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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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NASA NTRS1994

Extended and compact X-ray emission in powerful radio galaxies

We report ROSAT X-ray observations of two powerful radio galaxies. 3C 280 provides evidence for a mixture of unresolved and extended emission, with the latter produced by hot plasma of insufficient pressure to confine the radio lobes and insufficient density for a cooling flow to have begun. 3C 220.3 gives only an X-ray upper limit, but one consistent with our interpretation of the X-ray emission from powerful radio-loud active galactic nuclei (AGNs) in terms of obscured and unobscured components.

Worrall, D. M.↗
NASA NTRS1994

A study of ultraviolet absorption lines through the complete Galactic halo by the analysis of HST faint object spectrograph spectra of active Galactic nuclei, 1

The ultraviolet (1150 - 2850 A) spectra of a number of active galactic nuclei (AGNs) observed with the Hubble Space Telescope (HST) Faint Object Spectrograph (FOS) have been used to study the properties of the Galactic halo. The objects that served as probes are 3C 273, PKS 0454-220, Pg 1211+143, CSO 251, Ton 951, and PG 1351+640. The equivalent widths of certain interstellar ions have been measured, with special attention paid to the C IV/C II and Si IV/Si II ratios. These ratios have been intercompared, and the highest values are found in the direction of 3C 273, where C IV/C II = 1.2 and Si IV/Si II greater than 1. These high ratios may be due to a nearby supernova remnant, rather than to ionized gas higher up in the Galactic halo. Our data give some support to the notion that QSO metal-line systems may arise from intervening galaxies which contain high supernova rates, galactic fountains, and turbulent mixing layers.

Burks, Geoffrey S.↗
NASA NTRS1995

Hubble Space Telescope Images of Nearby Luminous Quasars: Results for Eight Quasars and Tests of the Detection Sensitivity - 2

Observations with the Wide-Field Camera of the Hubble Space Telescope (HST) are presented for eight intrinsically luminous quasars with redshifts between 0.16 and 0.29. These observations, when combined with a similar HST study of the quasar PKS 2349-014, show that luminous nearby quasars exist in a variety of environments. Seven companion galaxies brighter than M(V) = 16.5 (H(sub 0) = 100 km s(sup -1) Mpc(sup -1), Omega(sub 0) = 1.0) lie within a projected distance of 25 kpc of the quasars; three of the companions are located closer than 3'' (6 kpc projected distance) from the quasars, well within the volume that would be enclosed by a typical L* host galaxy. The observed association of quasars and companion galaxies is statistically significant and may he an important element in the luminous-quasar phenomenon. Apparent host galaxies are detected for three of the quasars: PG 1116+215, 3C 273, and PG 1444+407; the hosts have an average absolute magnitude of about 0.6 mag brighter than L*. The agreement between the previously published major-axis directions in ground-based images and in the present HST images of 3C 273 and PG 1444+407 constitutes important evidence supporting the reality of these candidate host galaxies. Upper limits are placed on the visual-band brightnesses of representative galactic hosts for all the quasars. These limits are established by placing galaxy images obtained with HST underneath the quasars and measuring at what faintness level the known galaxies are detected. On average, the HST spirals would have been detected if they were as faint as 1 mag below L*, and the early-type galaxies could have been detected down to a brightness level of about L*, where L* is the Schechter characteristic luminosity of field galaxies. Smooth, featureless galaxy models (exponential disks or de Vaucouleurs profiles) are fitted to the residual light after a best-fitting point source is subtracted from the quasar images. The results show that smooth spiral galaxies brighter than, on average, about L*, would have been detected. These upper limits, or possible detections, are consistent with, for example, the eight luminous quasars studied in this paper, occurring in host galaxies that have a Shechter luminosity function with a lower cutoff in the range 0.01-0.1 L*. Tests are performed to determine if our failure to detect, in some cases, luminous host galaxies could be an artifact caused by our analysis procedures. These tests include comparing the measured point-spread function (PSF) for our HST observations with the PSFs used in previous ground-based studies of host galaxies, measuring the fluctuations in the sky signals that were subtracted from the quasar images, evaluating empirically the effects of using different stellar PSFs in the analysis, carrying out the subtraction of the stellar (nuclear) source in different ways, creating and analyzing artificial active galactic nuclei (AGNs) with known surface brightnesses, and fitting the observed quasar light to an analytic model that includes a host galaxy.

