EBSD

Seeing is Believing?

Dr. René de Kloe, Applications Specialist, EDAX

A few weeks ago, I participated in a joint SEM – in-situ analysis workshop in Fuveau, France with Tescan electron microscopes and Newtec (supplier of the heating-tensile stage). One of the activities during this workshop was to perform a live in-situ tensile experiment with simultaneous EBSD data collection to illustrate the capabilities of all the systems involved. In-situ measurements are a great way to track material changes during the course of an experiment, but of course in order to be able to show what happens during such an example deformation experiment you need a suitable sample. For the workshop we decided to use a “simple” 304L austenitic stainless-steel material (figure 1) that would nicely show the effects of the stretching.

Figure 1. Laser cut 304L stainless steel tensile test specimen provided by Newtec.

I received several samples a few weeks before the meeting in order to verify the surface quality for the EBSD measurements. And that is where the trouble started …

I was hoping to get a recrystallized microstructure with large grains and clear twin lamellae such that any deformation structures that would develop would be clearly visible. What I got was a sample that appeared heavily deformed even after careful polishing (figure 2).

Figure 2. BSE image after initial mechanical polishing.

This was worrying as the existing deformation structures could obscure the results from the in-situ stretching. Also, I was not entirely sure that this structure was really showing the true microstructure of the austenitic sample as it showed a clear vertical alignment that extended over grain boundaries.
And this is where I contacted long-time EDAX EBSD user Katja Angenendt at the MPIE in Düsseldorf for advice. Katja works in the Department of Microstructure Physics and Alloy Design and has extensive experience in preparing many different metals and alloys for EBSD analysis. From the images that I sent, Katja agreed that the visible structure was most likely introduced by the grinding and polishing that I did and she made some suggestions to remove this damaged layer. Armed with that knowledge and new hope I started fresh and polished the samples once more. And I had some success! Now there were grains visible without internal deformation and some nice clean twin lamellae (figure 3). But not everywhere. I still had lots of areas with a deformed structure and whatever I tried I could not get rid of those.

Figure 3. BSE image after optimized mechanical polishing.

Back to Katja. When I discussed my remaining polishing problems she helpfully proposed to give it a try herself using a combination of mechanical polishing and chemical etching. But even after several polishing attempts starting from scratch and deliberately introducing scratches to verify that enough material was removed we could not completely get rid of the deformed areas. Now we slowly started to accept that this deformation was perhaps a true part of the microstructure. But how could that be if this is supposed to be a recrystallised austenitic 304L stainless steel?

Table 1. 304/304L stainless steel composition.

Let’s take a look at the composition. In table 1 a typical composition of 304 stainless steel is given. The spectrum below (figure 4) shows the composition of my samples.

Figure 4. EDS spectrum with quantification results collected with an Octane Elite Plus detector.

All elements are in the expected range except for Ni which is a bit low and that could bring the composition right at the edge of the austenite stability field. So perhaps the deformed areas are not austenite, but ferrite or martensite? This is quickly verified with an EBSD map and indeed the phase map below confirms the presence of a bcc phase (figure 5).

Figure 5. EBSD map results of the sample before the tensile test, IQ, IPF, and phase maps.

Having this composition right at the edge of the austenite stability field actually added some interesting additional information to the tensile tests during the workshop. Because if the internal deformation in the austenite grains got high enough, we might just trigger a phase transformation to ferrite (or martensite) with ongoing deformation.

Figure 6. Phase maps (upper row) and Grain Reference Orientation Deviation (GROD) maps (lower row) for a sequence of maps collected during the tensile test.

And that is exactly what we have observed (figure 6). At the start of the experiments the ferrite fraction in the analysis field is 7.8% and with increasing deformation the ferrite fraction goes up to 11.9% at 14% strain.

So, after a tough start the 304L stainless steel samples made the measurements collected during the workshop even more interesting by adding a phase transformation to the deformation. If you are regularly working with these alloys this is probably not unexpected behavior. But if you are working with many different materials you have to be aware that different types of specimen treatment, either during preparation or during experimentation, may have a large influence on your characterization results. Always be careful that you do not only see what you believe, but ensure that you can believe what you see.

Finally I want to thank the people of Tescan and Newtec for their assistance in the data collection during the workshop in Fuveau and especially a big thank you to Katja Angenendt at the Max Planck Institute for Iron Research in Düsseldorf for helpful discussions and help in preparing the sample.

