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Hudspeth M.,Purdue University | Sun T.,Argonne National Laboratory | Parab N.,Purdue University | Guo Z.,Purdue University | And 3 more authors.
Journal of Synchrotron Radiation | Year: 2015

Using a high-speed camera and an intensified charge-coupled device (ICCD), a simultaneous X-ray imaging and diffraction technique has been developed for studying dynamic material behaviors during high-rate tensile loading. A Kolsky tension bar has been used to pull samples at 1000s-1 and 5000s-1 strain-rates for super-elastic equiatomic NiTi and 1100-O series aluminium, respectively. By altering the ICCD gating time, temporal resolutions of 100ps and 3.37μs have been achieved in capturing the diffraction patterns of interest, thus equating to single-pulse and 22-pulse X-ray exposure. Furthermore, the sample through-thickness deformation process has been simultaneously imaged via phase-contrast imaging. It is also shown that adequate signal-to-noise ratios are achieved for the detected white-beam diffraction patterns, thereby allowing sufficient information to perform quantitative data analysis diffraction via in-house software (WBXRD-GUI). Of current interest is the ability to evaluate crystal d-spacing, texture evolution and material phase transitions, all of which will be established from experiments performed at the aforementioned elevated strain-rates. © 2015 International Union of Crystallography.

Wang L.,Peac Institute of Multiscale science | E J.C.,Peac Institute of Multiscale science | Cai Y.,Peac Institute of Multiscale science | Cai Y.,Anhui University of Science and Technology | And 3 more authors.
Journal of Applied Physics | Year: 2015

We investigate shock-induced deformation of columnar nanocrystalline Al with large-scale molecular dynamics simulations and implement orientation mapping (OM) and selected area electron diffraction (SAED) for microstructural analysis. Deformation mechanisms include stacking fault formation, pronounced twinning, dislocation slip, grain boundary (GB) sliding and migration, and lattice or partial grain rotation. GBs and GB triple junctions serve as the nucleation sites for crystal plasticity including twinning and dislocations, due to GB weakening, and stress concentrations. Grains with different orientations exhibit different densities of twins or stacking faults nucleated from GBs. GB migration occurs as a result of differential deformation between two grains across the GB. High strain rates, appropriate grain orientation and GBs contribute to deformation twinning. Upon shock compression, intra-grain dislocation and twinning nucleated from GBs lead to partial grain rotation and the formation of subgrains, while whole grain rotation is not observed. During tension, stress gradients associated with the tensile pulse give rise to intra-grain plasticity and then partial grain rotation. The simulated OM and SAED are useful to describe lattice/grain rotation, the formation of subgrains, GB migration and other microstructures. © 2015 AIP Publishing LLC.

Cai Y.,Anhui University of Science and Technology | Cai Y.,Peac Institute of Multiscale science | Wu H.A.,Anhui University of Science and Technology | Luo S.N.,Peac Institute of Multiscale science
Journal of Chemical Physics | Year: 2014

Large-scale molecular dynamics (MD) simulations are performed to investigate homogeneous nucleation and growth of nanovoids during cavitation in liquid Cu. We characterize in detail the atomistic cavitation processes by following the temporal evolution of cavities or voids, analyze the nucleation behavior with the mean first-passage time (MFPT) and survival probability (SP) methods, and discuss the results against classical nucleation theory (CNT), the Tolman equation for surface energy, independent calculation of surface tension via integrating the stress profiles, the Johnson-Mehl-Avrami (JMA) growth law, and the power law for nucleus size distributions. Cavitation in this representative metallic liquid is a high energy barrier Poisson processes, and the steady-state nucleation rates obtained from statistical runs with the MFPT and SP methods are in agreement. The MFPT method also yields the critical nucleus size and the Zeldovich factor. Fitting with the Tolman's equation to the MD simulations yields the surface energy of a planar interface (∼ 0.9 J-2) and the Tolman length (0.4-0.5 Å), and those values are in accord with those from integrating the stress profiles of a planar interface. Independent CNT predictions of the nucleation rate (1033 - 34 s -1 m-3) and critical size (3-4 Å in radius) are in agreement with the MFPT and SP results. The JMA law can reasonably describe the nucleation and growth process. The size distribution of subcritical nuclei appears to follow a power law with an exponent decreasing with increasing tension owing to coupled nucleation and growth, and that of the supercritical nuclei becomes flattened during further stress relaxation due to void coalescence. © 2014 AIP Publishing LLC.

Cai Y.,Anhui University of Science and Technology | Huang J.Y.,Anhui University of Science and Technology | Wu H.A.,Anhui University of Science and Technology | Zhu M.H.,Southwest Jiaotong University | And 3 more authors.
Journal of Physical Chemistry Letters | Year: 2016

It is well known that strain rate and size effects are both important in material failure, but the relationships between them are poorly understood. To establish this connection, we carry out molecular dynamics (MD) simulations of cavitation in Lennard-Jones and Cu liquids over a very broad range of size and strain rate. These studies confirm that temporal and spatial scales play equivalent roles in the tensile strengths of these two liquids. Predictions based on smallest-scale MD simulations of Cu for larger temporal and spatial scales are consistent with independent simulations, and comparable to experiments on liquid metals. We analyze these results in terms of classical nucleation theory and show that the equivalence arises from the role of both size and strain rate in the nucleation of a daughter phase. Such equivalence is expected to hold for a wide range of materials and processes and to be useful as a predictive bridging tool in multiscale studies. © 2016 American Chemical Society.

Wang M.C.,Peac Institute of Multiscale science | Wang M.C.,Southwest Jiaotong University | Qiao S.,CAS Shanghai Institute of Microsystem and Information Technology | Qiao S.,University of Shanghai for Science and Technology | And 5 more authors.
Physical Review Letters | Year: 2016

We report on a time-resolved ultrafast optical spectroscopy study of the topological insulator Bi2Se3. 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 ∼300 fs), 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 Eg 2 and A1g 1 phonon modes, respectively. These findings are suggestive of novel ways to manipulate the photoinduced coherent spins in topological insulators. © 2016 American Physical Society.

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