Bahcall, John N.↗
NASA NTRS1997

Multiwavelength Blazar Studies

The present report concerns the following projects 'High energy emission from hot accretion disks from active galactic nuclei'; 'OSSE spectral timing and monitoring observations of Cygnus X- 1'; 'OSSE observations of blazars'; and 'Search for correlated time variations of bright EGRET blazars'. The most favored model for the central engine inAGNs is the supermassive black hole hypothesis (Rees 1978). The observed luminosities range from approx. 10 logical and 44-45 erg /s for objects such as Mk 421 and Seyferts to as much as approx. 10logical and 47 for powerful QSOs such as 3C 273 and 3C 279, a large fraction of the observed bolometric luminosity being, in all likelihood, beamed. As such, these objects are strong X-ray emitters (cf. Makino et al. 1987) and often undergo gamma-ray flaring detected at the EGRET range (Hartman et al. 1992) and in the case of one of two known nearby BL Lacs, Mk 421, at TeV energies as well (Punch et al. 1992, Macomb et al. 1995). Previous campaigns emphasizing radio through X-ray and even gamma-ray observations have generally found that the multiwavelength spectrum is adequately fit by a standard synchrotron self-Compton (SSC-cf. Jones et al. 1974) model of a relativistic jet (e.g. Makino et al. 1987 and Macomb et al. 1995 for the BL Lac object Mk 421) or inhomogeneous relativistic jet (Mufson et al 1990). It also was examined the gamma-gamma transperency constraints inblazars.

Kafatos, Menas↗
NASA NTRS1997

Multifrequency Studies

The present report concerns the following projects 'Multifrequency observations of the BL Lacertae object MK 421"; "EUVE and multifrequency observations of the quasars 3C 273 and 3C 279"; "EUVE observations of the symbiotic star RAquarii"; and "Morphology of the local HI void and ionization of He in the local ISM". The common theme of the above projects is UV observations and analysis of a wide range of astrophysical classes or objects, from the diffuse interstellar medium, to symbiotic stars (R Aqr), to active galactic nuclei (AGNs). Understanding the local (primarily neutral hydrogen or HI) interstellar medium is of great importance to detecting nearby EUV sources. Specifically, the ionization of helium is strongly influencing the extinction in the extreme ultraviolet (EUV) and the optical depth can vary almost by an order of magnitude depending on the degree of ionization of various species including neutral helium Vel ) and singly ionized helium (Hell). The local interstellar medium (ISM) contains a giant void. Understanding the local ISM and the extent of the HI void engulfing the Sun is crucial to our understanding of the EUV emission from nearby strongly EUV-emitting stars like R Aqr, as well as subtracting local absorption effects from absorption effects in distant sources themselves such as quasars, BL Lacertae objects and AGNs in general. A compilation of known properties of the local ISM with appropriate references has been performed. R Aqr is a symbiotic stellar system comprised of a mass-losing approx.1 - 2 solar mass Mira- like long period variable with a 387 day period and a approx. solar mass,) hot companion/accretion disk that is believed to give rise to the symmetrical jet seen at UV, optical radio wavelengths. Recent observations of R Aqr have been conducted at continuum wavelengths with the Very Large Array (VLA) at 7 mm that have resolved the stellar components in RAqr and a geometrical distance of approx. 200pc has been obtained, confirming that R Aqr is the nearest symbiotic star system to the Sun. The status of the local ISM is, therefore, of great importance in attempting to understand the UV properties of the hot companion in R Aqr and its prominent jet. Here a multifrequency approach is required from both ground observations (e.g. with the VLA) as well as from space-borne observations (with the International Ultraviolet Explorer-IUE; and the Hubble Space Telescope-HST). The R Aqr jet has been observed with the VLA B-configuration at two epochs separated by approx. 13.2 yr. Comparison of the resulting 6 cm continuum images show that the radio jet has undergone a lateral counterclockwise rotation of approx. 6 deg - 12 deg on the plane of the sky. Comparison of the most recent radio image with a nearly contemporaneous HSTIFOC ultraviolet image at approx 2330 A suggests that the UV emission lies along the leading edge of the rotating radio jet.

