Transmission 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)

Resolving Matters

Dr. René de Kloe, Applications Specialist, EDAX

Every now and then a new critical parameter in EBSD analysis comes up. This parameter is often related to a new trend in materials science research or industrial development. One of the latest buzz-words is “nano”. Things are getting smaller and smaller and EBSD technology has to keep up to be able to provide the microstructural answers. This drive to investigate the tiniest details led, for example, to the emergence of transmission EBSD. But it seems that not everyone means the same thing when they talk about nano; Where does nano begin and where does it end? For example, is “nano” anything smaller than 1 micron, or anything below 100 nm, or perhaps only things smaller than 10 nm? The answer is of course: all of them. The feature scale really depends on your application and field of science. For example in natural geological materials, grain sizes smaller than 1 micron are not very common and such rocks could be described as nano-structures. But in semiconductor or photovoltaic applications nano may range from single atomic layers to perhaps 100 nm.

In the EBSD community the emergence of nano-analysis has sprouted the use of another buzz-word: ‘resolution’. But exactly as with nano, ‘resolution’ can have several different meanings depending on who you talk to. So let’s start with the basics, what is resolution?

resolution
Pronunciation: /rɛzəˈluːʃ(ə)n/
The smallest interval measurable by a telescope or other scientific instrument; the resolving power.
The degree of detail visible in a photographic or television image
A firm decision to do or not to do something
The action of solving a problem or contentious matter
(from http://www.oxforddictionaries.com/definition/english/resolution)

Of these definitions the first two have obvious relevance to EBSD analysis, but at the same time define something completely different. The first one deals with the feature size on the sample, whose limits are defined by the combination of SEM beam settings and physical sample properties. The second one describes the amount of detail observable in a diffraction pattern collected from an area of a sample, which is affected by the number of pixels available on the imaging sensor. These two definitions are easily confused, but are not directly related.

And of course there is a third definition of resolving something which is a well-known strength of EBSD. It is the power to resolve the difference between different phases. For example here is a geological material, a granitic rock with several minerals with low-symmetry crystal structures. In order to resolve the phase differences, the analysis of a rock like this does not require any high resolution settings. A detector resolution of only 120 pixels and step size of 0.5 micron was sufficient.


But things are not always what they seem at first glance as is illustrated by the sequence of images below. All these maps are collected with the same low detector resolution of only 80 pixels, but with very different spatial resolution, or step size, on the sample. The sample is a piece of the Gibeon meteorite, an iron meteorite that was discovered in Namibia in 1836. The structure is very coarse grained and the individual grains can easily be seen with the naked eye.

But looks can be deceiving. My first attempt to characterize the structure resulted in a speckled map that looked pretty bad and made me doubt my polishing skills. Subsequent maps with higher and higher spatial resolution (i.e. smaller step size) started to resolve a fine-grained structure with many grains being smaller than 100 nm. So for this example, high spatial resolution maps were obtained using low resolution camera settings.

For phase identification and characterization of deformation microstructures the pixel resolution of the detector becomes more important, but within reason. For phase identification you want to be able to identify small details in a pattern and this is also helpful when you are looking at small orientation changes due to dislocation structures in a material. This biotite pattern displays a pseudosymmetry where the difference between similar orientations is defined by the position of some relatively weak bands in the pattern. The correct indexing result is outlined in red.

Left: Biotite patterns with pseudosymmetric indexing results – red pattern is correct
Right: Kernel average misorientation map in deformed iron alloy illustrating precise locations of subgrain boundaries

In such cases, what you need to identify the difference between these orientations is having enough pixels in the band pattern across the weaker and thinner bands to be able to detect them automatically. Therefore the band detection capabilties dictate the resolution that you need to resolve the difference between these two orientations and not the number of pixels that you have available on your EBSD detector. Typically a pattern of 200 pixels is sufficient on any material. And that resolution is also enough to be able to measure orientation changes down to 0.05 degrees, which allows accurate identification of subgrain structures as shown in the kernel misorientation map.

If even smaller orientation changes are the target of your analysis or if you want to measure minute shape changes of the crystal lattice due to elastic strains, the standard band detect routines are insufficient. For such analyses you would need to use a pattern with more pixels and a dedicated technique based on cross correlation of sections of the diffraction patterns. For this tool, using many more pixels would appear to be better. But keep in mind that other variables in the system geometry such as the exact detector positioning, signal-to-noise level, lens artifacts, or pattern center calibration errors can introduce uncertainties that may easily exceed the improvements gained by using more pixels. In practice, patterns with 480 pixels or more have been used successfully. What you see is not always what you get.

The above examples have highlighted a number of different uses of the word resolution and it appears that you have to be pretty careful in describing what you really want to accomplish. Going for a high resolution camera will not give you better spatial resolution in your EBSD maps. Similarly low resolution maps may be measured using a high resolution camera which shows that the number of pixels on your detector is totally independent of the spatial resolution that may be obtained on your samples.

Therefore to conclude I would like to appeal to the last definition of resolution mentioned above: The action of solving a problem or contentious matter.
With the different meanings of resolution clouding the waters and creating confusion I would like to propose the use of “high resolution EBSD ” to describe the application of mapping with small step-size to resolve the fine details in a material. “High precision EBSD” would then describe the application to map out small orientation changes in a material in order to investigate and understand the deformation mechanisms and finally “high definition EBSD” to describe the technique of investigating minute changes in pattern geometry to characterize (elastic) strains in the crystal lattice by applying cross-correlation methods.

My $0,02

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 …