of 6 nm and demonstrates the superior resolution that can be attained with think samples and the TKD method.

29.2.10\ Application Example

Application of EBSD To Understand Meteorite Formation

EBSD has found application in many materials studies from ceramics, to semiconductors to metals and alloys. It has also has been applied to understanding metallic meteorites and

their thermal history. One example of this will be illustrated with the Gibeon meteorite. There has been interest in understanding the beautiful Widmanstatten pattern that is seen in these meteorites and how this two phase structure of ferrite (body-centered cubic crystal structure) and austenite (face-centered cubic crystal structure). Previous work had studied the formation of this structure and most of those studies had assumed that at high temperatures in the parent asteroid the meteorite consisted of very large grains of austenite. During the cooling of this meteorite in space over many millions of years the austenite was assumed

. Fig. 29.24  a EBSD inverse pole figure map with respect to the sample normal direction that is constructed by tiling 220 separate 1 × 1-mm maps. The interesting Widmanstatten pattern can be seen in the large ferrite plates that are running nearly the length of the image. This map contains both indexed austenite­ and ferrite;­ although at this scale only the larger ferrite is visible. b This is the same area as shown in .  Fig. 29.23a, but now we present­ the ferrite as a band contrast­ or a measure of the pattern­ sharpness and the austenite­ in the colors of the inverse pole figure map with respect to the sample normal direction­ . The amazing

observation­ from this EBSD data is that the ­austenite has the same orientation throughout the large 22 × 10-mm area, which leads

to the interpretation­ that the austenite­ is retained from the original parent body

a

b

to have fully transformed to ferrite. On further cooling, the ferrite, super-saturated with Ni, then was thought to decompose to the two phase ferrite plus austenite structure. However, EBSD has shown that this is not be the correct path for the observed microstructural evolution (Goldstein and Michael 2006).

.  Figure 29.24a is a large-area EBSD map acquired by mapping smaller areas of about 1 × 1 mm and tiling together 220 of these tiles into a large area map that covers 22 × 10 mm with a 3-μm step size. The sample was mechanically polished using standard metallographic practice followed by a few hours of vibratory polishing on colloidal silica. The entire map shown consists of a more than 25 million individual pixels. The general microstructure at a low magnification is clearly visible in .  Fig. 29.24a. .  Figure 29.24b shows a band contrast image of the ferrite and the austenite as an inverse pole figure map with respect to the sample surface normal. Note that all of the austenite has the same or very close to the same orientation, as shown by the austenite all of the same color in the inverse pole figure map. This is an important observation as the austenite could not have formed from precipitation from the ferrite but must be remnants of the original large austenite grains found in the parent meteorite body at elevated temperatures early in the meteorites life. This is further demonstrated by the pole figures shown in

.  Fig. 29.25. The austenite pole figures show that there is only a single orientation of austenite in the 22 mm × 10 mm area.

The ferrite pole figures are much more complicated and are a result of the many variants of ferrite that form from a single orientation of austenite.

There are also regions in .  Fig. 29.24a that are very fine grained and difficult to resolve with the 3-μm step size used. Further examination of the microstructure showed that these regions were extremely fine grained and required higher resolution than can be achieved with using bulk EBSD. Due to the small feature size in these areas, TKD is an excellent method to utilize. .  Figure 29.26 is a secondary electron image of a focused ion beam produced thin sample. Also shown is a scanning transmission electron image acquired at 30 kV which demonstrates that the sample is sufficiently thin for the transmission of 30 kV electrons. .  Figure 29.27 is the resulting TKD map and phase information obtained from the thin sample using an on-axis TKD detector. The step size for this image was 4 nm. It is now clear from these images that the fine-grained regions in .  Fig. 29.24a consist of regions of single crystal austenite that can be seen in .  Fig. 29.24b but also regions of ferrite that have begun to decompose during cooling to the equilibrium austenite plus ferrite that would be expected. The presence of twinned austenite precipitates is somewhat surprising, but may be explained by some of the stress in the sample during transformation.

This example shows how EBSD and TKD may be applied to complex microstructures and how the use of TKD is extremely complementary to EBSD. The visualization of the

. Fig. 29.25  Pole figures from the austenite (top) and the ferrite (bottom). In the austenite pole figures the single orientation of the austenite is shown by the arrangements of the poles that are shown. This was

also observed from .  Fig. 29.23b. The complexity of the ferrite pole figures is due to the many crystallographic variants of ferrite that form from the single orientation of austenite

. Fig. 29.26  A thin sample made for TKD of the fine two phase regions in the Gibeon meteorite. The sample was prepared with conventional FIB followed by low voltage ion FIB milling at 5 kV. a Secondary electron SEM

image of the thinned sample. b Scanning transmission electron image of the sample at 30 kV