Kafatos, Menas↗
NASA NTRS1998

Warm Absorbers in the ROSAT Spectra of Quasars

We present two ROSAT PSPC observations of the radio-loud, lobe-dominated quasar 3C 351, which shows an 'ionized absorber' in its X-ray spectrum. The factor 1.7 change in flux in the approx. 2 years between the observations allows a test of of models for this ionized absorber. The absorption feature at approx. 0.7 keV (quasar frame) is present in both spectra but with a lower optical depth when the source intensity - and hence the ionizing flux at the absorber - is higher, in accordance with a simple, single-zone, equilibrium photoionization model. Detailed modeling confirms this agreement quantitatively. The maximum response time of 2 years allows us to limit the gas density: n(sub e) greater than 2 x 10(exp 4)cm(exp -3); and the distance of the ionized gas from the central source R less than 19 pc. This produces a strong test for a photoionized absorber in 3C 351: a factor 2 flux change in approx. 1 week in this source must show non-equilibrium effects in the ionized absorber.

Fiore, Fabrizio↗
NASA NTRS1998

Design, Operation, and Modeling of a Vertical APCVD Reactor for Silicon Carbide Film Growth

An atmospheric pressure chemical vapor deposition (APCVD) reactor utilizing a unique vertical geometry which enables 3C-SiC films to be grown on two, 4-inch diameter Si wafers has been constructed. Contrary to expectations, 3C-SiC films grown in this reactor are thickest at the downstream end of the substrates. To better understand the reason for the thickness distribution on the wafers, an axisymmetric finite-element model of the gas flow in the reactor was constructed. The model uses the ANSYS53 Flowtran package and includes compressible and temperature-dependent fluid properties in laminar or turbulent flow. It does not include reaction chemistry or unsteady flow. The ANSYS53 results predict that the cool, inlet fluid falls through the inlet pipe and the warm, diffuser region like a jet. This jet impinges on top of the susceptor and gets diverted to the reactor side walls, where it flows to the bottom of the reactor, turns, and slowly rises along the face of the susceptor. This may explain why the SiC films are thickest at the downstream side of the wafers, as gas containing fresh reactants first passes over this region. Modeling results are presented for both one atmosphere and one half atmosphere reactor pressure.

DeAnna, Russell G.↗
NASA NTRS1999

Radio-Optical Alignments in a Low Radio Luminosity Sample

We present an optically-based study of the alignment between the radio axes and the optical major axes of eight z approximately 0.7 radio galaxies in a 7C sample. The radio galaxies in this sample are approximately 20-times less radio luminous than 3C galaxies at the same redshift, and are significantly less radio-luminous than any other well-defined samples studied to date. Using Nordic Optical Telescope images taken in good seeing conditions at rest-frame wavelengths just longward of the 4000A break, we find a statistically significant alignment effect in the 7C sample. Furthermore, in two cases where the aligned components are well separated from the host we have been able to confirm spectroscopically that they are indeed at the same redshift as the radio galaxy. However, a quantitative analysis of the alignment in this sample and in a corresponding 3C sample from HST (Hubble Space Telescope) archival data indicates that the percentage of aligned flux may be lower and of smaller spatial scale in the 7C sample. Our study suggests that alignments on the 50-kpc scale are probably closely related to the radio luminosity, whereas those on the 15 kpc scale are not. We discuss these results in the context of popular models for the alignment effect.

