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Results for “Phase Transformation”

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 253 records · Page 14
All sourcesNASA NTRSDOE OSTI
NASA NTRS1991

Phase transformations and the spectral reflectance of solid sulfur - Can metastable sulfur allotropes exist on Io?

Laboratory investigations have been conducted on the effects of variations in sulfur sample histories on their solid-state transformation rate and the corresponding spectral variation of freshly frozen sulfur. The temporal variations in question may be due to differences in the amount and type of metastable allotropes present in the sulfur after solidification, as well as to the physics of the phase-transformation process itself. The results obtained are pertinent to the physical behavior and spectral variation of such freshly solidified sulfur as may exist on the Jupiter moon Io; this would initially solidify into a glassy solid or monoclinic crystalline lattice, then approach ambient dayside temperatures. Laboratory results imply that the monoclinic or polymeric allotropes can in these circumstances be maintained, and will take years to convert to the stable orthorhombic crystalline form.

Moses, Julianne I.↗
NASA NTRS1994

Kinetics of phase transformation in glass forming systems

The objectives of this research were to (1) develop computer models for realistic simulations of nucleation and crystal growth in glasses, which would also have the flexibility to accomodate the different variables related to sample characteristics and experimental conditions, and (2) design and perform nucleation and crystallization experiments using calorimetric measurements, such as differential scanning calorimetry (DSC) and differential thermal analysis (DTA) to verify these models. The variables related to sample characteristics mentioned in (1) above include size of the glass particles, nucleating agents, and the relative concentration of the surface and internal nuclei. A change in any of these variables changes the mode of the transformation (crystallization) kinetics. A variation in experimental conditions includes isothermal and nonisothermal DSC/DTA measurements. This research would lead to develop improved, more realistic methods for analysis of the DSC/DTA peak profiles to determine the kinetic parameters for nucleation and crystal growth as well as to assess the relative merits and demerits of the thermoanalytical models presently used to study the phase transformation in glasses.

Ray, Chandra S.↗
DOE OSTI2020

Uranium-Titanium Alloys: The Effect of Ti Content on Phase Transformations, Microstructures, and Mechanical Properties

This report documents the findings of collaborative research done at Sandia National Laboratory and the Y-12 Development Division during the 1980s. Ongoing progress reports were presented at various U. S. Department of Energy (DOE) meetings throughout the 1980s, but the findings were never integrated into an inclusive final report. This report provides a summary of the key findings and implications of this research. This work was supported by the DOE National Nuclear Security Administration. The effect of titanium content on phase transformations, microstructures, and mechanical properties of uranium-titanium alloys has been investigated in alloys containing 0.3 wt.% to 2.0 wt.% Ti.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Collaborative or competitive phase transformation processes in an additively manufactured-maraging steel M300

Although retained austenite is present in minor amounts in as-built additively manufactured-maraging steels, its evolution during ageing is key to controlling the final microstructure. This study investigates the relationship between austenite reversion and precipitation evolution during ageing, particularly whether these processes compete or cooperate. Using Scanning/Transmission Electron Microscopy, Atom Probe Tomography and Thermo-Calc® PRISMA simulations, the presence and evolution of retained austenite, Ni 3 Ti and Fe 7 Mo 6 intermetallic phases was characterised across different ageing conditions. Experimental results revealed the presence of non-enriched austenite in the as-built condition. Nevertheless, with increasing ageing temperature, Ni and Mo segregation became evident. High-Resolution Transmission Electron Microscopy revealed that Ni 3 Ti precipitation primarily occurred within the martensitic matrix, while Fe 7 Mo 6 nucleated preferentially at the austenite-martensite interface. PRISMA simulations indicated early and rapid precipitation in solute-enriched areas (i.e., intercellular regions), with Ni 3 Ti forming prior to Fe 7 Mo 6 . Both experimental and simulation results suggested Ni diffusion controls retained austenite growth, while Ti and Mo drive Ni 3 Ti and Fe 7 Mo 6 precipitation, respectively. The evidence reported in this work supports a both collaborative and independent phase transformation mechanism, where austenite reversion and precipitation co-evolve.

