuum to create randomly oriented grains with an average size of 13 nm. Two static diffraction rings are simultaneously observed on a gated pixel array detector. Their centroids move following lower-intensity laser excitation, and utilizing an in-situ angular calibration, the transient lattice spacing is determined with femtometer accuracy, thereby allowing the measurement of charge carrier dynamics. Compared to bulk calculations, we find that carrier diffusion slows by more than one order of magnitude. This result provides evidence for the formation of potential energy barriers at the grain boundaries and demonstrates the capability of time-resolved x-ray scattering to probe nanoscale charge transport in materials other than near-perfect crystals.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Melts of CrCoNi-based high-entropy alloys: Atomic diffusion and electronic/atomic structure from ab initio simulation

High-entropy alloys (HEAs) are an emerging class of advanced structural alloys under extensive research; yet, the properties of the liquid states of these materials, which are relevant to their processing, have been far less explored. In this work, we utilize ab initio molecular dynamics simulations to investigate the melt properties of a representative HEA—the Cantor alloy—and its derivatives: CrMnFeCoNi, CrFeCoNi, and CrCoNi. The atomic dynamics of these melts at various temperatures are investigated, specifically to analyze their electronic and atomic structures, including charge transfer, pair distribution functions, and short-range order. Results are compared with existing information for the liquids of metallic glasses, which also typically contain multiple principal elements, but retain the amorphous state under moderate to fast cooling rates. The present results provide insights into the structural and bonding factors favoring solidification to single-phase solid solutions in HEAs.

36 MATERIALS SCIENCE↗
DOE OSTI2019

The purported square ice in bilayer graphene is a nanoscale, monolayer object

The phase diagram of water is complex, and interfacial effects can stabilize unusual structures at the nanoscale. In this paper, we employ bond order accelerated molecular dynamics simulations to show that upon encapsulation within bilayer graphene, water can spontaneously adopt a two-dimensional (monomolecular) layer of “square ice” at ambient conditions, instead of an encapsulated water droplet. Free energy calculations show that this motif is thermodynamically stable up to diameters of approximately 15 nm due to enhanced hydrogen bonding and favorable binding to the graphene sheets. Entropic losses due to solidification and reduced graphene–graphene binding enthalpy are opposing thermodynamic forces that conspire to limit the maximum size, but modification of any of these thermodynamic factors should change the range of stability. Finally, simulated core-level spectroscopy reveals unambiguous orientation dependent signatures of square ice that should be discernable in experiments.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Molten Chloride Technology Pathway to Meet the U.S. DOE SunShot Initiative with Gen3 CSP

Third-generation (Gen3) concentrating solar power (CSP) technologies require a thermally stable and inexpensive fluid to be used for heat transfer and thermal energy storage. For Gen3 CSP plants, a molten salt composed of MgCl2 - KCl - NaCl is a new candidate, but it is familiar enough to plants with existing molten-salt systems. To determine the best composition for the heat-transfer fluid, we mixed nine different ratios of the salts, dry/purified them following strict protocols, and tested them with a differential scanning calorimeter and a thermogravimetric analyzer. Our results showed that the lowest melting and solidification temperature of 385 degrees C +/- 1 degrees C was determined for the eutectic composition 44.7 MgCl2 - 25.8 KCl - 29.4 NaCl (mol.%). We determined that improper handling and preparation of the salt mixture highly affects its thermal properties because of moisture absorption. Preparation standards - particularly the dehydration of hydrous MgCl2 - merit further development.

14 SOLAR ENERGY↗