Lacy, Mark↗
NASA NTRS1999

Modeling of SiC Lateral Resonant Devices Over a Broad Temperature Range

Finite-element analysis (FEA) modal results of 3C-SiC lateral resonant devices anchored to a Si substrate are presented as resonant frequency versus temperature. The suspended elements are etched from a 2 micron, 3C-SiC film grown at 1600 K on a 500 micron-thick, Si substrate. The analysis includes, temperature-dependent properties, shape change due to volume expansion with temperature, and thermal stress caused by differential thermal expansion of different materials. Two designs are considered: type I has anchor locations close to the geometric centroid and a small shuttle; type 11 has a large shuttle with anchors far from the centroid, The resonant frequency decreases approximately 3.5% over a 1000 K temperature increase for the type-I device, and behaves according to theory. The resonant frequency of the type-11 device decreases by 2% over the first 400 K, then rises slightly over the remaining 600 K. This device deviates from theory because of the high thermal stress induced in the beams. The thermal stress is caused by the differential thermal expansion of the suspended element relative to the substrate. The results show that the device geometry must be properly chosen if the resonant frequency of that device will be used to calculate the temperature coefficient of Young's modulus. These results apply only to resonators of one material on a substrate of a different material.

DeAnna, Russell G.↗
NASA NTRS2004

Frequency-Dependent Time Lags in Three Blazars

The work completed includes the analysis of observations obtained during Cycle 6 of the Rossi X-ray Timing Explorer (RXTE). The project is part of a longer-term, continuing program to study the X-ray emission process in blazars in collaboration with Dr. Ian McHardy (U. of Southampton, UK). The goals of the program are to study the X-ray emission mechanism in blazars and the relation of the X-ray emission to changes in the relativistic jet. The program includes contemporaneous brightness and linear polarization monitoring at radio and optical wavelengths, total and polarized intensity imaging at 43 GHz with a resolution of 0.1 milliarcseconds with the VLBA, and well-sampled X-ray light curves obtained from a series of approved RXTE programs. During RXTE cycle 6, the project was awarded RXTE time to monitor three quasars: PKS 1510-089 two times per week, and 3C 273 and 3C 279 three times per week each. The X-ray data, including those from earlier cycles, were compared with radio measurements obtained 2-3 times per week in the centimeter-wave band with the Michigan radio telescope, monthly imaging observations with the VLBA at 43 GHz, and optical observations obtained at several telescopes around the world. Since it is the long-term X-ray light curve that is of greatest value, this report includes discussion of all the data collected through August 2003 rather than just the Cycle 6 data.

Marscher, Alan P.↗
NASA NTRS2004

Growth and Characterization of Silicon-Carbide Hetero-Polytype Structures

Films discussed in this repok were deposited at NASA Glenn Research Center on on- orientation 4H-Sic substrates. Substrates were patterned in order to form mesa structures of different shapes and sizes. The nominal thickness of the 3C-Sic film was approximately 10 microns although the thickness could change considerably between defect-free and defected mesa structures. The primary objective was to identify defect structures in defect-free mesas and in particular determine the degree and mode of strain relaxation in 3C films with thickness exceeding critical thickness.

Skowronski, Marek↗
NASA NTRS2006

Comparison of Orbital Parameters for GEO Debris Predicted by LEGEND and Observed by MODEST: Can Sources of Orbital Debris be Identified?