EBSD Detection Future Seems Bright and Fast

Dr. Patrick Camus, Director of Engineering, EDAX

EBSD sensors have changed as camera technologies have evolved. They began as intensified video cameras, then improved to scientific grade CCD cameras. These have provided both the high pixel count and high speed needed for EBSD acquisition requirements. Typical sensor sizes exceed 1M pixels at 10 fps for pattern quality applications and high-binning sensors are used for 1,500+ fps mapping applications at beam currents <=10 nA.

Newer technologies are appearing from imaging research labs that have similar pixel resolutions but significantly higher fps values. Unfortunately, the 1st generation of these devices do not have quite the sensitivity of the older devices, but each journal publication shows improvements.

  • R N Clough, et al, “Direct Detectors for Electron Microscopy”, 2014 J. Phys.: Conf. Ser. 522 012046
  • R. Clough, et al, “Direct Digital Electron Detectors” in Advances in Imaging and Electron Physics, Volume 198 # 2016 Elsevier Inc. ISSN 1076-5670
  • Angus J. Wilkinson, et al, “Direct Detection of Electron Backscatter Diffraction Patterns”, PRL 111, 065506 (2013)
  • S. Vespucci, et al, “Direct electron imaging of EBSD patterns using a CMOS hybrid pixel detector”, RMS-EBSD Workshop 2013
  • K.P. Mingard, et al, “Practical Application of Direct Electron Detectors to EBSD Mapping in 2D and 3D”, Ultramicroscopy (2017)
  • Angus Kirkland, “A Detector Revolution: Direct Silicon Detectors for Electron Microscopy”, EMAS 2017

Recent devices under test are providing binned patterns at 3,000 indexed fps, which provide 99% indexing quality but require a higher beam current of 40 nA. When performing higher resolution imaging at low beam energies (<=5 keV), less than 1 nA is required, which is significantly lower than previous devices, for full camera resolution at 100 fps.

5 kV EBSD pattern at 100 pA
5 kV EBSD pattern at 100 pA

OIM Map collected at 3,000 fps and 40 nA providing 99% indexing quality
OIM map collected at 3,000 fps and 40 nA providing 99% indexing quality

Many interesting developments are occurring with EBSD sensors. My colleagues will be reporting on those findings in the coming months.

EBSD in China

Sophie Yan, Applications Engineer, EDAX

EBSD in China is a big topic and it may sound as though I am not qualified to judge or to summarize the current situation. However, as I have worked with EBSD applications for several years, I have personal experience to share. More than ten years ago, I didn’t know about EBSD when I was studying the microstructure of materials. I was in Shanghai at that time and the environment was kind of open. It is probably that at that time in China: very few people knew about EBSD. Today the situation has changed enormously after just after 10+ years. Most researchers now try to put EBSD on their microscope. Microscopes including EDS and EBSD capability are standard in Chinese universities.

As an Applications Engineer, I visit research organizations, companies, and factories. I meet customers from many different backgrounds. Some of them are experts but more are new to microanalysis, especially students from science and engineering universities. They may each have a different focus, but they all have high expectations of EBSD. The professors care about the functions which can solve their issues. If there is currently no such function, then they often ask if we can add it. Entry level users prefer to learn how to operate the microscope and detectors quickly so that they get their results as soon as possible. The most frequent question asked is, what can EBSD do? Then I begin my introduction and I see that they become more and more interested. Sometimes they have high expectations. For example, when I demonstrate stress/strain analysis, I am often asked how to get stress value. This is a common misunderstanding because as an indirect way technique, EBSD can show the strain trend of materials, but it is beyond it to measure stress value.

My routine work includes introduction and training. Over a period of time, I can see a newcomer becoming more experienced and getting his own results, which makes me proud as a supporter. Whereas I care about the EBSD technology itself, the customers are more interested in learning how to use it in their work to solve some of their analysis challenges. They often give me new ideas and make me aware of other areas besides pure technology, for example, how to remove the users’ initial fear for EBSD. As a student majoring in material science I thought crystallography was very different from the reality I now understand. As a ‘teacher’ I am not focused on how to keep our users’ interest on EBSD and reminding to them to use it regularly. Fortunately, social media has improved the speed and consistency of our communication. When issues are solved quickly, people think the EBSD technique is less difficult. Effective communication contributes to the technology transfer.

