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Segregation-induced ordered superstructures at general grain boundaries in a nickel-bismuth alloy

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Science  06 Oct 2017:
Vol. 358, Issue 6359, pp. 97-101
DOI: 10.1126/science.aam8256
  • Fig. 1 Atomic-scale segregation–induced superstructures in the Ni-Bi system.

    (A to D) (100) crystallographic facets; (E to H) (110) crystallographic facets; (I to L) (111) crystallographic facets. These facets belong to grain boundaries #7, #2, and #5, respectively (table S1). HAADF-STEM images of Bi-based grain boundary superstructures [original images, (C), (G), and (K); averaged images, (D), (H), and (L)] show periodic arrangements of Bi atoms that are crystallographically related to the underlying nickel grains. Surface DFT calculations produce Bi-based superstructures [(A), (E), and (I)] that exhibit the same periodicity as the Bi adsorbate atoms in the HAADF-STEM images [(C), (D), (G), (H), (K), and (L)] when viewed from the side as two-dimensional projections [(B), (F), and (J)]. The HAADF-STEM images in (D), (H), and (L) have been averaged following the algorithm in (51) to help to determine the exact location of the Bi atoms. Note that the 3Bi/6Ni two-dimensional superstructure in (F) appears as 3Bi/6Ni when viewed in this direction if the lower-intensity, randomly centered Bi atoms are not counted.

  • Fig. 2 Results of DFT calculations for Ni-Bi free surfaces and a grain boundary.

    (A to C) DFT calculations of surface free energy as a function of chemical potential for various Bi-based superstructures on three different Ni surfaces: (A) Ni(100) surface, (B) Ni(110) surface, and (C) Ni(111) surface. (D to F) Charge density models based on DFT calculations: (D) unreconstructed [i.e., (1×1)] Bi monolayer on the Ni(100) surface; (E) C(2×2) superstructure on the Ni(100) surface; (F) Σ5-120 grain boundary. In these charge density models, red indicates increased charge density and green indicates charge depletion.

  • Fig. 3 Grain boundary facets in Bi-doped Ni.

    (A) HAADF-STEM image of a general grain boundary in Bi-doped Ni, which contains micrometer-sized facets. The grain boundary plane pairs (“grain boundary surfaces”) of one facet are marked as N1 and N2 (green) and those of a second facet are marked as N3 and N4 (yellow). (B to E) HAADF-STEM images of the four grain boundary facet planes (with inset atomic diagrams) showing the arrangement of Bi atoms at the grain boundary: (B) N1, (C) N2, (D) N3, (E) N4. (F) Schematic model of the atomic structure of these two grain boundary facets. In this model, the grain boundary is intentionally separated by nanometers to show the grain boundary facet surfaces. These HAADF-STEM images were obtained from grain boundary #2 (table S1).

  • Fig. 4 Grain boundary facet orientations and Bi-Bi distances at grain boundaries in Bi-doped Ni.

    (A) Statistical frequency of grain boundary plane orientations plotted on a stereographic triangle, which includes orientation information for 27 grain boundary facet planes analyzed from 11 general grain boundaries. Numbers of observations of each plane orientation appear within the squares; squares with the same number are colored alike. (B) Histogram of the projected in-plane Bi-Bi distances, showing a peak around 3.0 to 3.5 Å, which is between the first and second nearest neighbor Bi-Bi bond length in Bi metal. Inset: A representative STEM image in which the Bi-Bi distance is labeled.

Supplementary Materials

  • Segregation-induced ordered superstructures at general grain boundaries in a nickel-bismuth alloy

    Zhiyang Yu, Patrick R. Cantwell, Qin Gao, Denise Yin, Yuanyao Zhang, Naixie Zhou, Gregory S. Rohrer, Michael Widom, Jian Luo, Martin P. Harmer

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    • Materials and Methods 
    • Supplementary Text 
    • Figs. S1 to S30 
    • Tables S1 and S2 
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