arXiv subjects

P. Choi

Publications and source records attributed to P. Choi.

8 recordsLinked to original sources

arXiv2014

Chemical gradients across phase boundaries between martensite and austenite in steel studied by atom probe tomography and simulation

Partitioning at phase boundaries of complex steels is important for their properties. We present atom probe tomography results across martensite / austenite interfaces in a precipitation-hardened maraging TRIP steel (12.2 Mn, 1.9 Ni, 0.6 Mo, 1.2 Ti, 0.3 Al; at.%). The system reveals compositional changes at the phase boundaries: Mn and Ni are enriched while Ti, Al, Mo, and Fe are depleted. More specific, we observe up to 27 at.% Mn in a 20 nm layer at the phase boundary. This is explained by the large difference in diffusivity between martensite and austenite. The high diffusivity in martensite leads to a Mn-flux towards the retained austenite. The low diffusivity in the austenite does not allow accommodation of this flux. Consequently, the austenite grows with a Mn-composition given by local equilibrium. The interpretation is based on DICTRA and mixed-mode diffusion calculations (using a finite interface mobility).

cond-mat.mtrl-sci↗

arXiv2012

Nanoscale austenite reversion through partitioning, segregation, and kinetic freezing: Example of a ductile 2 GPa Fe-Cr-C steel

Austenite reversion during tempering of a Fe-13.6Cr-0.44C (wt.%) martensite results in an ultrahigh strength ferritic stainless steel with excellent ductility. The austenite reversion mechanism is coupled to the kinetic freezing of carbon during low-temperature partitioning at the interfaces between martensite and retained austenite and to carbon segregation at martensite-martensite grain boundaries. An advantage of austenite reversion is its scalability, i.e., changing tempering time and temperature tailors the desired strength-ductility profiles (e.g. tempering at 400°C for 1 min. produces a 2 GPa ultimate tensile strength (UTS) and 14% elongation while 30 min. at 400°C results in a UTS of ~ 1.75 GPa with an elongation of 23%). The austenite reversion process, carbide precipitation, and carbon segregation have been characterized by XRD, EBSD, TEM, and atom probe tomography (APT) in order to develop the structure-property relationships that control the material's strength and ductility.

cond-mat.mtrl-sci↗

arXiv2006

Mapping Global Star Formation in the Interacting Galaxy Pair ARP 32

A multi-wavelength set of photometric data including UV (GALEX), optical, near-IR, infrared (Spitzer) and radio (VLA 20cm) images and spectroscopic observations are used to map the dust-obscured and unobscured star formation in the galaxy pair ARP 32. The system consists of an actively star-forming galaxy and another one with depressed star formation. The most active galaxy has disrupted morphology and different sites of star formation. Spectroscopic data show hints of nuclear activity in its core, intense star formation in limited regions of the galaxy as well as an underlying population of stars witnessing a past episode of star formation. Current star formation rates are estimated from UV and bolometric IR luminosities.

astro-ph↗

arXiv2005

Spitzer Detection of PAH and Silicate Dust Features in the Mid-Infrared Spectra of z~2 Ultraluminous Infrared Galaxies

We report the initial results from a Spitzer GO-1 program to obtain low resolution, mid-infrared spectra of infrared luminous galaxies at z~1-2. This paper presents the spectra of eight sources observed with the Spitzer InfraRed Spectrograph (IRS). Of the eight spectra, six have mid-IR spectral features, either emission from Polycyclic Aromatic Hydrocarbon (PAH) or silicate absorption. Based on these mid-IR features, the inferred six redshifts are in the range of 1.8-2.6. The remaining two spectra detect only strong continua, thus do not yield redshift information. Strong, multiple PAH emission features are detected in two sources, and weak PAH emission in another two. These data provide direct evidence that PAH molecules are present and directly observable in ULIRGs at z~2. The six sources with measured redshifts are dusty, infrared luminous galaxies at z~2 with estimated $L_{bol} \sim 10^{13}L_\odot$. Of the eight sources, two appear starburst dominated; two with only power law continua are probably type I QSOs; and the remaining four are likely composite systems containing a buried AGN and a starburst component. Since half of our sample are optically faint sources with R>25.5mag (Vega), our results demonstrate the potential of using mid-infrared spectroscopy, especially the Aromatic and silicate features produced by dust grains to directly probe optically faint and infrared luminous populations at high redshift.

astro-ph↗

arXiv2004

Infrared Properties of Radio-Selected Submillimeter Galaxies in the Spitzer First Look Survey Verification Field

We report on submillimeter and infrared observations of 28 radio-selected galaxies in the Spitzer First Look Survey Verification field (FLSV). All of the radio-selected galaxies that show evidence for emission at 850um with SCUBA have Spitzer counterparts at 24um, while only half of the radio-selected galaxies without 850um emission have detectable counterparts at 24um. The data show a wide range of infrared colors (S70/S24 < 5--30, S8/S3.6 < 0.3--4), indicative of a mixture of infrared-warm AGN and cooler starburst dominated sources. The galaxies showing 850um emission have Spitzer flux densities and flux density ratios consistent with the range of values expected for high-redshift (z=1--4) ultraluminous infrared galaxies.

astro-ph↗

arXiv2004

Spitzer 24micron Observations of Optical/Near-IR Selected Extremely Red Galaxies: Evidence for Assembly of Massive Galaxies at z~1-2 ?

