arXiv subjects

C. T. Liu

Publications and source records attributed to C. T. Liu.

11 recordsLinked to original sources

arXiv2021

Highly Distorted Lattices in Chemically Complex Alloys Produce Ultra-Elastic Materials with Extraordinary Elinvar Effects

Conventional crystalline alloys usually possess a low atomic size difference in order to stabilize its crystalline structure. However, in this article, we report a single phase chemically complex alloy which possesses a large atomic size misfit usually unaffordable to conventional alloys. Consequently, this alloy develops a rather complex atomic-scale chemical order and a highly distorted crystalline structure. As a result, this crystalline alloy displays an unusually high elastic strain limit (~2%), about ten times of that of conventional alloys, and an extremely low internal friction (< 2E-4) at room temperature. More interestingly, this alloy firmly maintains its elastic modulus even when the testing temperature rises from room temperature to 900 K, which is unmatched by the existing alloys hitherto reported. From an application viewpoint, our discovery may open up new opportunities to design high precision devices usable even under an extreme environment.

cond-mat.mtrl-sci↗

arXiv2016

Correlations Between Local Elastic Heterogeneities and Overall Elastic Properties in Metallic Glasses

The common notion suggests that metallic glasses (MGs) are a homogeneous solid at the macroscopic scale; however, recent experiments and simulations indicate that MGs contain nano-scale elastic heterogeneities. Despite the fundamental importance of these findings, a quantitative understanding is still lacking for the local elastic heterogeneities intrinsic to MGs. On the basis of Eshelby's theory, here we develop a micromechanical model that correlates the properties of the local elastic heterogeneities, being very difficult to measure experimentally, to the measurable overall elastic properties of MGs, such as shear/bulk modulus and Poisson's ratio. Our theoretical modeling is verified by the experimental data obtained from various MGs annealed to different degrees. Particularly, we revealed that the decrease of Poisson's ratio upon annealing of MGs is associated with a much large shear softening over hydrostatic-pressure softening, and $vice$ $versa$ in local elastic inhomogeneities. The relative extent of the bulk versus shear modulus softens is extracted for different MGs, and is found to closely depend on the specific composition and their ductility. The implication of our results on the Poisson's ratio criterion on the ductility as well as the aging dynamics in MGs is discussed.

cond-mat.mtrl-sci↗

arXiv2016

Phase separation of metastable CoCrFeNi high entropy alloy at intermediate temperatures

The CoCrFeNi alloy is widely accepted as an exemplary stable base for high entropy alloys (HEAs). Although various investigations prove it to be stable solid solution, its phase stability is still suspicious. Here, we identified that the CoCrFeNi HEA was thermally metastable at intermediate temperatures, and composition decomposition occurred after annealed at 750oC for 800 hrs. The increased lattice distortion induced by minor addition of Al into the CoCrFeNi base accelerated the composition decomposition and a second fcc phase with a different lattice constant occurred in the long time annealed CoCrFeNiAl0.1 HEA. A Cr-rich σ phase also precipitated from the CoCrFeNiAl0.1 HEA. The Al element can induce the instability of CoCrFeNi HEA. The revealed metastable CoCrFeNi at intermediate temperatures will greatly change the way of HEAs development.

cond-mat.mtrl-sci↗

arXiv2016

Transformation-mediated Plasticity in CuZr based Metallic Glass Composites: A Quantitative Mechanistic Understanding

In this paper, we present a thorough stress analysis of the Cu-Zr metallic-glass composite with embedded B2 particles subject to a martensitic transformation. Within the framework of the Eshelby theory, we are able to explain, in a quantitative manner, (1) the formation of three types of shear bands with distinct morphologies as observed experimentally in the severely deformed Cu-Zr metallic-glass composite and (2) the work hardening ability of the Cu-Zr metallic-glass composite as related to the coupled effects of elastic back stress and elastic mismatch caused by the martensitic transformation. Furthermore, we also discuss the issues about the stress affected zone of the individual B2 phase and the stability of the crystalline-amorphous interface. Given the general agreement between the theoretical and experimental findings, we believe that the outcome of our current work can lead to a deeper understanding of the transformation-induced plasticity in the Cu-Zr based metallic glass composites, which should be very useful to the design of the metallic-glass composites with improved ductility.