The level of adoption of EBSD hardware in China is excellent, but the usage of and research into the technique is still in its infancy. I have spoken to many people about this issue. The interesting thing is that outsiders tend to give an optimistic perspective. An Australia professor told me several years ago that we should be taking a longer-term view and that there would probably be, a tremendous change in the next ten years. Quantitative results make a qualitative change. I hope he is right!

Fortunately, EBSD usage in China has increased greatly and continues to increase, which shows us a promising future.

 

 

 

Avoid a Distorted View

Dr. Stuart Wright, Senior Scientist EBSD, EDAX

In the world of “fake news” and “alternative facts”, it is important that we dig a little deeper than the headlines to understand the world around us and to avoid a distorted view those in power often want to give us. Ironically, the same is true at the microscale. I recently ran into some work concerning the effects of sample prep on x-ray measurements. It made me reflect on some early work we did to explore the effects of sample prep on EBSD results.

In order to prepare EBSD samples properly it is important to understand that surface finish is not the whole story. It is important that the layer of material sampled by EBSD be distortion free. Charts shown in many metals preparation handbooks clearly show that there can be significant deformation imparted into the sub-surface of a material during preparation. Consider the following chart adapted from a figure in a classic EBSD sample preparation paper: D. Katrakova & F. Mücklich (2001) “Specimen preparation for electron Backscatter Diffraction. Part I: Metals” Praktische Metallographie. 8:547-65. This plot clearly shows why sample prep for EBSD needs to be meticulous.

My longtime colleague, Matt Nowell, did a nice study comparing by grinding two samples, one ground to 240 grit and one to 1200 grit. He then cross-sectioned these samples and carefully prepared the cross-sectioned surfaces. Matt then did OIM scans on the two surfaces. Using a Kernel Average Misorientation (KAM) map, the degree of deformation in the 240 grit sample is clearly more pronounced that in the 1200 grit sample. Matt and I have always wanted to repeat this measurement for more grits and materials but have never found the time to pursue it again.

Many times, students who have asked me “which grinding and/or polishing steps can I skip?” Or, “how many times can I really use a grinding paper?” (I remember as a student we got one paper for each grit for the semester and we would hang them from a wire with clothes pins in the sample prep lab!). Or, “can’t I just do the final grinding step for a longer time and skip the coarser grinding steps?” One thing we’ve learned on our own and in conversations with the sample prep vendors is that the recipes developed with several steps for what intuitively may feel like short times really are the steps that lead to the best results -basically confirming the plot shown above.

The improvement in cameras, image processing and particularly NPAR™ should not be used as an excuse to take shortcuts in sample prep. While it may be possible to get patterns and reasonable maps, are you really looking at the representative microstructure of interest or a distorted version resulting from deformation induced by sample prep?

I believe EBSD has had a positive impact on the metallography community. EBSD has forced us to be more careful in sample preparation over that typically done for light microscopy or even scanning electron microscopy. Hopefully that extra care has resulted in more representative microstructural characterization.

Seeing the World a Little Differently.

Jonathan McMenamin, Marketing Communications Specialist, EDAX

When I started at EDAX five years ago, I knew very little about materials analysis. My education was in Management Information Systems and Computer Science and my work experience came from spending eight years in the Sports Information department at Rowan University. Little by little, I have learned more about the various analysis techniques and feel comfortable enough to write this blog.

One of the first things that caught my eye at EDAX was a series of maps generated from Energy Dispersive Spectroscopy (EDS) and Electron Backscatter Diffraction (EBSD) analysis. The vibrant colors and patterns are very beautiful and almost look like art. Lately, I have noticed objects in everyday life that remind me of these maps.

This past July, my wife and I and our friend took a trip to Ireland. We visited Slieve League in county Donegal, one of the highest sea cliffs in Europe (1,998 feet from the highest point). We decided to hike up the cliffs for a bit and on our way up the rocky pathway on this rainy, foggy day, I came across a large rock that grabbed my attention. It was covered in a pattern that reminded me of an EBSD map showing grain boundaries. I quickly snapped a few photos (below) to show to our EBSD product manager, Matt Nowell when I returned.

Photos of a rock taken at Slieve League in Donegal, Ireland.