We carried out the direct measurement of the fraction of dusty sources in a sample of extremely red galaxies with (R - Ks) > 5.3mag and Ks < 20.2mag, using 24um data from the Spitzer Space Telescope. Combining deep 24um, Ks- and R-band data over an area of 64sq.arcmin in ELAIS N1, we find that 50%+-6% of our ERO sample have measurable 24um flux above the 3sigma flux limit of 40uJy. This flux limit corresponds to a SFR of 12Msun/yr at z~1. The 24um-detected EROs have 24-to-2.2 and 24-to-0.7um flux ratios consistent with infrared luminous, dusty sources at z>1, and an order of magnitude too red to be explained by an infrared quiescent spiral or a pure old stellar population at any redshift. Keck optical spectroscopy of a sample of similarly selected EROs in the FLS field suggests that most of the EROs in ELAIS N1 are probably at z~1. The mean 24um flux (167uJy) of the 24um-detected ERO sample roughly corresponds to the total infrared luminosity of (3-10)x10^(11)Lsun at z=1-1.5, which is similar to that of local LIRGs and ULIGs. The corresponding SFR is ~(50-170)Msun/yr. If the time scale of this starbursting phase is on the order of 10^8yr as inferred for the local LIRGs and ULIGs, the lower limit on the masses of these 24um-detected EROs is (5-20)x10^9Msun. It is plausible that some of the starburst EROs are in the midst of violent transformation to become massive early type galaxies at the epoch of z ~ 1-2.

astro-ph↗

arXiv2004

Characterization of Extragalactic 24micron Sources in the Spitzer First Look Survey

In this Letter, we present the initial characterization of extragalactic 24um sources in the Spitzer First Look Survey (FLS) by examining their counterparts at 8um and R-band. The color-color diagram of 24-to-8 vs. 24-to-0.7um is populated with 18,734 sources brighter than the 3sigma flux limit of 110uJy, over an area of 3.7sq.degrees. The 24-to-0.7um colors of these sources span almost 4 orders of magnitudes, while the 24-to-8um colors distribute at least over 2 orders of magnitudes. In addition to identifying ~30% of the total sample with infrared quiescent, mostly low redshift galaxies, we also found that: (1) 23% of the 24um sources (~1200/sq.degrees) have very red 24-to-8 and 24-to-0.7 colors and are probably infrared luminous starbursts with L(IR)>3x10^(11)Lsun at z>1. In particular, 13% of the sample (660/sq.degrees) are 24um detected only, with no detectable emission in either 8um or R-band. These sources are the candidates for being ULIRGs at z>2. (2) 2% of the sample (85/sq.degrees) have colors similar to dust reddened AGNs, like Mrk231 at z~0.6-3. (3) We anticipate that some of these sources with extremely red colors may be new types of sources, since they can not be modelled with any familiar type of spectral energy distribution. We find that 17% of the 24um sources have no detectable optical counterparts brighter than R limit of 25.5mag. Optical spectroscopy of these optical extremely faint 24um sources would be very difficult, and mid-infrared spectroscopy from the Spitzer would be critical for understanding their physical nature (Abridged).

astro-ph↗

arXiv2000

A Lack of Planets in 47 Tucanae from an HST Search

We report results from a large Hubble Space Telescope project to observe a significant (~34,000) ensemble of main sequence stars in the globular cluster 47 Tucanae with a goal of defining the frequency of inner-orbit, gas-giant planets. Simulations based on the characteristics of the 8.3 days of time-series data in the F555W and F814W WFPC2 filters show that ~17 planets should be detected by photometric transit signals if the frequency of hot Jupiters found in the solar neighborhood is assumed to hold for 47 Tuc. The experiment provided high-quality data sufficient to detect planets. A full analysis of these WFPC2 data reveals ~75 variables, but NO light curves resulted for which a convincing interpretation as a planet could be made. The planet frequency in 47 Tuc is at least an order of magnitude below that for the solar neighborhood. The cause of the absence of close-in planets in 47 Tuc is not yet known; presumably the low metallicity and/or crowding of 47 Tuc interfered with planet formation, with orbital evolution to close-in positions, or with planet survival.

astro-ph↗