t-EBSD

Space Rocks Rock!

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.

Figure 1 – Large area EBSD map of Gibeon meteorite.

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.

Figure 2 – 100X EBSD map of Gibeon meteorite.

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.

Figure 3 – 1,000X EBSD map of Gibeon meteorite.

Figure 4 – 1,000X EBSD Phase map of Gibeon meteorite.

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.

Figure 5 – 8,000X EBSD map of Gibeon meteorite.

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.

Figure 6 – 3D EBSD data from Gibeon meteorite.

 

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.

Figure 8 – 3D Kamacite lamella grain.

Figure 9 – 3D interface in Gibeon meteorite.

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.

Figure 10 – EBSD Image Quality map of inclusion in Gibeon meteorite.

Figure 11 – Composite EDS map of inclusion in Gibeon meteorite.

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!

Figure 12 – EBSD map from meteorite (primarily Enstatite phase)

Notes from Madison: Atom Probe Tomography Users’ Meeting

Dr. Katherine Rice, Applications Scientist at CAMECA Instruments, Inc.

Dinner at the top of the Park with a view of the Wisconsin State Capitol

The Terrace at the University of Wisconsin

Last week was a great week up here in Madison for our bi-annual users’ meeting, with about 90 atom probe enthusiasts making the trek to Madison, WI to discuss the finer points of atom probe tomography (APT).   There were plenty of great sessions involving, for example, correlative microscopy, cryo-atom probe, and new ways to detect evaporated ions.  Lest anyone think that we are too serious up here in Wisconsin, we also enjoyed talks on atom probing rodent teeth and even beer, as well as having several social events where our attendees could sample local brews.

Demo attendees watching a map being taken

Many of the users have been implementing transmission EBSD (or TKD, as some folks prefer) on their needle-shaped atom probe specimens which are typically shaped by a focused ion beam (FIB) microscope.  This allows for identification of any grain boundaries present, and also helps position a grain boundary close to the specimen apex so there is a good chance it will be captured in an APT analysis.  Atom probe specimens usually have a radius of ~100 nm which makes them ideally sized for transmission EBSD at SEM voltages between 20-30 kV.   The users’ group meeting also marked another special event:  the debut of Atom Probe Assist (APA) mode in the TEAM™ software.  Transmission EBSD can be challenging, but APA mode makes the analysis faster and easier by implementing recipes for background subtraction developed by EDAX and by skipping mapping of areas not intercepted by the specimen.  We had about 20 users at the Tuesday demos of APA mode and another few at an additional demo on Friday.  CAMECA’s Dr. Yimeng Chen manned the FIB and quickly targeted a grain boundary for FIB milling while our EDAX friend Dr. Travis Rampton took maps after each milling step to make sure the grain boundary was contained in the specimen.

Yimeng Chen and Travis Rampton present a poster.

Sample holders that work well for t-EBSD and FIB were also on debut at the meeting.  Many of CAMECA’s atom probe users mount up each specimen to our Microtip coupons, which are 3 mm X 5 mm pieces of Si that hold 22 flat topped posts.  Our Microtip Holder (affectionately nicknamed the Moth) was developed to do transmission EBSD on each of 22 mounted specimens, and then transfer the stub portion directly into the atom probe.  Even if you don’t do APT, these microtip posts are a convenient way to mount multiple thin samples for transmission EBSD.

The moth sample holder containing a microtip coupon

It was incredible to see the explosion of transmission EBSD for atom probe, and the cool things that many LEAP users are discovering when they try it out on their atom probe samples.  Perhaps the greatest strength of this technique is how easy and integrated it is in the atom probe specimen preparation process.  You don’t even need to move your sample or the camera between steps when you are shaping a liftout wedge into a specimen that is atom probe ready.  I look forward to hearing about the new applications that are being discovered when combining t-EBSD and APT!

TRANSMISSION-EBSD – Taking Transparency to a New Level.

Dr. René de Kloe, Applications Specialist, EDAX

EBSD on electron transparent samples is quickly becoming the next big thing in orientation measurement techniques. In the literature you may find it described as TKD (Transmission Kikuchi Diffraction) or t-EBSD and there is actually a bit of a debate going on about what the proper name is for the technique, because calling a forward scattered transmission technique by a backscattered diffraction acronym may seem a bit incongruous. Personally I like the name transmission-EBSD best as that name describes exactly what the technique does: getting diffraction patterns in a transmission geometry that may be analyzed with a conventional EBSD system. And I think that a good descriptive name that people will recognize is worth a little incoherence. TKD for me points a little too much to a TEM imaging mode that many people may not recognize as a SEM-based technique.