Since 2002 the National Aeronautics and Space Administration (NASA) has carried out an optical survey of the debris environment in the geosynchronous Earth-orbit (GEO) region with the Michigan Orbital Debris Survey Telescope (MODEST) in Chile. The survey coverage has been similar for 4 of the 5 years allowing us to follow the orbital evolution of Correlated Targets (CTs), both controlled and un-controlled objects, and Un-Correlated Targets (UCTs). Under gravitational perturbations the distributions of uncontrolled objects, both CTs and UCTs, in GEO orbits will evolve in predictable patterns, particularly evident in the inclination and right ascension of the ascending node (RAAN) distributions. There are several clusters (others have used a "cloud" nomenclature) in observed distributions that show evolution from year to year in their inclination and ascending node elements. However, when MODEST is in survey mode (field-of-view approx.1.3deg) it provides only short 5-8 minute orbital arcs which can only be fit under the assumption of a circular orbit approximation (ACO) to determine the orbital parameters. These ACO elements are useful only in a statistical sense as dedicated observing runs would be required to obtain sufficient orbital coverage to determine a set of accurate orbital elements and then to follow their evolution. Identification of the source(s) for these "clusters of UCTs" would be advantageous to the overall definition of the GEO orbital debris environment. This paper will set out to determine if the ACO elements can be used to in a statistical sense to identify the source of the "clustering of UCTs" roughly centered on an inclination of 12deg and a RAAN of 345deg. The breakup of the Titan 3C-4 transtage on February 21, 1992 has been modeled using NASA s LEGEND (LEO-to-GEO Environment Debris) code to generate a GEO debris cloud. Breakup fragments are created based on the NASA Standard Breakup Model (including fragment size, area-to-mass (A/M), and delta-V distributions). Once fragments are created, they are propagated forward in time with a subroutine GEOPROP. Perturbations included in GEOPROP are those due to solar/lunar gravity, radiation pressure, and major geopotential terms. The question to be addressed: are the UCTs detected by MODEST in this inclination/RAAN region related to the Titan 3C-4 breakup? Discussion will include the observational biases in attempting to detect a specific, uncontrolled target during given observing session. These restrictions include: (1) the length of the observing session which is 8 hours or less at any given date or declination; (2) the assumption of ACO elements for detected object when the breakup model predicts debris with non-zero eccentricities; (3) the size and illumination or brightness of the debris predicted by the model and the telescope/sky limiting magnitude.

Barker, Edwin S.↗
NASA NTRS2007

Does the Blazar Gamma-ray Spectrum Harden with Increasing Flux? - Analysis of Nine Years of EGRET Data

The Energetic Gamma Ray Experiment Telescope (EGRET) on the Compton Gamma Ray Observatory (CGRO) discovered gamma-ray emission from more than 67 blazars during its nine-year lifetime. We conducted an exhaustive search of the EGRET archives and selected all the blazars that were observed multiple times and were bright enough to enable a spectral analysis using standard powerlaw models. The sample consists of 18 flat-spectrum radio quasars (FSRQs), 6 low-frequency-peaked BL Lacs (LBLs) and 2 high-frequency-peaked BL Lacs (HBLs). We do not detect any clear pattern in'the variation of spectral index with flux. Some of the blazars do not show any statistical evidence for spectral variability. The spectrum hardens with increasing flux in a few cases. There is also evidence for a flux-hardness anticorrelation at lo\v fluxes in five blazars. The well observed blazars (3C 279,3C 273, PKS 0528-i-134, PKS 1622-297, PKS 0208- 512) do not show any overall trend in the long-term spectral dependence on flux, but the sample shows a mixture of hard and soft states. We observed spectral hysteresis at weekly timescales in all the three FSRQs for which data from flares lasting for 3 approx. 4 weeks were available. All three sources show a counterclockwise rotation despite the widely different flux profiles. Hysteresis in the spectral index vs. flux space has never been observed in FSRQs in gamma-rays at weekly timescales. itre analyze the observed spectral behavior in the context of various inverse-Compton mechanisms believed to be responsible for emission in the EGRET energy range. Our analysis uses the EGRET skymaps that were regenerated to include the changes in performance during the mission.

