Search NASA⌕ Search

SEARCH · Search NASA

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.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 631 records · Page 35
All sourcesNASA NTRSDOE OSTI
DOE OSTI2019

Nanoscale self-healing mechanisms in shape memory ceramics

Abstract Shape memory ceramics, such as yttria-stabilized tetragonal zirconia (YSTZ), offer unique properties including ultra-high operating temperatures and high resistance to oxidation. However, they are susceptible to formation of defects during manufacturing and/or by mechanical deformation. To completely take advantage of their shape memory properties, it is necessary to fully understand the nano-structural evolution of defects under external stimuli. In this study, defect evolution behaviors in YSTZ nanopillars are investigated by atomistic simulations. Two characteristic orientations of $$\left[ {01\bar 1} \right]$$ 01 1 ¯ and [001] are selected to represent the dominant deformation mechanisms of phase transformation and dislocation migration, respectively. Volume expansion associated with the tetragonal to monoclinic phase transformation is observed to promote healing of crack and void. Atom stress analyses reveal stress concentrations along the newly formed monoclinic phase bands. A critical crack width is identified, less than which the crack can be fully closed in compression. For [001]-oriented YSTZ nanopillars, dislocation migration leads to formations of an amorphous phase, which also assist the crack and void closure process. The revealed crack/void healing mechanisms may provide a path for mitigating internal defects that influences the mechanical properties and deformation mechanisms of shape memory ceramics.

Zhang, Ning (ORCID:0000000240661837)↗
DOE OSTI2024

High-pressure phase transition in 3-D printed nanolamellar high-entropy alloy by imaging and simulation insights

Abstract We report on the high-resolution imaging and molecular dynamics simulations of a 3D-printed eutectic high-entropy alloy (EHEA) Ni 40 Co 20 Fe 10 Cr 10 Al 18 W 2 consisting of nanolamellar BCC and FCC phases. The direct lattice imaging of 3D-printed samples shows the Kurdjumov–Sachs (K–S) orientation relation {111} FCC parallel to {110} BCC planes in the dual-phase lamellae. Unlike traditional iron and steels, this alloy shows an irreversible BCC-to-FCC phase transformation under high pressures. The nanolamellar morphology is maintained after pressure cycling to 30 GPa, and nano-diffraction studies show both layers to be in the FCC phase. The chemical compositions of the dual-phase lamellae after pressure recovery remain unchanged, suggesting a diffusion-less BCC–FCC transformation in this EHEA. The lattice imaging of the pressure-recovered sample does not show any specific orientation relation between the two resulting FCC phases, indicating that many grain orientations are produced during the BCC–FCC phase transformation. Molecular dynamics simulations on phase transformation in a nanolamellar BCC/FCC in K–S orientation show that phase transformation from BCC to FCC is completed under high pressures, and the FCC phase is retained on decompression aided by the stable interfaces. Our work elucidates the irreversible phase transformation under static compression, providing an understanding of the orientation relationships in 3-D printed EHEA under high pressures.

3-D Printing↗
DOE OSTI2022

Crystal plasticity modeling of strain-induced martensitic transformations to predict strain rate and temperature sensitive behavior of 304 L steels: Applications to tension, compression, torsion, and impact

This paper advances crystallographically-based Olson-Cohen (direct γ → α’) and deformation mechanism (indirect γ→ε→α’) phase transformation models for predicting strain-induced austenite to martensite transformation. Here, the advanced transformation models enable predictions of not only strain-path sensitive, but also of strain-rate and temperature sensitive deformation of polycrystalline stainless steels (SSs). The deformation of constituent grains in SSs is modeled as a combination of anisotropic elasticity, crystallographic slip, and phase transformation, while the hardening is based on the evolution of dislocation density and explicit shifts in phase fractions. Such grain-scale deformation is implemented within the meso-scale elasto-plastic self-consistent (EPSC) homogenization model, which is coupled with the implicit finite element (FE) method to provide a constitutive response at each FE integration point for solving boundary value problems at the macro-scale. Parameters pertaining to the hardening and transformation models within FEEPSC are calibrated and validated on a suite of data including flow curves and phase fractions for monotonic compression, tension, and torsion as a function of strain-rate and temperature for wrought and additively manufactured (AM) SS304L. To illustrate the potential and accuracy of the integrated multi-level FE-EPSC simulation framework, geometry, mechanical response, phase fractions, and texture evolution are simulated during gas-gun impact deformation of a cylinder and quasi-static tension of a notched specimen made of AM SS304L. Details of the simulation framework, comparison between experimental and simulation results, and insights from the results are presented and discussed.

