Matt Nowell, EBSD Product Manager, EDAX
I’ve always liked learning about space. I remember in 2nd grade we had a test, and the final question was “If you think the Sun is hot, look in the sink”. It was very rewarding, and obviously memorable, to find a basket of candy there. With EDAX, I’ve had the opportunity to work with many groups involved with space-related research, including NASA and a visit to the Kennedy Space Center to train users. Recent confluence of events motivated me to write about my experiences using EBSD to analyze meteorites.
First, I had the opportunity to visit the Planetary Materials Research Group within the Lunar and Planetary Laboratory at the University of Arizona. You can read more about what this group does in both the recent EDAX Insight article and at their website, but it was interesting to learn about their research and their involvement with different NASA space missions. One of the students had a great quote, which inspired this post, “Earth rocks are OK, but space rocks are very cool.”
Second, Shawn Wallace recently joined EDAX as an Applications Engineer. Previously he had been at the American Museum of Natural History in New York City where his research focused on meteorites for understanding planet formation.
Third, I’ve spent some time testing ComboScan, where we montage multiple fields of view together for large area EBSD mapping. Turns out the Gibeon meteorite is a great sample for this application.
Fourth, and finally, I was recently at a workshop where Joe Michael from Sandia National Laboratory showed transmission-EBSD (t-EBSD, or TKD) results from this Iron meteorite. I found it very interesting that this same meteorite could be used with EBSD on areas of interest ranging from centimeters to nanometers, and show interesting and useful results.
Figure 1 shows an EBSD image quality and orientation map (subsequently referred to as EBSD map unless otherwise noted) collected from a polished slice of the Gibeon meteorite, an iron meteorite. The area mapped was approximately 27 mm x 15 mm with an 8µm step size. The microstructure shows a classic Widmanstätten pattern of geometrically arranged plates that correlated with the crystallography of the material. This structure develops as face-centered cubic taenite slowly cools and transforms into body-centered cubic kamacite at specific sites on the taenite crystal lattice. The orientation relationships that develop are easily observed and measured using EBSD.
Visually it is easy to see the long plates of kamacite at this magnification, but you can also see areas of finer microstructure. Figure 2 shows another EBSD map collected at 100X magnification with a 850 nm step size. Here you can see 3 different length scales of material, the large kamacite plates, a smaller field of both kamacite and taenite, and regions of fine-grained microstructure.
Pushing the magnification higher, Figure 3 shows an image collected at 1,000X magnification with a 100 nm step size. Figure 4 shows the same data presented as a phase map, with the kamacite phase colored blue and the taenite phase colored yellow. We can clearly resolve the taenite grains within the field of kamacite. You will also notice that the majority of the taenite grains have the same orientation in this region. This is an area of incomplete transformation, and is similar to retained austenite engineered into modern steel alloys. You can also see running through the upper left corner of the map a thin band of even finer microstructure.
This fine microstructure was analyzed with a 25 nm step size, and is shown in Figure 5. The same geometrical relationships observed on a mm-scale are reproduced on a nm-scale. This sample nicely shows how EBSD can be used to characterize materials across the spatial range.
Of course these are all 2D slices on what is a very interesting 3D microstructure. To go further, I used an FEI Quanta 3D Dual Beam instrument to collect EBSD data from FIB serial-sections, and used the tools in OIM Analysis to reconstruct the microstructure in three dimensions. I targeted a region where the fine microstructure was adjacent to one of the larger kamacite grains, and the 3D volume is shown in Figure 6. The large kamacite grain is easily seen, along with the interface layer bordering this grain. The majority of the displayed volume here is the fine dual-phase microstructure. Figure 7 shows an animation of the entire 3D volume.
Once the 3D data has been collected and reconstructed, it is possible to identify and select specific grains of interest for analysis. In Figure 8, I selected a kamacite plate, and in Figure 9 I selected the interface grain between the larger and smaller microstructures. The 3D grain information is able to show how these different plates grow and fit together, like pieces of a complex jigsaw puzzle.
Other regions of the Gibeon meteorite show interesting inclusions that can be analyzed with both EBSD and simultaneously collected EDS data. Figure 10 shows a large area Image Quality map with two visible inclusions. Figure 11 shows a composite EDS map, where the Iron signal is colored Blue, the Nickel signal is colored green, and the Sulfur signal is colored Red. The inclusion exhibits a Sulfur zoning profile, and can be correlated with the orientation microstructure.
Finally, this work has focused on results from one specific meteorite, the Gibeon meteorite. As Shawn would tell us, there are many, many more, which tell interesting tales about the Universe we live in. Figure 12 shows an EBSD map from a meteorite section we analyzed at the University of Arizona. This meteorite is primarily enstatite. I’m excited to learn what this microstructure has to tell us in their future work!