Time filter

Source Type

Zhong T.,City University of Hong Kong | Rao K.P.,City University of Hong Kong | Prasad Y.V.R.K.,City University of Hong Kong | Zhao F.,City University of Hong Kong | And 2 more authors.
Materials Science and Engineering A | Year: 2013

Processing map for hot working of hot extruded AZ31-1.5 vol% nano-alumina magnesium composite (AZ31-NAL) prepared by disintegrated metal deposition (DMD) technique has been developed in the temperature range of 250-500°C and strain rate range of 0.0003-10s-1. The starting composite microstructure is fine grained and is much less textured compared with the base AZ31 material prepared by similar technique (AZ31-DMD). The processing map for the composite exhibits three domains in the temperature and strain rate ranges: (1) 250-350°C/0.0003-0.01s-1; (2) 375-500°C/0.0003-0.01s-1; (3) 300-400°C/1-10s-1, which are all similar to those exhibited by the base alloy. In Domains #1 and #3, dynamic recrystallization occurs and results in fine grained microstructure, while in Domain #2, grain boundary sliding leading to wedge cracking in compression and intercrystalline cracking in tension have been identified. In comparison with the base alloy AZ31-DMD, the efficiency in the first domain has decreased, the third domain moved to lower temperatures and the effect on second domain is marginal. These differences are attributed to the lower intensity of starting texture in the nano-composite compared with the base alloy. A study of texture changes in the deformed specimens revealed that nano-alumina additions are helpful in reducing the preferred orientation in AZ31 alloy. © 2013 Elsevier B.V.

PubMed | Georgia Institute of Technology, CAS Shanghai Institute of Microsystem and Information Technology and The Peac Institute of Multiscale science
Type: Journal Article | Journal: Physical review letters | Year: 2016

We report on a time-resolved ultrafast optical spectroscopy study of the topological insulator Bi_{2}Se_{3}. We unravel that a net spin polarization cannot only be generated using circularly polarized light via interband transitions between topological surface states (SSs), but also via transitions between SSs and bulk states. Our experiment demonstrates that tuning photon energy or temperature can essentially allow for photoexcitation of spin-polarized electrons to unoccupied topological SSs with two distinct spin relaxation times (25 and 300fs), depending on the coupling between SSs and bulk states. The intrinsic mechanism leading to such distinctive spin dynamics is the scattering in SSs and bulk states which is dominated by E_{g}^{2} and A_{1g}^{1} phonon modes, respectively. These findings are suggestive of novel ways to manipulate the photoinduced coherent spins in topological insulators.

PubMed | The Peac Institute of Multiscale science and Sichuan University
Type: Journal Article | Journal: The Journal of chemical physics | Year: 2015

With molecular dynamics simulations, we systematically investigate melting of a set of 311070.53 tilt grain boundaries (GB) in Cu bicrystals, including coherent twin boundaries (CTBs), 12 asymmetric tilt grain boundaries (ATGBs), and symmetric incoherent twin boundaries (SITBs), in the order of increasing length weight of SITB or GB energy. ATGBs decompose into CTBs and SITBs, which migrate and coalesce as a result of internal stress relaxation. GBs can be superheated or premelted, and GB melting temperature decreases exponentially with increasing SITB weight, owing to the systematics in GB microstructure. GB melting nucleates at disordered CTB-SITB junctions, and grows along SITBs and then into grain interiors, with the solid-liquid interfaces preferentially aligned with {111}.

Dharmendra C.,City University of Hong Kong | Rao K.P.,City University of Hong Kong | Zhao F.,The Peac Institute of Multiscale science | Zhao F.,City University of Hong Kong | And 3 more authors.
Materials Science and Engineering A | Year: 2014

The effect of silicon (0.2-0.8wt%) addition on the hot working behavior and deformation mechanisms of the Mg-3Sn-2Ca-0.4Al (TX32-0.4Al) alloy has been evaluated by generating processing maps in the temperature and strain rate ranges of 300-500°C and 0.0003-10s-1. The processing map for the base TX32-0.4Al alloy exhibited two dynamic recrystallization (DRX) domains in the ranges (1) 300-360°C and 0.0003-0.001s-1 and (2) 400-500°C and 0.003-0.7s-1. While 0.2% Si addition did not result in any significant change in the processing map of the base TX32-0.4Al alloy, 0.4% Si addition has enhanced hot workability by widening the processing window(s) and by reducing flow instability. The rate controlling mechanism in Domain 1 is identified as climb, whereas it is cross-slip in Domain 2. When the Si content is increased to 0.6 and 0.8%, the volume fraction of hard intermetallic particles has increased nearly two fold. The processing map for the alloy with 0.6% Si addition exhibited an additional Domain 3 at higher temperatures and high strain rates (475-500°C and 0.01-10s-1). However, cracking has occurred in Domain 1 due to void formation at hard particles. In Domains 2 and 3, DRX occurred predominantly by basal slip with climb as a recovery process, as confirmed by the resulting basal texture and tilt type sub-boundary structure. This is attributed to the large back stress generated by the increased volume fraction of intermetallic particles due to which the extensive activation of basal slip required considerably high temperatures. Increase in the volume fraction of hard particles due to higher Si content reduces the flow instability by generating a high rate of entropy production through increasing the nucleation sites for power dissipation and enhances the occurrence of void formation and/or ductile fracture. © 2014.