42 ENGINEERING↗
DOE OSTI2019

EVPSC modeling of 316 stainless steel with and without observed phase transformation

The following is a description of modeling the constitutive behavior of the transforming and non-transforming 316 stainless steel measured in-situ at APS. Details of the elasto-viscoplastic self-consistent (EVPSC) polycrystal deformation model are described in the literature and only the salient information is given here. The EVPSC model is a general rate-sensitive self-consistent polycrystal plasticity model valid at arbitrary large deformations. The homogenizing method used is the self-consistent approach, originally proposed by Kröner, assuming that each grain is an ellipsoidal inclusion embedded in an infinite homogeneous equivalent medium (HEM), where the interaction is determined using Eshelby's solution. The hardening behavior is described by a Voce type strain hardening law, and the phase transformation uses a strain energy criterion for choosing a single daughter grain for each parent grain that reaches a critical value of the Von Mises equivalent strain. The orientation relationship between parent and daughter grains is determined based upon a rotation matrix that preserves an invariant line between the parent and daughter. The evolution law for the growth of the martensite phase is based on an empirical accumulated shear strain law proposed by Olson and Cohen. The single crystal elastic constants for the austenite phase are as reported for 316 stainless steel and the single crystal elastic constants of the martensite are taken from hardened SAE 1050 plain-carbon steel.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Uncertainty-aware molecular dynamics from Bayesian active learning for phase transformations and thermal transport in SiC

Abstract Machine learning interatomic force fields are promising for combining high computational efficiency and accuracy in modeling quantum interactions and simulating atomistic dynamics. Active learning methods have been recently developed to train force fields efficiently and automatically. Among them, Bayesian active learning utilizes principled uncertainty quantification to make data acquisition decisions. In this work, we present a general Bayesian active learning workflow, where the force field is constructed from a sparse Gaussian process regression model based on atomic cluster expansion descriptors. To circumvent the high computational cost of the sparse Gaussian process uncertainty calculation, we formulate a high-performance approximate mapping of the uncertainty and demonstrate a speedup of several orders of magnitude. We demonstrate the autonomous active learning workflow by training a Bayesian force field model for silicon carbide (SiC) polymorphs in only a few days of computer time and show that pressure-induced phase transformations are accurately captured. The resulting model exhibits close agreement with both ab initio calculations and experimental measurements, and outperforms existing empirical models on vibrational and thermal properties. The active learning workflow readily generalizes to a wide range of material systems and accelerates their computational understanding.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Selective electron beam melting of TiAl alloy: Microstructure evolution, phase transformation and microhardness

Highlights: cThe microstructure variation with build height of TiAl alloy was investigated. • The microhardness rapidly increased with build height increasing from 19 mm to 20 mm. • The mechanism of microstructure evolution was illuminated during SEBM. - Abstract: In this research, a Ti-47Al-2Cr-2Nb alloy was fabricated by selective electron beam melting (SEBM). The microstructures at different heights of SEBM-produced TiAl alloy sample and their contribution to microhardness were investigated. The results revealed that the last deposited layers displayed near lamellar structure with fine α{sub 2} lamellae. Lamellar colonies degraded at the height of 19 mm of the as-built sample, accompanied with the α{sub 2} lamellae decomposition and B2 precipitation along primary α{sub 2} lamellae. Meanwhile, equiaxed γ and blocky B2 phase dispersed on the colony boundaries. Lamellar colonies further degraded and colony boundaries further coarsen at the heights of 15 mm and 5 mm due to the in-situ heat treatment during SEBM process. Three kinds of phase transformations could be observed during SEBM process, which were α{sub 2} → γ, α{sub 2} → B2 and α{sub 2} + γ → B2. The microhardness exhibited slight fluctuation as the building height increased from 3 mm to 19 mm, and the microhardness rapidly increased with the building height increasing from 19 mm to 20 mm. Eventually, the mechanism of microstructure evolution during SEBM process was discussed in detail.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Mg and Al-induced phase transformation and stabilization of Ga 2 O 3 -based $\gamma$-phase spinels

Ga 2 O 3 films were deposited on (100) MgAl 2 O 4 spinel substrates at 550, 650, 750, and 850 °C using metal-organic chemical vapor deposition and investigated using x-ray diffraction and transmission electron microscopy. A phase-pure γ-Ga 2 O 3 -based material having an inverse spinel structure was formed at 850 °C; a mixture of the γ-phase and β-Ga 2 O 3 was detected in films grown at 750 °C. Only β-Ga 2 O 3 was determined in the films deposited at 650 and 550 °C. A β- to γ-phase transition occurred from the substrate/film interface during growth at 750 °C. The growth and stabilization of the γ-phase at the outset of film growth at 850 °C was affected by the substantial Mg and Al chemical interdiffusion from the MgAl 2 O 4 substrate observed in the energy-dispersive x-ray spectrum. Further, atomic-scale investigations via scanning transmission electron microscopy of the films grown at 750 and 850 °C revealed a strong tetrahedral site preference for Ga and an octahedral site preference for Mg and Al. It is postulated that the occupation of these atoms in these particular sites drives the β-Ga 2 O 3 to γ-phase transition and markedly enhances the thermal stability of the latter phase at elevated temperatures.