cond-mat.mtrl-sci↗

arXiv2014

Late-stage galaxy mergers in COSMOS to z~1

The role of major mergers in galaxy and black hole formation is not well constrained. To help address this, we develop an automated method to identify late-stage galaxy mergers before coalescence of the galactic cores. The resulting sample of mergers is distinct from those obtained using pair-finding and morphological indicators. Our method relies on median-filtering of high-resolution images in order to distinguish two concentrated galaxy nuclei at small separations. Using mock images, we derive statistical contamination and incompleteness corrections for the fraction of late-stage mergers. We apply our new method to a magnitude-limited (I < 23) sample of 44,164 galaxies from the COSMOS HST/ACS catalog. Using a mass-complete sample with $\log M_*/M_\odot > 10.6$ and $0.25 < z \leq 1.00$, we find ~5% of systems are late-stage mergers with separations between 2.2 and 8 kpc. Correcting for incompleteness and contamination, the fractional merger rate increases strongly with redshift as $(1+z)^{3.8\pm0.9}$, in agreement with earlier studies and with dark matter halo merger rates. Separating the sample into star-forming and quiescent galaxies shows that the merger rate for star-forming galaxies increases strongly redshift, $(1+z)^{4.5\pm1.3}$, while the merger rate for quiescent galaxies is consistent with no evolution, $(1+z)^{1.1\pm1.2}$. Limiting our sample to galaxies with spectroscopic redshifts from zCOSMOS, we find that the star formation rates and X-ray selected AGN activity in likely late-stage mergers are enhanced by factors of ~2 relative to a control sample. Combining our sample with more widely separated pairs, we find that $8\pm5\%$ of star formation and $20\pm8\%$ of AGN activity is triggered by close encounters (<143 kpc) or mergers, once more suggesting that major mergers are not the only channels for star formation and black hole growth. (abridged)

astro-ph.GA↗

arXiv2014

The zCOSMOS Redshift Survey: evolution of the light in bulges and discs since z~0.8

We studied the chronology of galactic bulge and disc formation by analysing the relative contributions of these components to the B-band rest-frame luminosity density at different epochs. We present the first estimate of the evolution of the fraction of rest-frame B-band light in galactic bulges and discs since redshift z~0.8. We performed a bulge-to-disc decomposition of HST/ACS images of 3266 galaxies in the zCOSMOS-bright survey with spectroscopic redshifts in the range 0.7 < z < 0.9. We find that the fraction of B-band light in bulges and discs is $(26 \pm 4)%$ and $(74 \pm 4)%$, respectively. When compared with rest-frame B-band measurements of galaxies in the local Universe in the same mass range ($10^{9} M_{\odot}\lessapprox M \lessapprox 10^{11.5} M_{\odot}$), we find that the B-band light in discs decreases by ~30% from z~0.7-0.9 to z~0, while the light from the bulge increases by ~30% over the same period of time. We interpret this evolution as the consequence of star formation and mass assembly processes, as well as morphological transformation, which gradually shift stars formed at half the age of the Universe from star-forming late-type/irregular galaxies toearlier types and ultimately into spheroids.

astro-ph.GA↗

arXiv2012

The dominant role of mergers in the size evolution of massive early-type galaxies since z ~ 1

In this paper we measure the merger fraction and rate, both minor and major, of massive early-type galaxies (M_star >= 10^11 M_Sun) in the COSMOS field, and study their role in mass and size evolution. We use the 30-band photometric catalogue in COSMOS, complemented with the spectroscopy of the zCOSMOS survey, to define close pairs with a separation 10h^-1 kpc <= r_p <= 30h-1 kpc and a relative velocity Delta v <= 500 km s^-1. We measure both major (stellar mass ratio mu = M_star,2/M_star,1 >= 1/4) and minor (1/10 <= mu < 1/4) merger fractions of massive galaxies, and study their dependence on redshift and on morphology. The merger fraction and rate of massive galaxies evolves as a power-law (1+z)^n, with major mergers increasing with redshift, n_MM = 1.4, and minor mergers showing little evolution, n_mm ~ 0. When split by their morphology, the minor merger fraction for early types is higher by a factor of three than that for spirals, and both are nearly constant with redshift. Our results show that massive early-type galaxies have undergone 0.89 mergers (0.43 major and 0.46 minor) since z ~ 1, leading to a mass growth of ~30%. We find that mu >= 1/10 mergers can explain ~55% of the observed size evolution of these galaxies since z ~ 1. Another ~20% is due to the progenitor bias (younger galaxies are more extended) and we estimate that very minor mergers (mu < 1/10) could contribute with an extra ~20%. The remaining ~5% should come from other processes (e.g., adiabatic expansion or observational effects). This picture also reproduces the mass growth and velocity dispersion evolution of these galaxies. We conclude from these results that merging is the main contributor to the size evolution of massive ETGs at z <= 1, accounting for ~50-75% of that evolution in the last 8 Gyr. Nearly half of the evolution due to mergers is related to minor (mu < 1/4) events.

astro-ph.CO↗

arXiv2010

A Multiwavelength Study of a Sample of 70 micron Selected Galaxies in the COSMOS Field II: The Role of Mergers in Galaxy Evolution