304L steels↗
DOE OSTI2018

Microsegregation Measurement: Methods and Applications

Microsegregation is chemical variation at the microstructural level in a solidified ingot, which can cause undesirable effects on material properties. Its formation is driven by different levels of solute elemental solubility between solid and liquid phases, resulting in solute buildup or depletion in the liquid phase during solidification. Characterization of microsegregation can be essential for understanding its formation and removing it through heat treatment. Measurement is frequently carried out via scanning electron microscope (SEM), though metallographic analysis, mechanical testing and optical radiography have been used. SEM analysis can consist of point measurements on single dendrites, line scans across or along dendrites, elemental maps, and extended line-based or grid-based analyses extending over many dendrites, which can generate large amounts of data. Line-based or grid-based data can be analyzed using scatterplots of element concentrations to visualize segregation behavior and sorting/ranking of data by a single element or a combination of elements, to give compositional profiles as a function of extent of solidification. For both qualitative and quantitative analyses, effects of phase transformations and secondary phases need to be considered as well as sorting of measurement errors when generating compositional profiles. Applications of analysis include determination of partition ratios, examination of solute–element interactions, determination of effects of cooling rate and back diffusion, applications to freckling studies, validations of modeling, and development of heat-treatment profiles.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Roles of surface Mn(III) and MgOH + /CaOH + complexation in photochemically assisted Mn 2+ oxidation and Todorokite formation

Todorokite, a Mn oxide phase commonly found in seafloor Mn nodules, is one of the most abundant Mn oxides in nature and has wide applications in environmental engineering, including heavy metal remediation, contaminant adsorption for water purification, and catalysis for organic pollutant degradation. However, traditional abiotic formation of todorokite reported to date typically requires high temperature, pressure, and pH outside circumneutral conditions, or involves several days of phase transformations from precursor phases. In this study, we demonstrate rapid todorokite formation within 6 h via photochemically assisted oxidation of Mn 2+ (aq) in the presence of Mg 2+ or Ca 2+ , notably at ambient temperature and pressure. The concentrations of aqueous MgOH + and CaOH + , as well as the pH (initial 9 or 6), had significant collective influences on the crystalline phases of the formed Mn oxide solids. The proportions of the Mn oxidation states (IV, III, and II) and the extent of Mn(III) surface complexation with MgOH + or CaOH + controlled the crystalline phases (todorokite, feitknechtite, and birnessite) and the extent of cation incorporation into the resulting Mn oxide solids. Specifically, the oxidation state of Mn influenced the crystallization of different Mn oxide phases, and the formed Mn(III) complexed with MgOH + or CaOH + facilitated the formation of todorokite, along with cation incorporation. X-ray pair distribution function analysis revealed that incorporated Mg species were located at the inside corner sites of the 5 × 3 MnO 6 octahedral tunnel structure of todorokite, suggesting complexation between MgOH + and Mn(III) at the corners of the tunnel. Furthermore, this study provides novel insights into the elusive formation of todorokite and its potential for applications in advanced water treatment and environmental remediation.

Abiotic Mn2+ oxidation↗
DOE OSTI2017

Constitutive modeling of the dynamic-tensile-extrusion test of PTFE

Use of polymers in defense, aerospace and industrial applications under extreme loading conditions makes prediction of the behavior of these materials very important. Crucial to this is knowledge of the physical damage response in association with phase transformations during loading and the ability to predict this via multi-phase simulation accounting for thermodynamical non-equilibrium and strain rate sensitivity. The current work analyzes Dynamic-Tensile-Extrusion (Dyn-Ten-Ext) experiments on polytetrafluoroethylene (PTFE). In particular, the phase transition during loading and subsequent tension are analyzed using a two-phase rate sensitive material model implemented in the CTH hydrocode. The calculations are compared with experimental high-speed photography. Deformation patterns and their link with changing loading modes are analyzed numerically and correlated to the test observations. It is concluded that the phase transformation is not as critical to the response of PTFE under Dyn-Ten-Ext loading as it is during the Taylor rod impact testing.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Modulation of dielectric properties of hexagonal/cubic boron nitride composites

In this work, we synthesized mixed-phase hexagonal-boron nitride (h-BN)/cubic-BN (c-BN) composites with varying ratios and investigated their frequency and temperature-dependent dielectric properties. As the ratio of c-BN increased, we observed a corresponding increase in the dielectric constant of the composites. Furthermore, we used spark-plasma sintering (SPS) to treat the mixed-phase composite, which resulted in a phase transformation from mixed phase to pure h-BN phase. Remarkably, the composite exhibited an increase in dielectric constant after the SPS process, which can be attributed to the densification of the composite and the enhancement in grain size. Our approach presents a promising strategy for effectively modulating the dielectric properties of BN, which is crucial for advanced electronics.