42 ENGINEERING↗
DOE OSTI2018

Precipitation behavior and phase transformation mechanism of super austenitic stainless steel S32654 during isothermal aging

Highlights: • σ phase formed following a sequence: intergranular, cellular and intragranular. • Cellular σ phase was firstly detected in S32654 aged at 800–900 °C. • Cellular σ growth is controlled by both intergranular and bulk Cr, Mo diffusion. • N-rich zone and Cr diffusion near σ phase induced the nucleation of Cr{sub 2}N. • Local needle-like σ transformed into Cr{sub 2}N and R phase through Cr, Mo diffusion. - Abstract: Thermodynamic calculation as well as microstructure characterization was carried out to investigate the precipitation behavior of super austenitic stainless steel S32654 aged at 700–1100 °C. The corresponding precipitation characteristics and transformation mechanisms of secondary phases were proposed. The results demonstrate that σ phases with various morphologies formed during isothermal aging, and followed a sequence of intergranular, cellular and intragranular with aging temperature. The intergranular σ phase nucleated much faster than the intragranular one due to the lower nucleation energy and higher element diffusion rate at grain boundary. Most intragranular σ phase grew into needle-like shape because of the great difference in mobility of the interfaces formed around it. The cellular σ phase was firstly detected in S32654, and its growth process was successively controlled by intergranular diffusion and then by bulk diffusion of Cr and Mo. Both Cr and Mo bulk diffusion and large needle-like σ phase resulted in the incompleteness of cellular precipitation of σ phase. Another new phenomenon was found that σ phase could induce the precipitation of Cr{sub 2}N ahead of cell boundary, between lamellar σ phases and at the σ/γ interfaces. Meanwhile, local parts of needle-like σ phase gradually transformed into Cr{sub 2}N and R phase through Cr and Mo diffusion from the former to the latters.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Ordering in liquid and its heredity impact on phase transformation of Mg-Al-Ca alloys

It is a long-sought goal to achieve desired mechanical properties through tailoring phase formation in alloys, especially for complicated multi-phase alloys. In fact, unveiling nucleation of competitive crystalline phases during solidification hinges on the nature of liquid. Here we employ ab initio molecular dynamics simulations (AIMD) to reveal liquid configuration of the Mg-Al-Ca alloys and explore its effect on the transformation of Ca-containing Laves phase from Al 2 Ca to Mg 2 Ca with increasing Ca/Al ratio (r Ca/Al ). There is structural similarity between liquid and crystalline phase in terms of the local arrangement environment, and the connection schemes of polyhedras. The forming signature of Mg 2 Ca, as hinted by the topological and chemical short-range order originating from liquid, ascends monotonically with increasing r Ca/Al . However, Al 2 Ca crystal-like order increase at first and then decrease at the crossover of r Ca/Al = 0.74, corresponding to experimental composition of phase transition from Al 2 Ca to Mg 2 Ca. The origin of phase transformation across different compositions lies in the dense packing of atomic configurations and preferential bonding of chemical species in both liquid and solid. The present finding provides a feasible scenario for manipulating phase formation to achieve high performance alloys by tailoring the crystal-like order in liquid.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Nanosecond Optically Induced Phase Transformation in Compressively Strained BiFeO 3 on LaAlO 3

Above-band-gap optical illumination of compressively strained BiFeO 3 induces a transient reversible transformation from a state of coexisting tilted tetragonal-like and rhombohedral-like phases to an untilted tetragonal-like phase. Time-resolved synchrotron x-ray diffraction reveals that the transformation is induced by an ultrafast optically induced lattice expansion that shifts the relative free energies of the tetragonal-like and rhombohedral-like phases. The transformation proceeds at interfaces between regions of the tetragonal-like phase and regions of a mixture of tilted phases, consistent with the motion of a phase boundary. The optically induced transformation demonstrates that there are new optically driven routes towards nanosecond-scale control of phase transformations in ferroelectrics and multiferroics.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Crystal Morphology and Phase Transformation of LiAlO 2 : Combined Experimental and First-Principles Studies

