Microscopic characterization

Low magnification TEM imaging was used to assess the general shape and regularity of the various GBs studied. For typical observation, one side of the GB was oriented to a <001> or <011> zone axis, with the latter only being achievable through specimen tilting in the JEOL JEM-2000FX for edge on GB orientation specimens. The alignment of the Cu crystal near the GB to a common low index zone axis allowed for repeatable imaging conditions across multiple specimens, a convenient starting point for any imaging analysis to be performed on the specimens. For undoped GBs it was observed that the GB lines tended to be extremely straight and almost completely featureless with a lack of faceting or any second phase particles. Undoped GBs were observed in two separate conditions; the first observation condition being as received, and the second observation condition being as annealed. The as received condition

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was identical to the condition that the mechanical testing specimens were tested under, have undergone no heat treatment except that related to the mounting procedure. A second set of pure bicrystal specimens were observed after undergoing a heat

treatment with the same temperature and time as that of the Bi doped specimens. This second observation was to rule out any changes to the GB structure due to relaxation or the heat treatment environment to ensure that all observed boundary structures were due to the presence of Bi or a combination of Bi and the heat treatment environment.

Figure 41 and Figure 42 show images of the GBs from two 6° Cu bicrystal specimens;

the first being that of a Bi doped specimen, and the second being that of a pure Cu specimen that has undergone an identical heat treatment to the Bi doped specimen.

The heat-treated specimen showed no increase in second phase particles along the GB or any evidence of GB relaxation, meaning that the heating during the doping

procedure, by itself, cannot be responsible for any observed changes in mechanical behavior of the Cu specimens.

Upon observation of the pure specimens in the as received condition and the as heat treated condition, there was very little difference in the structure of the GB via SEM observation and low magnification TEM observation. Comparison between the pure and Bi doped specimens, however, revealed a change in the GB appearance for the 13° and the 33° GBs that was observable at low magnification (<200 kX) in the TEM. A summary of the qualitative yet observable morphology change of the different GBs is presented in Table 5. The 33° GB showed the largest change in morphology after Bi doping. GB faceting was observed along the length of Bi doped

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33° Cu GBs. This faceting is thought to be the result of GB relaxation to accommodate the large Bi atoms on the GB and relieve stress in and around the GB by changing the GB plane [79]. Large second phase particles periodically appear on the GB throughout the specimen thickness. Figure 43 shows a BF-TEM image of the GB in a 33° Bi doped Cu bicrystal fabricated in a near plane-view orientation. The second phase particles can be seen between the points labeled A and B indicating the beginning of each side of the GB. XEDS analysis, discussed further in section 5.6, revealed these second phase particles to be a Bi heavy oxide most likely Bi2O3.

Table 5 – Observable Change in GB Morphology upon Bi Doping

GB Misorientation Angle Change in GB Morphology

6 No

13 Yes

33 Yes

Viewing specimens fabricated in a near edge-on orientation as shown in Figure 44 and Figure 45, it is possible to see the different GB plane sections present along the GB due to faceting from the addition of Bi and subsequent rearrangement on the atomic-level (which may lead to relaxation) during the heat treatment. The HAADF image in Figure 44 also shows bright areas along the top and bottom surface of the specimen in this transmission view. These bright areas correspond directly to an increase in the average atomic number of the elements present in that area and are in

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fact due to an increase in Bi concentration at the GB. Figure 45 shows a STEM-BF image of the same GB displaying sharper contrast features on the top and bottom of the GB than are observable in the HAADF image of the same GB. The increase in contrast in the STEM-BF image is due to the additional contribution of the diffraction of electrons transmitted through the sample as well as the effect of incoherent

scattering due to any changes in atomic number in areas close to the GB. With BF-STEM it is often easier to observe certain crystal defects that may disrupt the

periodicity of the crystal lattice leading to local changes in the diffraction conditions.

Defects such as GBs, stacking faults, and dislocations are then very easy to discern in a BF-STEM image.

GBs fabricated with a slight inclination of the GB plane to the specimen surface, making it easier to view the GB in a near plane-view orientation, provided some insight into the structure of the GB. Operating a TEM at 80 kV allowed the dislocation structure to be viewed under very specific conditions. Figure 46 shows a DF-TEM image of a 33° GB where dislocations on the GB can be seen. There appear to be three separate types of dislocations: two orthogonal to one another and a

dislocation line that runs parallel to the GB. Burgers vector analysis was not capable of being performed on these dislocations as sufficient two-beam conditions were not achievable on the GB area.

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Figure 41 – BF-TEM image of a Bi doped 6° Cu bicrystal GB

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Figure 42 – BF-TEM image of a pure 6° Cu bicrystal GB taken from a post-heat treatment sample

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Figure 43 – BF-TEM image of a near plane-view orientation specimen showing second phase particles along the GB with two arrows indicating points A and B, the GB region of the specimen.

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Figure 44 – HAADF-STEM image of a 33° [100] twist Cu GB doped with Bi imaged at a location between two second phase GB particles rich in Bi on an edge on GB orientation specimen.

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Figure 45 - BF-STEM image of a 33° [100] twist Cu GB doped with Bi imaged at a location between two second phase GB particles rich in Bi on an edge on GB orientation specimen.

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Figure 46 – DF-TEM image (200) reflections from lower grain of 33° Cu bicrystal GB showing 3 different dislocation lines in red.

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