Month: May 2019

This Time of Year

Matt Nowell, EBSD Product Manager, EDAX

My wife is a schoolteacher, and 2 of my boys are still at home in 6th and 8th grade, so at this time of year we start counting the number of days of school left and looking forward to summer vacation. This year is especially poignant to me, as my 8th grader is finishing 8th grade and graduating up to high school. Of course, this time of year is the time of graduations, and makes me think back to my graduation. I graduated in 1995, and immediately went to work at TSL. If my math is correct, that means it’s been 24 years directly involved in EBSD and the microanalysis world.

As my wife would tell you, I’m not always the best planner. I got into materials science because I was curious about making fishing rods. That dream lasted until an organic chemistry class and developing a fear of memorizing molecules. Luckily, I was able to work in the university Electron Microscopy lab as a student, where I had a great opportunity to learn to operate microscopes. That lead to the opportunity at TSL where they wanted someone with a material science background (check), SEM experience (check), and who wanted to travel (I’m close to a million miler so while I won’t claim to love travel, I’ve held up my end of the deal). I certainly didn’t know I’d still be working at the same place 24 years later, but it’s been an enjoyable ride watching the growth and development of the EBSD technique from this perspective.

In reminiscing, it’s easy to recognize all the changes that have occurred in this time: email, iPhones, faster EBSD cameras. I found it interesting though, there are several fundamental truths that really haven’t changed. For this blog, I thought I’d share some examples.

You can communicate a lot about a microstructure with an IPF orientation map. They say a picture is worth a thousand words, and I firmly believe one of the driving forces behind the growth of EBSD is the pretty maps that can be created. An IPF map is colorful of course, with the colors representing crystallographic orientations relative to some sample reference frame, but it also often communicates information about the grain size and shape, any internal deformation, and preferred orientation (or texture).

IPF map of ECAP deformed copper

Sample preparation is still key for EBSD. EBSD is a diffraction technique, so for it to be effective, we need diffraction to occur. The surface layers of the crystal must be in a condition to diffract for the EBSD pattern to form. This is often a hard one to accept. We look at hardware and software design to try to make as much of the EBSD workflow as easy as possible, but we don’t have control over this aspect of the process. I had to learn the importance of this too. My training in metallographic preparation was minimal. I did lots of experimentation with mechanical polishing and electropolishing in my first few years just to try and get a pattern. One of my favorite memories is buying a Pyrex baking dish and a tablecloth to hand polish a sample with colloidal silica for an important demo. We got patterns and we got the sale. I’d like to say we then made a cake, but we probably just went out for pizza instead.

Training courses are learning experiences for both the teachers and the students. I’ve been in the EDAX Mahwah home office this week, and the discussion of being an expert came up. There is plenty I know about EBSD, but also plenty I don’t really have good answers for. As I come back to certain questions over and over, I find myself refining my thoughts, perhaps getting closer to a solution I’m satisfied with. New users though, evaluate those answers through a different perspective. I still enjoy when I can help someone else learn and apply a new concept, and enjoy the process of learning to teach these concepts better.

Travel is always interesting. As I mentioned, when I signed up, I knew some travel would be involved. My initial job offering listed customers in London, Paris, and Australia. Suffice it to say, these were not the first places that I visited. I love seeing new places, exploring the sites, and meeting new people. As for the travel process, I’m grateful for in-flight WiFi and on-demand movies to help pass the time. I haven’t forgotten the days of waiting for the movie to start or having to pack 4-5 books to last through my trip. I’ve been invited over to the home of graduate students for homemade meals, fallen asleep on a Japanese train and woken up without knowing where I was and which direction I was going, been stuck in an elevator between floors, accidentally winning a trick-shot pool competition by leaving a dent in a wall of a customer’s basement (and having the trophy to show for it), been questioned by the Prague transit patrol, and had my wallet stolen in Paris. Sometimes trips are memorable, sometimes the hotel rooms blend all together, but it’s always interesting.

Tricky Shot Trophy

There are many more I can share, and I’m sure many more that will become more visible as my career continues to progress. I take great pleasure in sharing my acquired knowledge of EBSD with customers and seeing them learn and apply this to further their work. I hope everyone has a wonderful summer.

Back to Basics

Dr. René de Kloe, Applications Specialist, EDAX

When you have been working with EBSD for many years it is easy to forget how little you knew when you started. EBSD patterns appear like magic on your screen, indexing and orientation determination are automatic, and you can produce colourful images or maps with a click of a mouse.

Image 1: IPF on PRIAS™ center EBSD map of cold-pressed iron powder sample.

All the tools to get you there are hidden in the EBSD software package that you are working with and as a user you don’t need to know exactly how all of it happens. It just works. To me, although it is my daily work, it is still amazing how easy it sometimes is to get high quality data from almost any sample even if it only produces barely recognisable patterns.

Image 2: Successful indexing of extremely noisy patterns using automatic band detection.

That capability did not just appear overnight. There is a combination of a lot of hard work, clever ideas, and more than 25 years of experience behind it that we sometimes just forget to talk about, or perhaps even worse, expect everybody to know already. And so it is that I occasionally get asked a question at a meeting or an exhibition where I think, really? For example, some years ago I got a very good question about the EBSD calibration.

Image 3: EBSD calibration is based on the point in the pattern that is not distorted by the projection. This is the point where the electrons reach the screen perpendicularly (pattern center).

