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Browsing by Author "Zhou, X."

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    Composition-dependent apparent activation-energy and sluggish grain-growth in high entropy alloys
    (Taylor & Francis, 2019) Gwalani, B.; Salloom, R.; Alam, T.; Valentin, S. G.; Zhou, X.; Thompson, G.; Srinivasan, S. G.; Banerjee, R.; University of North Texas System; University of North Texas Denton; University of Alabama Tuscaloosa; United States Department of Energy (DOE); Pacific Northwest National Laboratory
    Experimental results reveal that the apparent activation-energy for grain-growth in an fcc-based AlxCoCrFeNi high entropy alloy (HEA) system increases from 179 to 486kJ/mol when the Al content increases from x=0.1 to 0.3. These unexpectedly high apparent activation-energy values can be potentially attributed to solute clustering within the fcc solid-solution phase that develops with increasing Al content in this HEA. Detailed microstructural analysis using atom-probe tomography and density functional theory (DFT) calculations strongly indicate the presence of such nanoscale clusters. This phenomenon can change grain-growth from a classical solute-drag regime to a much more sluggish cluster-drag based mechanism in these HEAs. [GRAPHICS] IMPACT STATEMENTFirst report on a composition dependent change in apparent activation-energy for grain-growth in high entropy alloys. A novel cluster drag effect inhibiting grain-growth kinetics is suggested.
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    The hidden structure dependence of the chemical life of dislocations
    (American Association for the Advancement of Science, 2021) Zhou, X.; Mianroodi, J. R.; da Silva, A. Kwiatkowski; Koenig, T.; Thompson, G. B.; Shanthraj, P.; Ponge, D.; Gault, B.; Svendsen, B.; Raabe, D.; Max Planck Society; RWTH Aachen University; University of Alabama Tuscaloosa; University of Manchester; Imperial College London
    Dislocations are one-dimensional defects in crystals, enabling their deformation, mechanical response, and transport properties. Less well known is their influence on material chemistry. The severe lattice distortion at these defects drives solute segregation to them, resulting in strong, localized spatial variations in chemistry that determine microstructure and material behavior. Recent advances in atomic-scale characterization methods have made it possible to quantitatively resolve defect types and segregation chemistry. As shown here for a Pt-Au model alloy, we observe a wide range of defect-specific solute (Au) decoration patterns of much greater variety and complexity than expected from the Cottrell cloud picture. The solute decoration of the dislocations can be up to half an order of magnitude higher than expected from classical theory, and the differences are determined by their structure, mutual alignment, and distortion field. This opens up pathways to use dislocations for the compositional and structural nanoscale design of advanced materials.
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    Structure and Magnetic Properties of Exchange-biased Polycrystalline Fe/MnPd Bilayers
    (2000) Tang, Y. J.; Roos, B. F. P.; Mewes, T.; Frank, A. R.; Rickart, M.; Bauer, M.; Demokritov, S. O.; Hillebrands, B.; Zhou, X.; Liang, B. Q.; Chen, X; Zhan, W. S.; University of Alabama Tuscaloosa
    We report the fabrication of polycrystalline Fe/MnPd bilayers, which show the exchange-bias effects. We show that the exchange-bias effect, described by unidirectional anisotropy, is also accompanied by induced uniaxial and fourfold in-plane anisotropy contributions in the system. Using the Stoner-Wohlfarth model, all in-plane anisotropy contributions to the total free energy are determined for the polycrystalline samples from the fit of the in-plane angular dependence of Heb and the coercivity Hc . A mechanism explaining the presence of the induced higher-order anisotropy contributions in an exchange-bias system is proposed.
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    Variability and stability in blazar jets on time-scales of years: optical polarization monitoring of OJ 287 in 2005-2009
    (Oxford University Press, 2010) Villforth, C.; Nilsson, K.; Heidt, J.; Takalo, L. O.; Pursimo, T.; Berdyugin, A.; Lindfors, E.; Pasanen, M.; Winiarski, M.; Drozdz, M.; Ogloza, W.; Kurpinska-Winiarska, M.; Siwak, M.; Koziel-Wierzbowska, D.; Porowski, C.; Kuzmicz, A.; Krzesinski, J.; Kundera, T.; Wu, J. -H.; Zhou, X.; Efimov, Y.; Sadakane, K.; Kamada, M.; Ohlert, J.; Hentunen, V. -P.; Nissinen, M.; Dietrich, M.; Assef, R. J.; Atlee, D. W.; Bird, J.; DePoy, D. L.; Eastman, J.; Peeples, M. S.; Prieto, J.; Watson, L.; Yee, J. C.; Liakos, A.; Niarchos, P.; Gazeas, K.; Dogru, S.; Donmez, A.; Marchev, D.; Coggins-Hill, S. A.; Mattingly, A.; Keel, W. C.; Haque, S.; Aungwerojwit, A.; Bergvall, N.; University of Turku; Ruprecht Karls University Heidelberg; Pedagogical University of Cracow; Jagiellonian University; University of Toronto; Chinese Academy of Sciences; National Astronomical Observatory, CAS; Russian Academy of Sciences; Crimean Astrophysical Observatory; Osaka University of Education; University System of Ohio; Ohio State University; Texas A&M University System; Texas A&M University College Station; National & Kapodistrian University of Athens; Canakkale Onsekiz Mart University; University of Shumen; University of Alabama Tuscaloosa; University West Indies Mona Jamaica; University West Indies Saint Augustine; Naresuan University; University of Warwick; Uppsala University
    OJ 287 is a BL Lac object at redshift z = 0.306 that has shown double-peaked bursts at regular intervals of similar to 12 yr during the last similar to 40 yr. We analyse optical photopolarimetric monitoring data from 2005 to 2009, during which the latest double-peaked outburst occurred. The aim of this study is twofold: firstly, we aim to analyse variability patterns and statistical properties of the optical polarization light curve. We find a strong preferred position angle in optical polarization. The preferred position angle can be explained by separating the jet emission into two components: an optical polarization core and chaotic jet emission. The optical polarization core is stable on time-scales of years and can be explained as emission from an underlying quiescent jet component. The chaotic jet emission sometimes exhibits a circular movement in the Stokes plane. We find six such events, all on the time-scales of 10-20 d. We interpret these events as a shock front moving forwards and backwards in the jet, swiping through a helical magnetic field. Secondly, we use our data to assess different binary black hole models proposed to explain the regularly appearing double-peaked bursts in OJ 287. We compose a list of requirements a model has to fulfil to explain the mysterious behaviour observed in OJ 287. The list includes not only characteristics of the light curve but also other properties of OJ 287, such as the black hole mass and restrictions on accretion flow properties. We rate all existing models using this list and conclude that none of the models is able to explain all observations. We discuss possible new explanations and propose a new approach to understanding OJ 287. We suggest that both the double-peaked bursts and the evolution of the optical polarization position angle could be explained as a sign of resonant accretion of magnetic field lines, a 'magnetic breathing' of the disc.