The atomic structure and solute segregation
of grain boundaries are often analyzed by transmission electron microscopy (TEM) in ceramics
(2, 14–18) and metals ( 4, 19–21); more recently,
atom probe tomography (APT) has become a
useful tool for grain boundaries because of its
complementary analytical strengths ( 22–24). However, only a handful of studies have examined the
structural arrangement of adsorbates at general
grain boundaries in polycrystalline materials [e.g.,
( 15–18, 20, 21)]. Some degree of grain boundary
adsorbate periodicity was evident in a few of these
studies, such as along one side of nanometer-thick
intergranular “glassy” films in Si3N4 ( 16–18), but no
evidence of widespread adsorbate-induced grain
boundary superstructures has been reported. In
contrast to these studies, TEM studies on special
high-symmetry boundaries, such as tilt (2, 4, 14, 19)
and twist ( 25) boundaries in artificial bicrystals,
often reveal striking, periodic patterns of segregated
elements—for example, in Bi-doped Cu ( 4) and
rare earth–doped alumina (2). We also note that
an interface reconstruction has been observed at a
Ni-Al2O3 phase boundary ( 26).
Unlike the grain boundaries in high-symmetry
tilt or twist bicrystals, the majority of grain bound-
aries in polycrystalline materials are of mixed
twist and tilt character. Mixed boundaries are
sometimes called “random” or “general” grain
boundaries ( 27). The common definition of a gen-
eral grain boundary is one with a large inverse
coincidence (S > 29) ( 8). Although this S-based
definition has received criticism ( 28), it is widely
used ( 29), so for the purposes of the present work,
a general boundary is simply one that lacks ap-
preciable lattice coincidence. General grain bound-
aries are populous in polycrystalline engineering
materials and are often weaker mechanically and
chemically than higher-symmetry special grain
boundaries, and thereby can limit macroscopic
properties and performance. Hence, understand-
ing these performance-limiting grain boundaries
is critical to enhancing our ability to engineer
next-generation materials ( 8, 30).
Our previous work on liquid-metal embrittlement of Ni by Bi showed two Bi-rich layers visible
at the grain boundary with linear periodicity ( 21).
However, it was unclear whether the segregated
adsorbate atoms had superstructures. Here, we
present experimental results from scanning TEM
(STEM) and simulated results from density functional theory (DFT) to demonstrate that a variety
of periodic adsorbate superstructures form at
naturally occurring, randomly selected general
grain boundaries in Bi-infused polycrystalline
nickel. We discovered that the grain boundary
reconstructions at these boundaries are not driven
by the grain boundary misorientation, as commonly
believed, but by the crystallographic orientation
of the grain boundary plane. In this way, these
grain boundary reconstructions are strongly analogous to surface reconstructions, which are also
driven by the crystallography of the terminating
surface and which alter the two-dimensional translational symmetry of the interface, influencing
surface diffusion coefficients ( 31), electronic characteristics ( 32), and other physical properties.
We randomly selected 12 grain boundaries
from a Bi-infused Ni polycrystalline specimen and
examined them by aberration-corrected STEM
(table S1 and figs. S1 to S14) ( 33). The Ni-Bi alloy
has the equilibrium solidus composition at 700°C,
which we estimated to be 0.22 atomic percent Bi
in Ni based on recent CALPHAD data ( 34). The
misorientation of all 12 randomly selected grain
boundaries was determined via a detailed Kikuchi
diffraction pattern analysis. Calculations ( 35) done
with these misorientation data showed that 11 of
the 12 boundaries were general grain boundaries
with S values greater than 500 (table S1). One
of the 12 boundaries was determined to be a S3
twin boundary. Bi adsorbate superstructures were
discovered at many of the general grain bounda-
ries, and the Bi segregation was confirmed by
energy-dispersive x-ray spectroscopy (fig. S15).
We use two different categories of notation to
describe the Bi adsorbate superstructures: (i)
When discussing the arrangement of Bi atoms
within the grain boundary plane, we use Wood’s
notation ( 36) [e.g., Fig. 1 shows a C(2×2) recon-struction] or matrix notation ( 37) (e.g., Figure 2
includes results for the [5–511] reconstruction,
which cannot be clearly represented by Wood’s
notation), both common in surface science. (ii)
When discussing how the Bi adsorbate atoms
appear when viewed from the side in projection
(i.e., as in TEM images), we refer to the number
of Bi atoms that appear to be sitting on top of a
given number of Ni atoms (e.g., Fig. 1B shows a
2Bi/4Ni superstructure, the side view of Fig. 1A
parallel to [010]).
We observed the simplest Bi adsorbate pattern
of 2Bi/4Ni for the ( 100) grain boundary facet
(Fig. 1, A to D). The ( 110) grain boundary facet
(Fig. 1, E to H) exhibits a 3Bi/6Ni superstructure
when viewed from the side parallel to ½1;11;. Both
of these superstructures reduce to an apparent
1Bi/2Ni superstructure when viewed from the
(1x1)
(2x2)−RandC
Bi Ni
top view
3Bi/6Ni
[ 110]
[ 111]
(1x1)
Bi Ni
[010]
top view [ 100]
2Bi/4Ni
[010] [ 111]
side view
top view
(1x1)
(8x8)
Bi Ni
[ 111]
1 nm
[ 110]
1 nm 1 nm
10 Bi on 16
Ni
superstructu
re
uc
[ 110]
side view side view
10Bi/16Ni
Bi
randomly
occupied
C(2x2)
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
DFTcalculations 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.