Figure 1: schematic t-EBSD measurement geometry

Anyway, knowing what a name means does not necessarily describe what you can actually do with a technique. A few years ago t-EBSD started to gain popularity due to the improved spatial resolution that it offers. For standard EBSD on bulk samples the lateral resolution is mainly determined by the formation depth of a virtual point source in the sample combined with the effective beam diameter. Typically the resolution ranges from about 15 to perhaps 75nm. One of the parameters that affects the resolution in standard EBSD is the 70 degree tilt of the analysis surface as that causes the intersection of the beam with the sample to become an ellipse that is three times higher than wide. When you collect EBSD patterns in a transmission geometry the parameters that govern the effective lateral resolution are different: the accelerating voltage, the specimen foil thickness, and material density, all of which together define the effective beam diameter at the point of exit from the sample. And that corresponds to the obtainable lateral resolution. The beam shape is less important. When the sample is mounted horizontally the beam-sample intersection is circular, but even at a -40 degree tilt, the aspect ratio is still only 1 : 1.3. And that is a nice bonus as the intensity distribution on the EBSD detector is much more homogeneous at -20 to -40 degree tilt which allows for excellent indexing performance.

Figure 2: Copper t-EBSD pattern and high resolution mapping result with 2.5 nm steps, scalebar is 100 nm

The smaller information volume will give the user a significant resolution improvement which allows successful orientation analysis of previously inaccessible nanomaterials and extremely deformed samples. But that is not all you can do with t-EBSD. It also opens up a host of combined applications where electron transparent samples are involved. The most obvious to me is looking at TEM specimens before doing any detailed work in the transmission electron microscope.

Figure 3: SEM SE image of an electro-polished TEM foil of AlTi3-TiAl3 alloy. The grain microstructure is not easily recognizable. (specimen courtesy of Klemens Kelm – DLR Cologne, Germany).

Many imaging modes in materials science TEM analysis such as weak beam dark field dislocation analysis depend on specific grain orientations. Finding these orientations or crystallographic zone axes required for certain diffraction conditions in the TEM can be very time consuming, and thus expensive. Furthermore not all grains that you try to orientate may actually have the desired zone axis within the tilting range of your specimen holder. And when that happens all the work and time spent to find the orientation for that particular grain has been wasted. This is normally not too bad for highly symmetric cubic materials, but it will become a challenge on for example hexagonal Ti and Mg alloys and many low symmetry ceramics and minerals that have a limited number of “usable” zones.

And this is where t-EBSD can help as identifying orientations is exactly what EBSD is designed to do for you and rapidly too. Just imagine that you can analyze your sample with low resolution t-EBSD to prepare a full orientation map of all electron transparent areas so that you know which grains are usable for your diffraction analysis before going to the TEM.

Figure 4: SEM image of the electro-polished TEM foil and corresponding EBSD IPF crystal direction map of the entire electron transparent area

When you want to do this there are a few things to keep in mind. First, the EBSD pattern quality varies strongly with specimen thickness and you will need some special background processing to obtain indexable patterns.

Figure 5: TEM sample overview with raw and processed patterns over a range of foil thickness

Also, EBSD patterns are projected more or less sideways onto the EBSD detector and not straight down as in the TEM. So if you want to use the EBSD indexing results to identify specific zone axes that would be accessible in the TEM you cannot just look at the crystallographic zones that are visible in the pattern. You always need to rotate a simulation of the measured pattern to the TEM geometry to identify the zones within a certain angular distance to the optical axis.

   
Figure 6: Actual EBSD pattern after special background processing with indexing result and pattern simulation. The corresponding TEM projection can then be obtained by rotating the simulation. When the tilting range of the used TEM specimen holder is now superimposed on the rotated simulation, the zones that are within tilting range may be determined together with the required rotation angles.

When the required zones are not generally accessible in all grains, you can use the full EBSD map of the sample and generate a crystal orientation or crystal direction map to identify which grains can be used for your TEM diffraction analysis.

An important thing that still remains is the correlation between the SEM and the TEM images. And for that purpose a series of dark-field STEM images may be collected using the PRIAS imaging mode. From this sequence of images a custom combination can be selected to display the best grain contrast to help navigating the sample in the TEM.

Figure 7: Left: PRIAS center image of the analysis area displaying strong orientation contrast. Right: Superimposed crystal direction map of the AlTi3 phase only showing grains that have the [111] direction within 20°of the optical axis

The application described above is just one example of the use of t-EBSD to assist another technique and I am sure that many more hybrid t-EBSD applications will be found. So if you know of other applications where EBSD or t-EBSD may be used to facilitate or improve materials analysis on other instruments please send me a short message so we can all save a bit of time. I cannot help but wish that I had known about this one when I was doing my own TEM diffraction analysis for my PhD thesis. It might have saved me quite a few weekends being hidden away in a dark room …