We analyze the morphological properties of a large sample of 1503 70 micron selected galaxies in the COSMOS field spanning the redshift range 0.01 1 being difficult to classify and subject to the effects of band pass shifting, therefore, these numbers can only be considered lower limits. At z<1, where the morphological classifications are most robust, major mergers clearly dominate the ULIRG population (50-80%) and are important for the LIRG population (25-40%). At z>1 the fraction of major mergers is at least 30-40% for ULIRGs. Although the general morphological trends agree with what has been observed for local (U)LIRGs, the fraction of major mergers is slightly lower than seen locally. This is in part due to the difficulty of identifying merger signatures at high redshift. We argue that given the number of major gas-rich mergers observed and the relatively short timescale that they would be observable in the (U)LIRG phase that it is plausible for the observed red sequence of massive ellipticals (<10^12 M_sun) to have been formed entirely by gas-rich major mergers.

astro-ph.CO↗

arXiv2009

A Multiwavelength Study of a Sample of 70 micron Selected Galaxies in the COSMOS Field I: Spectral Energy Distributions and Luminosities

We present a large robust sample of 1503 reliable and unconfused 70microm selected sources from the multiwavelength data set of the Cosmic Evolution Survey (COSMOS). Using the Spitzer IRAC and MIPS photometry, we estimate the total infrared luminosity, L_IR (8--1000 microns), by finding the best fit template from several different template libraries. The long wavelength 70 and 160 micron data allow us to obtain a reliable estimate of L_IR, accurate to within 0.2 and 0.05 dex, respectively. The 70 micron data point enables a significant improvement over the luminosity estimates possible with only a 24 micron detection. The full sample spans a wide range in L_IR, L_IR ~ 10^8-10^14 L_sun, with a median luminosity of 10^11.4 L_sun. We identify a total of 687 luminous, 303 ultraluminous, and 31 hyperluminous infrared galaxies (LIRGs, ULIRGs, and HyLIRGs) over the redshift range 0.01<z<3.5 with a median redshift of 0.5. Presented here are the full spectral energy distributions for each of the sources compiled from the extensive multiwavelength data set from the ultraviolet (UV) to the far-infrared (FIR). Using SED fits we find possible evidence for a subset of cooler ultraluminous objects than observed locally. However, until direct observations at longer wavelengths are obtained, the peak of emission and the dust temperature cannot be well constrained. We use these SEDs, along with the deep radio and X-ray coverage of the field, to identify a large sample of candidate active galactic nuclei (AGN). We find that the fraction of AGN increases strongly with L_IR, as it does in the local universe, and that nearly 70% of ULIRGs and all HyLIRGs likely host a powerful AGN.

astro-ph.CO↗

arXiv2007

COSMOS : Hubble Space Telescope Observations

The Cosmic Evolution Survey (COSMOS) was initiated with an extensive allocation (590 orbits in Cycles 12-13) using the Hubble Space Telescope (HST) for high resolution imaging. Here we review the characteristics of the HST imaging with the Advanced Camera for Surveys (ACS) and parallel observations with NICMOS and WFPC2. A square field (1.8$\sq$°) has been imaged with single-orbit ACS I-F814W exposures with 50% completeness for sources 0.5\arcsec in diameter at I$_{AB} $ = 26.0 mag. The ACS imaging is a key part of the COSMOS survey, providing very high sensitivity and high resolution (0.09\arcsec FWHM, 0.05\arcsec pixels) imaging and detecting 1.2 million objects to a limiting magnitude of 26.5 (AB). These images yield resolved morphologies for several hundred thousand galaxies. The small HST PSF also provides greatly enhanced sensitivity for weak lensing investigations of the dark matter distribution.

astro-ph↗

arXiv2005

Correlations between bright submillimetre sources and low-redshift galaxies

We present evidence for a positive angular correlation between bright submillimetre sources and low-redshift galaxies. The study was conducted using 39 sources selected from 3 contiguous, flux-limited SCUBA surveys, cross-correlated with optical field galaxies with magnitudes R<23 (with a median redshift of z~0.5). We find that the angular distribution of submm sources is skewed towards overdensities in the galaxy population, consistent with 25+-12 per cent being associated with dense, low-redshift structure. The signal appears to be dominated by the brightest sources with a flux density S_850>10mJy. We conduct Monte-Carlo simulations of clustered submm populations, and find that the probability of obtaining these correlations by chance is less than 0.4 per cent. The results may suggest that a larger than expected fraction of submm sources lie at z~0.5. Alternatively, we argue that this signal is most likely caused by gravitational lensing bias, which may be entirely expected given the steep submm source counts. Implications for future submm surveys are discussed.

astro-ph↗