A few weeks later, I was at a restaurant in the St. Louis Lambert International Airport having dinner with a few of my coworkers following a successful Microscopy & Microanalysis (M&M) show. While we were waiting for the waitress to return with our food, I looked up at the light hanging over the table next to ours and noticed that it resembled an EBSD pattern. I found another example of a beautiful glass piece when I was decorating my Christmas tree with my wife a few weeks ago. My wife’s grandmother and aunt give her gorgeous hand-blown glass ornaments from Cape Cod every year. As I was hanging one on the tree, I took a photo and explained to her that it looked like the maps we produce at work.

Light in a restaurant at the St. Louis Lambert International airport. One of my family’s hand-blown glass Christmas ornaments.

Ever since I was little, I have had a fascination with the ocean and sharks in particular. One of my favorite species is the Rincodon typus, more commonly known as a whale shark. It is not only the largest living nonmammalian vertebrate, but the whale shark has a very particular pattern of pale yellow spots and stripes on its skin. When I was putting together the EDAX Interactive Periodic Table of Elements (http://www.edax.com/resources/interactive-periodic-table), I came across a map of nickel nanopillars on indium at 3 kV demonstrating low kV microanalysis, and I immediately though it resembled a whale shark.

Rincodon typus, commonly known as a whale shark. Low kV microanalysis: nickel nanopillars on indium at 3 kV.

My final example comes from one of my favorite television shows, The Curse of Oak Island on the History Channel. The show follows a group of men that are in search of treasure that is supposedly buried on a small island off the coast of Nova Scotia, Canada. Several theories exist as to what the treasure is exactly, ranging from Knights Templar hiding sacred religious relics to pirates burying gold and jewels. The group has uncovered many clues and they have found various interesting items including ancient coins, bones from the 1600s, and pieces of wood, ceramic, and paper. On an episode in the current season (season 5), the group discovered a rose-headed spike while metal detecting near the coast line. If I had watched this prior to working at EDAX, I probably wouldn’t have thought anything about it. However, now that I know much more about microanalysis, I immediately thought to myself that they should use EDS analysis to find out what elements the metal was comprised of, to possibly date it based on what materials were used during that period for creating spikes. As it turns out, that is exactly what they did. The team found out that the spike was comprised of 90% iron and 10% carbon with no traces of manganese or sulfur. This showed that it was pre-1840s when manganese was used in metal work and that it was smelted with charcoal before the use of fossil fuels in the 1700s. All pointing to the fact that people were on Oak Island hundreds of years ago.

Rose-headed spike.

The world of microanalysis is extremely interesting and present all around us, you just have to keep your eyes open to see where it pops up in your daily life.

EBSD and the Real World

Shawn Wallace, Applications Specialist, EDAX

One of the powers of EBSD is showing how microstructures are created by the processing of a material and how these microstructures can change the material properties of a sample. Explaining this connection to novice users or potential customers can be difficult. Luckily for me, my sidewalk has given me a perfect example.

It is made up of oriented bricks. Some are placed square side up. Some are placed rectangular side up. But look at the color? Why are some bricks wet while others are dry? The square sides tend to be dry, rectangular still wet.

Now let’s start building up a case as to why this happens.

The first step is understanding how these bricks are made and what they are made of. You take clay, you slap it into a mold. You press the rectangular side to compress it to fill the mold. Fire it and tada, you have a brick. A lot is going on in these steps that you can’t see with the naked eye.

The main thing is that the squeezing step is really having a profound effect on the brick. You are taking randomly oriented platy minerals (Figure 1) and giving them a preferred orientation by squeezing them (Figure 2). It is like a house of cards that has fallen down. You now have grains lying down. Water can’t break through the new “sheets”, but turn the brick on its side and you have pathways to drain the water.

Figure 1. Clay minerals in bricks are often platy in shape. Without any outside forces, the grains are randomly orientated.

FIgure 2. By compressing the material to form a brick shape, the grains are laid flat relative to each other. This is much like EBSD samples with a texture.

This is what you are seeing here. The square bricks have clays that are oriented to wick the water deeper in to the brick, while on the rectangular faces, the water has nowhere to go (Figure 3). Square bricks wick away and are dry, while rectangular faces are still wet.

Figure 3. After the brick is pressed, there are no connecting paths for the water to flow into the material from the top. But from the side, there are channels leading into the bricks. This is what allows the water to wick inside the brick from the square side, but not the rectangular side.