Nandikotkur, Giridhar↗
NASA NTRS2009

Reflectance Spectra Comparison of Orbital Debris, Intact Spacecraft, and Intact Rocket Bodies in the GEO Regime

A key objective of NASA s Orbital Debris program office at Johnson Space Center (JSC) is to characterize the debris environment by way of assessing the physical properties (type, mass, density, and size) of objects in orbit. Knowledge of the geosynchronous orbit (GEO) debris environment in particular can be used to determine the hazard probability at specific GEO altitudes and aid predictions of the future environment. To calculate an optical size from an intensity measurement of an object in the GEO regime, a 0.175 albedo is assumed currently. However, identification of specific material type or types could improve albedo accuracy and yield a more accurate size estimate for the debris piece. Using spectroscopy, it is possible to determine the surface materials of space objects. The study described herein used the NASA Infrared Telescope Facility (IRTF) to record spectral data in the 0.6 to 2.5 micron regime on eight catalogued space objects. For comparison, all of the objects observed were in GEO or near-GEO. The eight objects consisted of two intact spacecraft, three rocket bodies, and three catalogued debris pieces. Two of the debris pieces stemmed from Titan 3C transtage breakup and the third is from COSMOS 2054. The reflectance spectra of the Titan 3C pieces share similar slopes (increasing with wavelength) and lack any strong absorption features. The COSMOS debris spectra is flat and has no absorption features. In contrast, the intact spacecraft show classic absorption features due to solar panels with a strong band gap feature near 1 micron. The two spacecraft are spin-stabilized objects and therefore have solar panels surrounding the outer surface. Two of the three rocket bodies are inertial upper stage (IUS) rocket bodies and have similar looking spectra. The slopes flatten out near 1.5 microns with absorption features in the near-infrared that are similar to that of white paint. The third rocket body has a similar flattening of slope but with fewer features of white paint - indicating that the surface paint on the SL-12 may be different than the IUS. This study shows that the surface materials of debris appear different spectrally than intact rocket bodies and spacecraft and therefore are not believed to be solar panel material or pristine white paint. Further investigation is necessary in order to eliminate materials as possible choices for the debris pieces.

Barker, Ed↗
NASA NTRS2008

The LHT (Lunar Highlands Type) Regolith Simulant Series

Three NU-LHT (NASA/USGS-Lunar Highlands Type) regolith simulants have been produced to date: NU-LHT-1M, -ID, and -2M. A fourth simulant is currently in production: NU-LHT-3C. The "M" (medium) designation indicates a simulant with a grain size of <1 mm, "D" (dust) a simulant with a grain size of <36 microns, and "C" (coarse) a simulant with a 10 cm maximum particle size. The composition of these simulants is based on a NASA average Apollo 16 regolith chemical composition, However, the mixing model used to create our simulants is based on cationic nonnative mineral proportions derived from the target chemical composition to approximate lunar modal mineralogy rather than chemical composition per se. Accordingly, the amount of plagioclase, pyroxenes, olivine, and trace minerals in the simulant crystalline fraction approximates that of the lunar regolith. We also added synthetic agglutinate in amounts approximate for low-medium regolith maturity. A pure glass fraction was also added to simulate other types of lunar glasses present in the regolith. In addition, the 3C simulant will include synthetic impact melt breccia clasts for the >1 cm particles. The bulk raw materials used to create these simulants include clinopyroxene-norite, anorthosite, hartzburgite and noritic mill waste from the Stillwater Mine, Nye, MT, and olivine from the Twin Sisters dunite, WA. Added trace minerals include beach sand ilmenite, chromite, synthetic p-tricaicium phosphate (whitiockite), gem grade fluor-apatite, and pyrite. The agglutinate, glasses, and synthetic breccia were designed and prepared at an industrial plasma melting facility in Boulder, CO, using Stillwater mill waste feedstock for the melt. These simulants do not include nanophase-feO. The M and C simulant grain size distribution (down to 0.4 microns) approximates that of Apollo 16 regolith and the regolith in general.