As you probably suspect EBSD calibration is not some kind of magic that ensures that you can index your patterns. It is a precise geometrical correction that distorts the displayed EBSD solution so that it fits the detected pattern. I always compare it with a video-projector. That is also a point projection onto a screen at a small angle, just like the EBSD detection geometry. And when you do that there is a distortion where the sides of the image on the screen are not parallel anymore but move away from each other. On video projectors there is a smart trick to fix that: a button labelled keystone correction which pulls the sides of the image nicely parallel again where they belong.

Image 4: Trapezoid distortion before (left) and after (right) correction.

Unfortunately, we cannot tell the electrons in the SEM to move over a little bit in order to make the EBSD pattern look correct. Instead we need to distort the indexing solution just so that it matches the EBSD pattern. And now the question I got asked was, do you actually adjust this calibration when moving the beam position on the sample during a scan? Because otherwise you cannot collect large EBSD maps. Apparently not everybody was doing that at that time, and it was being presented at a conference as the invention of the century that no EBSD system could do without. It was finally possible to collect EBSD data at low magnification! So, when do you think this feature will be available in your software? I stood quiet for a moment before answering, well, eh, we actually already have such a feature that we call the pattern centre shift. And it had been in the system since the first mapping experiments in the early 90’s. We just did not talk about it as it seemed so obvious.

There are more things like that hidden in the software that are at least as important, such as smart routines to detect the bands even in extremely noisy patterns, EBSD pattern background processing, 64-bit multithreading for fast processing of large datasets, and efficient quaternion-based mathematical methods for post-processing. These tools are quietly working in the background to deliver the results that the user needs.
There are some other original ideas that date back to the 1990’s that we actually do regularly talk about, such as the hexagonal scanning grid, triplet voting indexing, and the confidence index, but there is also some confusion about these. Why do we do it that way?

The common way in imaging and imaging sensors (e.g. CCD or CMOS chips) is to organise pixels on a square grid. That is easy and you can treat your data as being written in a regular table with fixed intervals. However, pixel-to-pixel distances are different horizontally and diagonally which is a drawback when you are routinely calculating average values around points. In a hexagonal grid the point-to-point distance is constant between all neighbouring pixels. Perhaps even more importantly, a hexagonal grid offers ~15% more points on the same area than a square grid, which makes it ideally suited to fill a surface.

Image 5: Scanning results for square (left) and hexagonal (right) grids using the same step size. The grain shape and small grains with few points are more clearly defined in the hexagonal scan.

This potentially allows improvements in imaging resolution and sometimes I feel a little surprised that a hexagonal imaging mode is not yet available on SEMs.
The triplet voting indexing method also has some hidden benefits. What we do there is that a crystal orientation is calculated for each group of three bands that is detected in an EBSD pattern. For example, when you set the software to find 8 bands, you can define up to 56 different band triangles, each with a unique orientation solution.

Image 6: Indexing example based on a single set of three bands – triplet.

Image 7: Equation indicating the maximum number of triplets for a given number of bands.

This means that when a pattern is indexed, we don’t just find a single orientation, we find 56 very similar orientations that can all be averaged to produce the final indexing solution. This averaging effectively removes small errors in the band detection and allows excellent orientation precision, even in very noisy EBSD patterns. The large number of individual solutions for each pattern has another advantage. It does not hurt too much if some of the bands are wrongly detected from pattern noise or when a pattern is collected directly at a grain boundary and contains bands from two different grains. In most cases the bands coming from one of the grains will dominate the solutions and produce a valid orientation measurement.

The next original parameter from the 1990’s is the confidence index which follows out of the triplet voting indexing method. Why is this parameter such a big deal that it is even patented?
When an EBSD pattern is indexed several parameters are recorded in the EBSD scan file, the orientation, the image quality (which is a measure for the contrast of the bands), and a fit angle. This angle indicates the angular difference between the bands that have been detected by the software and the calculated orientation solution. The fit angle can be seen as an error bar for the indexing solution. If the angle is small, the calculated orientation fits very closely with the detected bands and the solution can be considered to be good. However, there is a caveat. What now if there are different orientation solutions that would produce virtually identical patterns? This may happen for a single phase where it is called pseudosymmetry. The patterns are then so similar that the system cannot detect the difference. Alternatively, you can also have multiple phases in your sample that produce very similar patterns. In such cases we would typically use EDS information and ChI-scan to discriminate the phases.

Image 8: Definition of the confidence index parameter. V1 = number of votes for best solution, V2 = mumber of votes for 2nd best solution, VMAX= Maximum possible number of votes.

Image 9: EBSD pattern of silver indexed with the silver structure (left) and copper structure (right). Fit is 0.24″, the only difference is a minor variation in the band width matching.

In both these examples the fit value would be excellent for the selected solution. And in both cases the solution has a high probability of being wrong. And that is where the confidence index or CI value becomes important. The CI value is based on the number of band triangles or triplets that match each possible solution. If there are two indistinguishable solutions, these will both have the same number of triangles and the CI will be 0. This means that there are two or more apparently valid solutions that may all have a good fit angle. The system just does not know which of these solutions is the correct one and thus the measurement is rejected. If there is a difference of only 10% in matched triangles between alternative orientation solutions in most cases the software is capable of identifying the correct solution. The fit angle on its own cannot identify this problem.

After 25 years these tools and parameters are still indispensable and at the basis of every EBSD dataset that is collected with an EDAX system. You don’t have to talk about them. They are there for you.