Browsing by Author "Shen, J"

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    Inhomogeneous structure and glass-forming ability in Zr-based bulk metallic glasses
    (Elsevier, 2010-01-01) Sun, YL; Qu, DD; Sun, YJ; Liss, KD; Shen, J
    Recently, a series of quaternary Zr-based bulk metallic glasses (BMGs), i.e., Zr53Cu18.7Ni12Al16.3, Zr51.9Cu23.3Ni10.5Al14.3 and Zr50.7Cu28Ni9Al12.3, have been developed and their glass-forming ability (GFA) increases with Cu concentration. In this work, atomic structures of the three BMGs were rebuilt by reverse Monte Carlo simulations based on the reduced pair distribution functions measured by high energy X-ray diffraction. The results show that a certain amount of substitution of short Zr-Cu, Cu-Cu pairs with long Zr-Zr and Zr-Al pairs enhances the local denser packing of Kasper polyhedral centered by Zr and Al atoms. A cell sub-divided method is proposed to describe the fluctuation of local number density which is found to follow the normal distribution function. The largest possible free volume in the three alloys is found to approaches to 3.8 angstrom. For the three alloys, the enhancement of GFA with increasing Cu content is due to the increase in the fluctuation degree of local density instead of the dense packing. © 2010, Elsevier Ltd.
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    On the atomic anisotropy of thermal expansion in bulk metallic glass
    (Wiley-Blackwell, 2011-09-01) Qu, DD; Liss, KD; Yan, K; Reid, M; Almer, JD; Wang, Y; Liao, XZ; Shen, J
    Glass transition temperature and plastic yield strength are known to be correlated in metallic glasses. We have observed by in situ synchrotron high energy X-ray diffraction anisotropy of the thermal expansion behavior in the nearest neighbor and second nearest neighbor atomic distances in the building blocks of Zr-Cu-Ni-Al based bulk metallic glass, leading inevitably to shear. Mechanical yielding of the latter on the atomic scale leads to the glass transition and the increase of the free volume. These experimental results uncover the mechanism, how glass transition and yield strength are linked. © 2011, Wiley-Blackwell.
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    On the atomic anisotropy of thermal expansion in bulk metallic glass
    (Australian Institute of Physics, 2011-02-03) Liss, KD; Qu, DD; Reid, M; Shen, J
    Glass transition temperature and plastic yield strength are known to be correlated in metallic glasses. We have observed by in-situ synchrotron high energy X-ray diffraction anisotropy in the thermal expansion behavior of the nearest neighbor and second nearest neighbor atomic distances in the building blocks of Zr-Cu-Ni-Al based bulk metallic glass, leading inevitably to local shear stresses. Mechanical yielding of the latter on the atomic scale leads to the glass transition and the increase of the free volume. These experimental results uncover the mechanism, how glass transition and yield strength are linked.
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    Structural origins for the high plasticity of a Zr-Cu-Ni-Al bulk metallic glass
    (Pergamon-Elsevier Science Ltd, 2012-09-11) Qu, DD; Liss, KD; Sun, YJ; Reid, M; Almer, JD; Yan, K; Wang, Y; Liao, XZ; Shen, J
    The structural origins for the high plasticity of a Zr(53)Cu(18.7)Ni(12)Al(16.3) (at.%) bulk metallic glass are explored. Under plastic flow conditions, in situ synchrotron high-energy X-ray diffraction reveals that the atomic strain saturates to the closest packing in the longitudinal direction of the applied load while atoms yield in the transverse plane. Scanning electron microscopy investigation reveals that global plasticity benefits from abundant shear band multiplication and interactions. Atomic level flows are seen to accompany profuse shear bands. The plasticity enhancement of this metallic glass benefits from such atomic level flows. Atomic level flow facilitates the activation of shear transformation zones that further self-assemble to promote shear band multiplication. On the other hand, it also mitigates the shear band propagation that prevents catastrophic shear band extension. (C) 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. © 2012, Elsevier Ltd.
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    Zr-Cu-Ni-Al bulk metallic glasses with superhigh glass-forming ability.
    (Elsevier, 2009-02) Sun, YJ; Qu, DD; Huang, YJ; Liss, KD; Wei, XS; Xing, DW; Shen, J
    Zr-Cu-Ni-Al quaternary amorphous alloy compositions with varying glass-forming ability are developed by an efficient method of proportional mixing of binary eutectics. The critical diameter of the glassy sample is improved from 6 mm for Zr53Cu18.7Ni12Al16.3 to 14 mm for Zr50.7Cu28Ni9Al12.3 by straightforwardly adjusting the eutectic unit's coefficients. The drastic improvement in GFA is attributed to balancing, the chemical affinities of the Zr, Cu, Ni and Al components in the melt prior to solidification which makes the precipitation of competing crystalline phases more difficult. As the glass-forming ability increases, the concentration Of Cu ill the alloys exhibits a same trend. Based on synchrotron radiation high-energy X-ray diffraction analysis and Miracle's structural model, it is envisioned that the substitution of additional Cu Morris for Zr atoms in the investigated alloys stabilizes the efficient cluster packing structure of the amorphous alloys, leading to the pronounced increase ill their glass-forming ability. © 2008, Elsevier Ltd.