Stoeser, Douglas↗
NASA NTRS2019

Safe, High Power/Voltage Battery Module Design Challenges

NASA seeks to demonstrate a path for achieving safe, high power, and high performing Li-ion battery designs for the purpose of establishing design guidelines for our aeronautic and spacecraft applications. Safe means passively resistant to thermal runaway propagation of any single cell catastrophic thermal runaway. High power means capable of 3C continuous discharge without overheating. High performing means achieving > 160 Wh/kg, 200 Wh/L. The biggest challenge has been balancing a high flux light weight path for cell heat dissipation during the high rate discharge that minimizes thermal gradients between cells and also protects adjacent cells from the heat load of thermal runaway cell. Our solution includes an oscillating heat pipe spine that contacts every 18650 cell in the pack and careful cell design selection to maximize the range of initial temperatures conditions where the battery can safely complete the 3C discharge.

Thermal runaway↗
DOE OSTI2024

Utah FORGE: GES Well 16A(78)-32 and Well 16B(78)-32 Stimulation Seismic Event Catalogs

This dataset contains seismic event catalogs from the hydraulic stimulation of wells 16A(78)-32 and 16B(78)-32 at the Utah FORGE site in April 2024. The data was collected by Geo Energy Suisse (GES) using a variety of seismic monitoring technologies, including 3-component (3C) geophones and distributed acoustic sensing (DAS) systems. These technologies were deployed across several locations, including wells 16A, 16B, and Delano-1, with sensor arrays at multiple depths to capture microseismic activity during the stimulations. The catalogs provide both real-time and manually checked seismic event locations, with detailed parameters such as trigger conditions, velocity models, and data acquisition settings. The dataset includes information on the stimulation stages, event rates, and hydraulic injection conditions for each well, with a report detailing the data acquisition configuration and seismic event location methodologies. Users will need to reference the included report for a complete understanding of the sensor network, data processing techniques, and accuracy considerations.

15 GEOTHERMAL ENERGY↗
DOE OSTI2024

Effect of Zn Addition on Phase Evolution in AlCrFeCoNiZn High–Entropy Alloy

The addition of Zn to AlCrFeCoNi high-entropy alloy (HEA) poses intriguing questions as to how it would affect phase evolution. Herein, the phase evolution in AlCrFeCoNiZn is studied using a combination of experimental techniques (X-ray diffraction, scanning electron microscopy, energy-dispersive spectroscopy, and differential scanning calorimetry) and computational (density-functional theory [DFT], calculation of phase diagrams, and machine-learning) methods. Mechanically alloyed and spark-plasma-sintered AlCrFeCoNiZn assumes a metastable single-phase, body-centered-cubic (BCC) structure that undergoes diffusion-controlled phase separation upon subsequent heat treatment to form separate (Al, Cr)-rich, (Fe, Co)-rich, and (Zn, Ni)-rich phases. The formation of (Al, Cr)-rich phase, not reported previously in AlCrFeCoNi-based HEAs, is attributed to strong clustering tendency of Cr–Zn and Cr–Ni pairs, combined with the strong ordering of Zn–Ni pair, driving out Cr that in turn combines with Al to form a (Al, Cr)-rich phase. In the DFT results, the formation of thermodynamically stable L1 2 phase is shown wherein Cr–Fe–Zn [Al–Ni-Co] preferably occupy1a (000) [3c (0 ½ ½)] positions. Furthermore, the sluggish diffusional transformation to L1 2 phase from BCC precursors is attributed to the small stacking-fault energy of AlCrFeCoNiZn. The equilibrated HEA exhibits a high microhardness of 8.24 GPa with an elastic modulus of 184 GPa.

36 MATERIALS SCIENCE↗