Huang J.Y.,Southwest Jiaotong University | Lu L.,Anhui University of Science and Technology | Fan D.,The Peac Institute of Multiscale science | Sun T.,Argonne National Laboratory | And 4 more authors.
Scripta Materialia | Year: 2015

Dynamic compression experiments are conducted on micron-sized SiC powders of different initial densities with a split Hopkinson pressure bar. Digital image correlation is applied to images from high-speed X-ray phase contrast imaging to map dynamic strain fields. The X-ray imaging and strain field mapping demonstrate the degree of heterogeneity in deformation depends on the initial powder density; mesoscale strain field evolution is consistent with softening or hardening manifested by bulk-scale loading curves. Statistical analysis of the strain probability distributions exhibits exponential decay tail similar to those of contact forces, which are supposed to lead to the grain-scale heterogeneity of granular materials. © 2015 Acta Materialia Inc.

Sheu Y.M.,Los Alamos National Laboratory | Trugman S.A.,Los Alamos National Laboratory | Yan L.,Los Alamos National Laboratory | Qi J.,The Peac Institute of Multiscale science | And 3 more authors.
Physical Review X | Year: 2014

Using ultrafast optical spectroscopy, we show that polaronic behavior associated with interfacial antiferromagnetic order is likely the origin of tunable magnetotransport upon switching the ferroelectric polarity in a La0.7Ca0.3MnO3/BiFeO3 (LCMO/BFO) heterostructure. This is revealed through the difference in dynamic spectral weight transfer between LCMO and LCMO/BFO at low temperatures, which indicates that transport in LCMO/BFO is polaronic in nature. This polaronic feature in LCMO/BFO decreases in relatively high magnetic fields due to the increased spin alignment, while no discernible change is found in the LCMO film at low temperatures. These results thus shed new light on the intrinsic mechanisms governing magnetoelectric coupling in this heterostructure, potentially offering a new route to enhancing multiferroic functionality.

Lu L.,Anhui University of Science and Technology | Lu L.,Southwest Jiaotong University | Lu L.,The Peac Institute of Multiscale science | Huang J.W.,The Peac Institute of Multiscale science | And 7 more authors.
Acta Materialia | Year: 2016

In situ synchrotron x-ray imaging and diffraction are used to investigate anisotropic deformation of an extruded magnesium alloy AZ31 under uniaxial compression along two different directions, with the loading axis (LA) either parallel or perpendicular to the extrusion direction (ED), referred to as LA∥ED and LA⊥ED, respectively. Multiscale measurements including stress–strain curves (macroscale), x-ray digital image correlation (mesoscale), and diffraction (microscale) are obtained simultaneously. Electron backscatter diffraction is performed on samples collected at various strains to characterize deformation twins. The rapid increase in strain hardening rate for the LA∥ED loading is attributed to marked {101¯2} extension twinning and subsequent homogenization of deformation, while dislocation motion leads to inhomogeneous deformation and a decrease in strain hardening rate. © 2016 Acta Materialia Inc.

PubMed | Argonne National Laboratory, The Peac Institute of Multiscale science and Los Alamos National Laboratory
Type: Journal Article | Journal: The Review of scientific instruments | Year: 2016

We develop a mini gas gun system for simultaneous, single-pulse, x-ray diffraction and imaging under high strain-rate loading at the beamline 32-ID of the Advanced Photon Source. In order to increase the reciprocal space covered by a small-area detector, a conventional target chamber is split into two chambers: a narrowed measurement chamber and a relief chamber. The gas gun impact is synchronized with synchrotron x-ray pulses and high-speed cameras. Depending on a cameras capability, multiframe imaging and diffraction can be achieved. The proof-of-principle experiments are performed on single-crystal sapphire. The diffraction spots and images during impact are analyzed to quantify lattice deformation and fracture; fracture is dominated by splitting cracks followed by wing cracks, and diffraction peaks are broadened likely due to mosaic spread. Our results demonstrate the potential of such multiscale measurements for studying high strain-rate phenomena at dynamic extremes.

PubMed | Argonne National Laboratory and The Peac Institute of Multiscale science
Type: Journal Article | Journal: Journal of synchrotron radiation | Year: 2016

Dynamic compression experiments are performed on single-crystal Si under split Hopkinson pressure bar loading, together with simultaneous high-speed (250-350ns resolution) synchrotron X-ray Laue diffraction and phase-contrast imaging. A methodology is presented which determines crystal rotation parameters, i.e. instantaneous rotation axes and angles, from two unindexed Laue diffraction spots. Two-dimensional translation is obtained from dynamic imaging by a single camera. High-speed motion of crystals, including translation and rotation, can be tracked in real time via simultaneous imaging and diffraction.

PubMed | The Peac Institute of Multiscale science and Anhui University of Science and Technology
Type: Journal Article | Journal: The Journal of chemical physics | Year: 2015

Homogeneous nucleation and growth during crystallization of supercooled liquid Cu are investigated with molecular dynamics simulations, and the microstructure is characterized with one- and two-dimensional x-ray diffraction. The resulting solids are single-crystal or nanocrystalline, containing various defects such as stacking faults, twins, fivefold twins, and grain boundaries; the microstructure is subject to thermal fluctuations and extent of supercooling. Fivefold twins form via sequential twinning from the solid-liquid interfaces. Critical nucleus size and nucleation rate at 31% supercooling are obtained from statistical runs with the mean first-passage time and survival probability methods, and are about 14 atoms and 10(32) m(-3)s(-1), respectively. The bulk growth dynamics are analyzed with the Johnson-Mehl-Avrami law and manifest three stages; the Avrami exponent varies in the range of 1-19, which also depends on thermal fluctuations and supercooling.

Loading The Peac Institute of Multiscale science collaborators
Loading The Peac Institute of Multiscale science collaborators