Crystal Orientation Mapping

[Pages:2]Technical Note: 51923

Crystal Orientation Mapping: Orientation Coloring Scheme

Patrick Camus, Ph.D., Thermo Fisher Scientific, Madison, WI, USA

Key Words ? Color Scheme ? EBSD ? Orientation Maps

Background

Electron backscatter diffraction (EBSD) orientation mapping programs, like QuasOr, collect and measure the individual crystal orientations with respect to the sample or detector axes. An easy way to describe these orientations is through the use of Euler angles. A set of Euler angles consists of 3 successive rotations about specific directions to align the crystal axes with the reference axes. Color coding is used to illustrate these angular changes. In fact, 2 orientation maps are required to fully describe the crystal orientations with respect to the reference axes.

Description

The easiest way to illustrate the orientation of a crystal is through the use of an oriented unit cell "cube". However, this graphical representation is best suited for showing the orientation of a single point, not for many points as in a map. The easiest way to map the Euler angles of a crystal orientation to the reference axes is to use colors mapped to a stereographic triangle. A usual convention is to place

Color map for orientations within a stereographic triangle

Unit cell "cube" for specific Euler angles 1, , 2

red at the , green at the , and blue at the directions. This coloring convention provides a color crystal orientation map (COM) of the Euler angles within a sample.

However, a single map does not provide the complete answer. A single map can actually have multiple crystal rotations for a single color representation. An example can be described as follows: a map with respect to the sample normal contains a grain which is colored red for a orientation. This color can be assigned for a variety of unit cell orientations. For instance, the unit cell "cube" could be oriented horizontally or rotated 45 and still be given the color red.

Valid "cube" orientations for a red pixel in a "normal" COM

For illustration purposes only (below), a COM can be annotated with unit cell "cubes." Any regions which are similar in color have one similar orientation with respect to the reference axis, but the rotation about that axis is random. This can be seen by examining all the "cubes" associated with the red grains in a "sample normal" reference direction. All these "cubes" should have a cube-face upward, but with a random rotation about this "out-of-paper" direction. This rotation shows that a constant red color does NOT mean that the orientations of all commonly colored grains are identical. Therefore, additional information is required to fully describe the crystal orientations.

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Normal

Reference

Transverse

COM images of same data set colored using the stereographic triangle scheme with respect to the indicated reference axis. Note the variation of color for the same grain between the images necessitating multiple displays for correct interpretation of the orientation data.

A second COM is usually displayed to illustrate this additional orientation information (see figure above). This COM has the same color scheme based on the stereographic triangle, but it is with respect to a different reference axis. The reference axes include "transverse" (vertical), "reference" (horizontal), and "normal" (out-ofpaper). Close examination of the colors in the two COM's to determine the rotations about the specific axes can predict the orientation of the "cube".

Conclusion

A single COM is not sufficient to adequately describe the orientations measured by QuasOr. Thus, two COM's with respect to different reference axes fully describes the orientations within the sample.

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