Nucleation of fractal nanocrystallites upon annealing of Fe-based metallic glass
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Nucleation of fractal nanocrystallites upon annealing of Fe-based metallic glass Jiecheng Diao School of Materials Science and Engineering, TongJi University, Shanghai 201804, China
Bo Chena) School of Materials Science and Engineering, TongJi University, Shanghai 201804, China; and London Centre for Nanotechnology, University College London, London WC1H 0AH, U.K.
Qiang Luo, Wei Lin, Xianping Liu, and Jun Shen School of Materials Science and Engineering, TongJi University, Shanghai 201804, China
Ian Robinson School of Materials Science and Engineering, TongJi University, Shanghai 201804, China; London Centre for Nanotechnology, University College London, London WC1H 0AH, U.K.; and Division of Condensed Matter Physics and Materials Science, Brookhaven National Laboratory, Upton, NY 11973, USA (Received 26 August 2016; accepted 16 February 2017)
Bragg coherent X-ray diffraction imaging has been used to determine the structure of the initial clusters of a-Fe nano crystals which form upon annealing of an iron-based amorphous alloy or metallic glass. The method is able to identify the shapes and strain of these crystallites without any need for cutting the sample, so can visualize them in three dimensions in their intact state. In this way, the delicate dendritic structures on the exterior of the crystallites can be seen and its density versus radius relationship identifies a fractal dimension of the porous region that is consistent with diffusion-limited aggregation models. The crystal sizes were found to be around 60 nm after annealing at 700 °C growing to about 330 nm after annealing at 750 °C. This article introduces the BCDI method and describes its application to characterize previously recrystallized samples of iron-based amorphous alloys. It paves the way for a possible future in situ nucleation/growth investigation of the relationship between kinetics and nanostructure of metallic glass.
I. INTRODUCTION
X-ray diffraction (XRD) methods have a central role in materials science since its foundation. Powder diffraction provides not only crystallographic structures of materials and mixtures of phases but also size and microstrain information about the constituent grains. Powder diffraction depends on getting a sufficiently accurate average over many grains of the sample. A newer method, introduced to exploit the relatively high coherence of the X-ray beams generated at the latest synchrotron sources, called Bragg coherent diffractive imaging (BCDI), attempts to undo this powder average and examine the size and strain information separately from each individual grain. It therefore has the advantage of being much more specific in providing a 3D image of the grain and a projection of the strains present inside it, but loses the averaging capability of powder diffraction. Here we apply BCDI to an important
Contributing Editor: Chris Nicklin a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2017.79
question in materials science, the structure of the crystalline grains formed af
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