Physics↗
DOE OSTI2020

Ab Initio Study of the Effect of Mono-Vacancies on the Metastability of Ga-Stabilized δ-Pu

Most experimental studies on metallic Pu are on the room temperature monoclinic α-phase or the fcc Ga stabilized δ-phase. Stabilized δ-phase Pu-Ga alloys are metastable and exhibit a martensitic phase transformation to α’-phase at low temperatures, or applied shear, with concentrations lower than three atomic percent Ga. By using first principles, we explore the metastability of δ-phase by investigating the structural and electronic behavior induced by Ga alloying and by a mono-vacancy point defect. We find that a site substitutional Ga induces a tetragonal distortion in the lattice affected by hybridization of Ga 4p and Pu 6d states. With the addition of a mono-vacancy, a monoclinic or tetragonal distortion forms locally (dependent on its distance from Ga), and decoupling of the Pu 5f and 6d states and broadening of the 6d states occurs. This response enables hybridization of Pu 6d with the Ga 4p states affecting the mono-vacancy formation energy. Thus, stabilization of the fcc lattice correlates with hybridization of Pu 6d states with Ga 4p states, and this becomes more evident in the presence of a mono-vacancy.

38 RADIATION CHEMISTRY, RADIOCHEMISTRY, AND NUCLEA↗
DOE OSTI2025

Multi-scale modeling of wastage layer formation in metallic fuel cladding

Fuel-cladding chemical interaction (FCCI) is a major concern for U-Zr metallic fuels' performance, primarily due to the formation of a brittle layer (wastage) in the cladding. This brittle layer, resulting from intermetallic compounds between cladding constituents Fe, Cr, and lanthanide fission products, significantly impacts the cladding's mechanical integrity. Recent efforts focus on developing a mechanistic modeling framework to understand lanthanide production, transport to the fuel-cladding interface, and phase transformation to intermetallic phases. A multi-scale computational approach has been used to calculate lanthanide transport rates, with atomistic calculations determining Nd diffusivities through the solid fuel matrix and along pore surfaces. These diffusivities inform a mesoscale model to determine an effective diffusion coefficient, accounting for porosity and infiltration with bond sodium. This effective diffusivity is used in engineering-scale simulations via the BISON fuel performance code, which has been validated against EBR-II and FFTF reactor experiments.

11 - NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2016

Unraveling the Age Hardening Response in U-Nb Alloys

Complicating factors that have stymied understanding of uranium-niobium’s aging response are briefly reviewed, including (1) niobium inhomogeneity, (2) machining damage effects on tensile properties, (3) early-time transients of ductility increase, and (4) the variety of phase transformations. A simple Logistic-Arrhenius model was applied to predict yield and ultimate tensile strengths and tensile elongation of U-4Nb as a function of thermal age. Lastly, fits to each model yielded an apparent activation energy that was compared with phase transformation mechanisms.

36 MATERIALS SCIENCE↗
DOE OSTI2018

A facile pathway to prepare VO{sub 2} and V{sub 2}O{sub 3} powders via a carbothermal reduction process

The development of a highly efficient and low cost production process is instrumental for the wide application of VO{sub 2} and V{sub 2}O{sub 3}. In this paper, VO{sub 2} and V{sub 2}O{sub 3} powders are successfully prepared via carbothermal reduction of V{sub 2}O{sub 5} in a flowing Ar atmosphere at 973 K and 1423 K, respectively. Compared with the traditional methods, graphite is used as a reductant, which resulted in a low cost. The experimental results show that a high temperature assists in improving the kinetic rate. The phase transformation during the low-temperature reduction process between 948 K and 973 K occurs in the following sequential order: V{sub 2}O{sub 5}→V{sub 6}O{sub 13}→VO{sub 2}. When the temperature is 1423 K or 1473 K, the phase transformation can be described as follows: V{sub 2}O{sub 5}→V{sub 3}O{sub 5}→V{sub 2}O{sub 3}. Owing to the shrink of the volume, the as-prepared VO{sub 2} and V{sub 2}O{sub 3} particles could not maintain the original micro-morphology of V{sub 2}O{sub 5}. Relative to VO{sub 2}, the as-prepared V{sub 2}O{sub 3} particles are porous. It is also concluded that the V{sub 2}O{sub 3} can be easily oxidized to VO{sub 2}, when the partial pressure of O{sub 2} is in the range of 10{sup −8} atm to 10{sup −1} atm. Therefore, a reasonable reaction temperature and reaction time are important to prepare high purity V{sub 2}O{sub 3}.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