Layered α-LiAlO 2 matrix often suffers from detrimental structural and morphological changes during its lifetime, especially under H 2 containing fuel inlet environments of molten carbonate fuel cell systems. However, the underlying mechanism of this phase and morphological transformations has rarely been explored. In this study, transmission electron microscopy (TEM) techniques were used to determine the changes in structure and morphology of LiAlO 2 samples lifted by a focused ion beam (FIB). In accordance with X-ray diffraction (XRD) analysis, TEM images show that under H-rich conditions the rhombohedral α-LiAlO 2 transforms to tetragonal γ-LiAlO 2 phase with the appearance of the (101) terminated octahedral shaped γ-LiAlO 2 crystallites. We further support, and plausibly rationalize, the observed transformations using density functional theory (DFT) calculation. The DFT computed surface energies of γ-LiAlO 2 reveal that the {101} surface becomes the lowest energy surface upon H-adsorption, thus leading to formation of observed octahedral geometry. Contrary to stabilization of γ-LiAlO 2 surfaces upon H passivation, DFT revealed H-adsorption on α-LiAlO 2 surfaces to be energetically unfavorable. Furthermore, this contrasting behavior of α-LiAlO 2 and γ-LiAlO 2 under H-rich environments could be a potential driving force for the observed α-LiAlO 2 to γ-LiAlO 2 phase transformation.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Multiple contributions to electrostrain in high performance PbTiO 3 –Bi(Ni 1/2 Hf 1/2 )O 3 piezoceramics triggered by phase transformation

Piezoelectric materials, which mechanically respond to electrical stimulation and vice versa, typically exhibit high piezoelectricity at a morphotropic phase boundary (MPB). A thorough structural description is essential for understanding the origin of excellent piezoelectric properties. In this work, a comparative study of electric-field-driven structure and phase evolution has been conducted on MPB and non-MPB tetragonal composition in high performance system of PbTiO 3 -Bi(Ni 1/2 Hf 1/2 )O 3 piezoceramics using in situ high-energy synchrotron X-ray diffraction. The results show that the MPB composition exhibits electric-field-driven reversible tetragonal-to-monoclinic transformation, whereas non-MPB composition does not. Direct evidence shows that the extensive phase transformation triggers multiple contributions to enhancing piezoelectric performance. Namely, it not only enables the unit cell parameters of coexisting phases easily change with an external electric field, but also enhances the domain switching and lattice strain. These results offer a unique perspective on MPB associated piezoelectricity enhancement, and will be helpful for developing high performance piezoelectric materials.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Elucidating the phase transformations and grain growth behavior of O3-type sodium-ion layered oxide cathode materials during high temperature synthesis

Understanding the formation mechanism of layered oxide cathodes via solid-state synthesis is imperative to achieving controllability over their materials properties and electrochemical behaviors. In this work, we investigate the phase and microstructure evolution during the synthesis of NaNi 1/3 Fe 1/3 Mn 1/3 O 2 , a model sodium-ion layered oxide cathode, using a combination of imaging, diffraction, and spectroscopic techniques. We unravel the synthetic mechanistic pathways involved in the high-temperature calcination reaction, as well as elaborate the synthesis-microstructure-performance relationship of this material. The formation of the final layered oxide phase involves a gradual transformation through a sodiated oxyhydroxide intermediate. During the reaction, the precursor dehydration reaction dominates at 250–550 °C, while the major sodiation reaction occurs at 550–850 °C. Alongside multiple stages of phase transformations, the final grain structure formation occurs through the continuous growth of the (003) and (104) facets. During the reaction, Mn acts as the charge-compensating element and exhibits depth-dependent characteristics. When the sodiation reaction dominates over dehydration, the reaction intermediates undergo gradual electronic structure changes with increasing temperature, as indicated by the spectral features of TM3d-O2p hybrid states. Calcination duration is also a critical parameter governing the microstructure, surface reactivity, phase fraction distribution and electrochemical performance of the material. The optimal calcination duration was determined to be 18 hours at 850 °C under the conditions evaluated here. Calcination beyond this duration was found to be detrimental to electrochemical performance due to Na and O loss and heterogeneous sodium distribution throughout the particles. Our work sheds light on the complex crystallographic-chemical-microstructural evolution of sodium ion layered oxide cathodes and provides insight into precisely tuning material properties which are intimately linked to battery performances.