On a high level, this is what EBSD is all about. You are seeing how these processes forming a material are now controlling how the material behaves. For EBSD, these can be electrical, thermal, or mechanical properties, but EBSD is the driving force to truly understanding how and why your material behaves the way it does.

 

Materials Selection While Black Friday Shopping

Matt Nowell, Product Manager EBSD, EDAX

I’m writing this blog the Monday after the Thanksgiving holiday, and having survived a Black Friday shopping adventure that started just a couple of hours after finishing the turkey last Thursday. While waiting in line for the doors to open and the tryptophan to wear off, I worked on plotting a strategy through the store to find a robotic vacuum cleaner, an Amazon Echo, some LED lights for outside, and the latest Minecraft toy for my youngest son. As the clock ticked towards 6 P.M., I felt confident in my plan and ready to go.

When the doors opened, and folks started streaming in, I grabbed a cart. This is always a tricky decision, as it immediately limits your mobility and possible escape routes. However, I knew ironically that the vacuum robot wasn’t going to push himself around quite yet. With cart in hand, I had to take a wider path, so I went a circuitous route to avoid the anticipated crowds, and ended up in housewares near where I expected the robot to be.

The first thing that caught my eye though wasn’t the vacuum cleaners, or the shiny Christmas plates, it was the cooking pans. It wasn’t the color, or size, or even price that piqued my interest, it was the material on the label: Titanium.

Now I’m no gourmet chef, but I generally don’t think of Titanium as a material used for pans. I’m more familiar with its applications in aerospace engine applications, in medical implants (see https://edaxblog.com/2014/01/22/bringing-oim-analysis-closer-to-home/), and in golf clubs. I’ve certainly polished more Titanium samples than I want to remember. Seeing these Titanium pans, it got me thinking about material selection, how material scientists must balance different materials properties (and cost) to match a material with an application, and where Titanium fits in the world of cooking.

One of my favorite cookbooks is “The Food Lab”, by J. Kenji Lopez-Alt, which has the sub-title “Better Home Cooking Through Science”. I’ve enjoyed reading this book because the author systematically tackles questions like “Is New York pizza better because of the water?” using the Scientific Method, and writes humorously about the results as well as providing delicious recipes. I’ve also taken to following him on Twitter (@TheFoodLab), and a recent post shows the pictures, (shown here as Figure 1), using an Infrared (IR) camera, of skillets made of different materials.

Figure 1. Heat distribution in pans made of different materials.

I found it a fascinating picture visualizing the heat distribution, derived from the thermal conductivity of the materials. Stainless steel is non-reactive, so you can cook anything in it. However, it doesn’t have the greatest thermal conductivity. Cast iron has a similar issue, and takes a while to warm up but once it’s hot, it stays hot, which is great for searing meat. Aluminum has better thermal conductivity, but also soft, scratchable, and can react with some foods. Copper is another material with excellent thermal conductivity, but it is reactive to certain foods. When confronted with these types of property challenges, material scientists like the best of both worlds, so composite pans have been made where copper and/or aluminum are sandwiched with steel to try and combine thermal performance with a non-reactive surface.

So what advantages does Titanium bring to this application? As with the aerospace and recreational applications, Titanium has a good strength to weight ratio. It’s lighter than steel and stronger than aluminum, as well as being corrosion-resistant. This means it’s the lightest cookware you can buy. Not necessarily the most important feature in the kitchen, but it does have value for cookware designed for camping.

All this thinking about optimization of thermal conductivity made me think about work done on thermoelectric materials. These materials convert a temperature differential into an electrical potential. Unlike these cooking pans, these materials want to minimize thermal conductivity while maximizing electrical conductivity. This is an interesting challenge. Thermoelectric properties can be optimized by increasing grain boundary density to disrupt phonon heat transfer. Figure 2 shows an EBSD IPF map of a Bismuth Telluride thermoelectric material that was made by shock-wave consolidation. This manufacturing process was investigated as a way to consolidate thermoelectric powders while retaining the nanostructure. More information can be found in our paper at: https://link.springer.com/article/10.1007/s11664-011-1878-4.

Figure 2. EBSD IPF map of a Bismuth Telluride thermoelectric material that was made by shock-wave consolidation

In the end, I decided not to buy a pan, but I did get the robot vacuum cleaner. I look forward to asking Alexa EBSD-related questions, just to see what happens. I also hope Santa brings me something that is microstructurally interesting, that perhaps I’ll use in my next blog.