TOF MS Investigation of Nickel Oxide CVD

NiO layers were deposited by metal-organic chemical vapor deposition using bis-(ethylcyclopentadienyl) nickel (EtCp){sub 2}Ni and oxygen or ozone. As a continuation of kinetic study of NiO MOCVD the gas-phase, transformations of (EtCp){sub 2}Ni were studied in the temperature range of 380–830 K. Time of reactions corresponding to the residence time of the gas stream in hot zone of the reactor was about 0.1 s under conditions studied. The interaction of (EtCp){sub 2}Ni with oxygen started at 450 K and its conversion rate reached the maximum at 700 K. The interaction of (EtCp){sub 2}Ni with ozone started at 400 K and its conversion rate reached the maximum at 600 K. Transformations of the gas phase with the temperature in the reaction zone were studied, the model reaction schemes illustrating (EtCp){sub 2}Ni transformations in the reaction systems containing oxygen and ozone have developed. In the reaction system (EtCp){sub 2}Ni–O{sub 2}–Ar the main gas-phase products at 380–500 K were CO, CO{sub 2}, HCO, C{sub 2}H{sub 5}OH, CpCOOH, and CpO. Formation of the C{sub 2}H{sub 2}O, C{sub 3}H{sub 4}O, and C{sub 5}H{sub 8}O was found at 630–830 K. The same gas-phase species, (C{sub 4}H{sub 3}O){sub 2}Ni and dialdehydes was formed in the reaction system (EtCp){sub 2}Ni–O{sub 3}–O{sub 2}–Ar. .

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Fundamental Insight into Zr Modification of Li- and Mn-Rich Cathodes: Combined Transmission Eelctron Microscopy and Electrochemical Impedance Spectroscopy Study

Zr surface modification was performed on Li- and Mn-rich (LMR) cathode material (Li1.2Ni0.13Co0.13Mn0.54O2) to suppress the detrimental phase transformation upon cycling and enhance its electrochemical performance. Using a wet chemical method, a Zr-based rock-salt layer with a thickness of 1~2 nm was formed on the surface of the LMR. Controlled studies reveal that the Zr-based layers can effectively suppress the phase transformation, leading to an enhanced long-term cycling performance and rate capability. At C/3, the Zr-modified LMR material was found to deliver an initial discharge specific capacity of 211 mAh g-1, and maintain a capacity of 182 mAh g-1 after 100 cycles. Even at high rates of 1, 2 and 3C, it still displays high discharge capacities of 178, 156 and 143 mAh g-1, respectively. This work reports for the first time the detailed mechanisms at atomic scale of how Zr modification can be effectively used to tailor the surface microstructures of high energy cathode materials to improve their long-term stability and cycle life.

Li- and Mn-rich cathode, Li1.2Ni0.13Co0.13Mn0.54O2↗
DOE OSTI2019

Graphite to diamond transformation under shock compression: Role of orientational order

To gain insight into the role of orientational order on the shock-induced graphite to diamond phase transformation, three pyrolytic graphite types having different orientational orders were shock-compressed along the average c -axis to peak stresses between 35 and 69 GPa. The materials studied were ZYB-grade highly oriented pyrolytic graphite (HOPG), ZYH-grade HOPG, and as-deposited pyrolytic graphite (PG) having mosaic spreads of 0.8° ± 0.2°, 3.5° ± 1.5°, and ~45°, respectively. Wave profiles, obtained using laser interferometry, show a multiple-wave structure with a distinct, rapid (<10 ns) rise to the high-pressure phase for each graphite type. Multiple-wave profiles, first observed in this study for the less ordered ZYH-grade HOPG and PG samples, show that somewhat poorly oriented pyrolytic graphites also undergo a well-defined phase transformation. Previously, rapid transformation was reported for ZYB-grade but not ZYH-grade HOPG. The measured wave profiles for both HOPG grades are very similar and both grades show a ~22 GPa transformation stress. In contrast, the PG wave profiles are quite different and show a ~46 GPa transformation stress. The continuum results (stress-density states) presented here cannot distinguish between the different high-pressure phases [hexagonal diamond (HD) or cubic diamond] reported in recent x-ray studies. Because ZYB-grade HOPG was recently shown to transform to HD and due to the similar peak states for both HOPG grades, it seems likely that ZYH-grade also transforms into HD. In conclusion, the very different shock responses of PG and HOPG suggest different transformation mechanisms for PG and HOPG, but the high-pressure PG phase remains unclear in the present work.