25 ENERGY STORAGE↗
DOE OSTI2025

Mn(II)-induced phase transformation of Mn(IV) oxide in seawater

Manganese (Mn) oxides are key components of oceanic and lacustrine Mn nodules and influence metal cycling through oxidation and adsorption processes. Layered Mn oxides (LMOs) are the most common minerals in these nodules and the immediate products of microbially mediated Mn(II) oxidation by O 2 . LMOs can transform into tunneled Mn oxides (TMOs), Mn oxyhydroxides (MnOOH), or Mn(II,III) phase (Mn 3 O 4 ). LMOs often concur with Mn(II) in the environment and the adsorption and oxidation of Mn(II) by LMOs can greatly promote the transformation of LMOs to those phases. However, the Mn(II)-promoted transformation of LMOs in seawater—rich in various cations (300 mM Na + , 10 mM K + , 50 mM Ca 2+ , and 10 mM Mg 2+ ) remains poorly understood. We examined the transformation of δ-MnO 2 in artificial seawater (pH 8.2) under anoxic conditions with the Mn(II)/MnO 2 ratio (r) ranging from 0.08 to 3.83. To assess the effect of ionic strength (IS), parallel experiments were conducted in a mixed 530 mM NaCl and 10 mM KCl solution (having seawater ionic strength but without Ca 2+ and Mg 2+ ) and in 100 mM NaCl solution as a control. At low r (0.08), δ-MnO 2 transformed into triclinic birnessite and a 4 × 4 TMO in 100 mM NaCl solution, which, however, was suppressed in seawater due to strong interactions of Ca 2+ /Mg 2+ with δ-MnO 2 . In the mixed 530 mM NaCl and 10 mM KCl solution (the same ionic strength as of seawater), the transformation occurred extensively but the products had lower crystallinity compared to in 100 mM NaCl solution. At the high Mn(II)/MnO 2 ratios (0.5 ≤ r ≤ 3.83), δ-MnO 2 transformed extensively into MnOOH phases and hausmannite (Mn 3 O 4 ) in 100 mM NaCl solution. The seawater suppressed the transformation, but the suppression became weaker with increasing Mn(II)/MnO 2 ratio. For example, the transformation was completely suppressed at r = 0.5 but essentially negligible at r = 3.83. The suppression at these high Mn(II)/MnO 2 ratios was mainly ascribed to the influence of Ca 2+ and Mg 2+ rather than of the high IS, and the weaker suppression at the higher Mn(II)/MnO 2 ratio suggests stronger competition of Mn(II) with Ca 2+ /Mg 2+ for interacting with δ-MnO 2 . Moreover, the composition and crystallinity of the transformation products (i.e., the relative abundance of MnOOH (α, β, and γ) and Mn 3 O 4 ) were influenced by both the high ionic strength and the presence of Ca 2+ and Mg 2+ . Therefore, even though Ca 2+ and Mg 2+ concentrations are much lower than Na + in seawater, their impacts on the transformation are dominant. Our study explains why MnOOH phases, hausmannite, and TMOs are less common than LMOs in oceanic environments, partially because seawater chemistry suppresses their formation. LMOs are the most reactive for metal adsorption and oxidation among all Mn oxides. Thus, the high stability of LMOs in an oceanic environment confers the high impacts of Mn oxides on metal cycling in the ocean.

Divalent manganese↗
DOE OSTI2020

Phase transformation in boron under shock compression

Using first-principles molecular dynamics, we calculated the equation of state and shock Hugoniot of various boron phases. Here, we find a large mismatch between Hugoniots based on existing knowledge of the equilibrium phase diagram and those measured by shock experiments, which could be reconciled if the α-B 12 /β phases are significantly over-pressurized in boron under shock compression. Our results also indicate that there exist an anomaly and negative Clapeyron slope along the melting curve of boron at 100 GPa and 1500–3000 K. These results enable an in-depth understanding of matter under shock compression, in particular the significance of compression-rate dependence of phase transitions and kinetic effects in experimental measurements.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
NASA NTRS1992

Phase transformations in SrAl2Si2O8 glass

Bulk glass of SrAl2Si2O8 composition crystallized at temperatures below 1000 C into hexacelsian, a hexagonal phase which undergoes a reversible, rapid transformation to an orthorhombic phase at 758 C, and at higher temperatures crystallized as celsian, a monoclinic phase. The glass transition temperature and crystallization onset temperature were determined to be 883 C and 1086 C, respectively, from DSC at a heating rate of 20 C/min. Thermal expansion of the various phases and density and bend strengths of cold isostatically pressed glass powder bars, sintered at various temperatures, were measured. The kinetics of the hexacelsian-to-celsian transformation for SrAl2Si2O8 were studied. Hexacelsian flakes were isothermally heat treated at temperatures from 1025-1200 C for various times. Avrami plots were determined by quantitatively measuring the amount of monoclinic celsian formed at various times using x ray diffraction. The Avrami constant was determined to be 1.1, suggesting a diffusionless, one dimensional transformation mechanism. The activation energy was determined from an Arrhenius plot of 1n k vs. 1/T to be 125 kilocal/mole. This value is consistent with a mechanism which transforms the layered hexacelsian structure to a three dimensional framework celsian structure and involves the breaking of Si-O bonds.

Drummond, Charles H., III↗