Introduction. Like a house consisting of rooms separated by walls, a crystalline material consists of grains separated by grain boundaries. Within a grain, the arrangement of atoms is the same and this arrangement determines the orientation of the grain. When moving from one grain to the next, the orientation changes. Depending on the orientation of adjacent grains, sometimes grain boundaries are formed that result in special properties. These grain boundaries are called ‘special’ or ‘Σ coincidence site lattice (CSL)’ boundaries. The Σ3 CSL boundary in face-centered cubic materials, like nickel, is also a ‘twin’ boundary—that is, the orientation of the grain on one side of the boundary mirrors that on the other side of the boundary. In this work, electron backscatter diffraction (EBSD) was used to determine the effects of annealing electrodeposited (ED) nanocrystalline (nc) nickel, with an average grain size of 20 nm and no initial twin structures, at different temperatures for 1 hr on the percentages of CSL boundaries, particularly twin boundaries, in the material.

Results. EBSD, a technique associated with scanning electron microscopy (SEM), recognizes and maps the orientation of grains within a crystalline material. Each orientation is assigned a specific color (as shown in Figures 1 and 3), thus allowing us to identify grains. The difference in orientation between adjacent grains—that is, the misorientation angle at each grain boundary—can also be determined as well as the location and percentages of CSL boundaries (Figure 2). When compared to conventional transmission electron microscope (TEM) images (Figure 3b), EBSD maps (Figures 1 and 3a) are shown to provide accurate representations of microstructural features. It is known that twin boundaries look like straight boundaries in TEM. In EBSD, these straight boundaries are identified as Σ3 (twin) boundaries (examples marked with T in Figures 2 and 3). Therefore, confirming that presence of twin boundaries in our material.

Significance. The benefit of performing EBSD, in addition to TEM, is that it is an automated technique that provides information regarding not only grain orientation and grain size, but also regarding the frequency and location of CSL boundaries, which result in the improvement of important properties of materials such as strength, hardness and even corrosion resistance. [1]

References. 1. I. Roy et al., “Possible origin of superior corrosion resistance for electrodeposited nanocrystalline Ni.” Scripta Mater. 59:305-308 (2008)

The Application Of Electron Backscatter Diffraction To The Study Of Special Boundaries In Electrodeposited Nanocrystalline NickelFarghalli A. Mohamed, University of California, Irvine, DMR 0702978

Figure 1. EBSD map of a 220°C annealed sample. The white regions in (a) represent areas of nanocrystalline grains that are smaller than the step size (20nm).

Figure 2. CSL frequency distribution for the 220°C annealed sample. T indicates twin structures.

Figure 3. (a) EBSD map of annealed nc-Ni sample. (b) TEM image of annealed nc-Ni sample. T

indicates twin structures.

CSL Boundaries (220oC)

Σ values

Fre

qu

ency

(%

)

1 μm

T

T

50 nm

T

(b)

T

Σ3