58 GEOSCIENCES↗
DOE OSTI2021

Thermal Stability and Lattice Strain Evolution of High-Nb-Containing TiAl Alloy under Low-Cycle-Fatigue Loading

The micromechanical behavior and the effect of temperature on the micromechanical mechanism of high-Nb-containing TiAl alloy during low-cycle fatigue still remain uncertain. Herein, in situ and ex situ synchrotron-based high-energy X-ray (HEXRD) experiment results reveal that the gamma and omega(o) phases suffer compressive lattice strains but the lattice strain in the alpha(2) phase evolves from tensile to compressive during low-cycle fatigue at 900 degrees C. In addition, the three phases suffer compressive lattice strains during cooling to room temperature, which could result in larger compressive lattice strains in gamma and omega(o) phases and the change of the lattice strain state in the alpha(2) phase. The peak-broadening results show gamma recrystallization is dominant in the interrupted low-cycle-fatigue samples, whereas inhomogeneous deformation occurs in the failed low-cycle-fatigue samples. The performed synchrotron diffraction experiments offer a deeper insight into the phase transformations and micromechanism of TiAl alloy during low-cycle fatigue.

cyclic stress-strain behavior↗
DOE OSTI2018

Pre-processing related recrystallization behavior in β annealing of a near-β Ti-5Al-5Mo-5V-3Cr-1Zr titanium alloy

Highlights: • Recrystallization behavior of Ti-55531 alloy in β annealing under four typical pre-processing conditions is studied. • Recrystallization nucleation mode is governed by the size and distribution of substructures. • The sequence of phase transformation and β recrystallization significantly affects recrystallized grain size. • The uniform substructures produce a higher fraction of low angle grain boundaries after full recrystallization. - Abstract: Recrystallization behavior of Ti-55531 alloy in β annealing is explored for grain refinement during primary hot working. Four typical pre-processing conditions were adopted to characterize the diverse processing routes of primary hot working: deformation and annealing in the two-phase region, deformation of globularized structure in two-phase region, deformation of lamellar structure in two-phase region and deformation in single β region. The microstructure evolution in β annealing was quantified by Optical Microscopy (OM), Electron Backscattered Diffraction (EBSD), X-ray diffraction (XRD) and quantitative metallography. The results indicate that the nucleation mode of static recrystallization is determined by the size and distribution of substructures prior to annealing which is related to pre-processing route. The uniform substructures in as-deformed globularized structure produce a higher fraction of low angle grain boundaries after full recrystallization. Fine β recrystallized structure can be obtained when β recrystallization is accompanied by α-to-β phase transformation.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Structural and Magnetic Properties of Ordered/Disordered Sendust Powders Prepared by Mechanical Alloying

In this study, Sendust alloy (Fe{sub 85}Si{sub 9.6}Al{sub 5.4}) was fabricated by mechanical alloying process, and its crystal structure and magnetic properties were investigated. Sendust alloy has excellent soft magnetic properties and is widely used in different applications. Raw materials (Fe, Si, and Al) were milled by a planetary ball mill at different milling times, from 5 to 20 h. The resulted alloys were annealed at 1373 and 1473 K for 2 h, and the effect of annealing temperature was investigated on the magnetic properties. Crystal structure of the samples was investigated by X-ray diffractometery method. The results indicated that saturation magnetization of 10-h milled sample was at maximum, and it decreased both by increasing milling time and by annealing. These variations were discussed based on phase transformation from ordered to disordered phases and/or vice versa. The maximum saturation magnetization obtained, 156 emu/g, was comparable by those reported for Sendust alloy, which was prepared by other methods. Magnetization–temperature curves of the alloys and annealed ones were recorded by a Faraday balance. Two clear drops were observed on each M-T curve of the as-milled samples that displayed the existence of two different magnetic phases. On the other hand, there is just one drop on the M-T curves of the annealed samples, indicating that these samples were in a less disordered phase.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