There is a Screw Loose

Dr. René de Kloe, Applications Specialist, EDAX

I dare say that in everyday life, most people do not think about crystallography very often. Equally, when we think of grains, a familiar image that comes to mind is children playing on the beach, building sandcastles (or in good Dutch tradition, perhaps a dam to keep the sea out).

Children already know about building things. They know you must use moist sand to make nice figurines. They also know that when you dig too deep on the beach, that water may come in and wreck your castle. You have to know your stuff when you start building things. Parents stimulate these construction experiments by supplying the building materials for some serious out-of-the-box thinking. The children start small, developing new, and intriguing concept cars (Figure 1), and then move on to bigger ideas and perhaps they build robots (Figure 2).

A concept car made of Duplo blocks.

Figure1. A concept car made of Duplo blocks.

Robots made from carton boxes.

Figure 2. Robots made from carton boxes.

Why should this stop when you grow up? Some people might say there is a screw loose inside if you occupy yourself with carton robots (I designed the robots for a children’s vacation camp 😊). Still, the fascination with building beautiful things remains at all ages. A while ago, my neighbor asked me to take a look at this impressive tower built of Anker stones without using any glue (Figure 3, https://anchor-stone.eurosourcellc.com/).

An Anker stone model of the Grunewaldturm in Berlin.

Figure 3. An Anker stone model of the Grunewaldturm in Berlin.

Engineers have never outgrown the desire to put bits together to build things, and with the knowledge they gained during their education and experience, amazing things have been created. But as with the Anker tower, to have a stable structure, you need to keep paying attention to detail. If you have ever built anything yourself, you know how important it is to use the right components and ensure that all the parts fit together.

During my work at EDAX, I often work with engineers who are creating and testing new materials. Such materials are typically being deliberately developed for certain purposes by mixing components and then treating them just so, but sometimes also found by accident. And of course, it is not only the composition of a material that defines its properties, it is also the microstructure that makes a material suitable for specific applications. When you take care to pick the proper starting material for your product, you can successfully build something. However, sometimes corners are cut, and things go wrong.

For example, take a look at the two montage EBSD maps of iron screws in Figure 4.

An EBSD IPF (Y) on image quality maps of a) a coarse grained screw and b) a fine-grained screw. All the green grains are aligned with one of the edges of the unit-cell cube facing towards the tip of the screw.

Figure 4. An EBSD IPF (Y) on image quality maps of a) a coarse grained screw and b) a fine-grained screw. All the green grains are aligned with one of the edges of the unit-cell cube facing towards the tip of the screw.

These are two multi-million-point EBSD maps showing the microstructure in two screws. The greenish color indicates that in both screws the crystallographic [011] direction lies along the length of the screw. This is indicative of the production process of the metal rods from which the screws are cut. The different purple colors in the head are caused by the stamp that shapes it and pushes the cross into the top of the screw. But that is where the similarities end.

The top screw shows a very coarse grain structure, while the bottom screw has a much finer interconnected grain structure. This difference in grain structure has consequences. When we zoom in on the shaft of the coarse-grained screw (Figure 5a), the large grains appear flattened in between the threads, and there is a strong change in grain size from the center to the edge of the shaft. In between the threads, some of these larger grains have even been forced apart to form cracks. This combination is bad news for the strength of the screw. When you tighten this screw, the force gets “focused” on the weak areas between the threads, and the screw breaks easily. In the fine-grained screw (Figure 5b), a minor grain size reduction is visible right at the edge of the shaft, but the internal structure is constant over the entire screw. This homogeneous structure distributes the force evenly over the screw, and it does not break easily.

Grain maps of the two screws shown at the same scale illustrating the difference in grain size. a) Shows a coarse-grained microstructure and b) depicts a fine-grained microstructure.

Figure 5. Grain maps of the two screws shown at the same scale illustrating the difference in grain size. a) Shows a coarse-grained microstructure and b) depicts a fine-grained microstructure.

A final detail scan of the grain structure shows an additional difference (Figure 6). In the coarse-grained screw, long trails of carbide particles can be observed in between the grains, which effectively separate the grains and facilitate cracking. In the fine-grained screw, the grains show a lamellar martensitic microstructure with very few carbides. These microstructures exacerbate the difference in strength between the screws.

Detail maps of the grain structure in a) the weak and b) the strong screw.

Figure 6. Detail maps of the grain structure in a) the weak and b) the strong screw.

The investigation example shown above was born out of frustration when I tried to build something, and the screws just kept breaking while I thought I was doing nothing wrong. So, I decided to cut up one of the failing screws and compare it with a screw from another box that had never given me trouble.

This was just about a screw used in a DIY project to put a wooden panel to a wall. Nothing crucial, you would think. But just imagine when this screw would have been used to hold up something a bit more impressive, like that big, heavy chandelier 10 meters above your head in the lobby of a hotel? Then suddenly, the microstructure of a humble construction component, such as a screw, becomes crucial, and thinking about the crystallography and grain structure of everyday items turns out to be really important.

Care and Upkeep of Your Standards

Shawn Wallace, Applications Engineer, EDAX

As I prepared for some analytical work yesterday, I had to repolish a standard block. This made me think about how important these little blocks are and how often they are not cared for properly. With that in mind, I thought it might be useful to pass on some little nuggets of information I have gathered over the years from many sources.

The most important thing about caring for a block is knowing what is in it. Standard blocks can be purchased as a whole or personally made. No matter what, you need to know what you have! To do so, you should keep several copies of the following for every standard you have:

  • Optical light images of the whole block
  • SEM Montage image of the whole block (BSE and SE)
  • Individual image of each standard material
  • Composition of each standard material with sources
  • Notes on each standard
Each of our standard blocks has a name and a duplicate document. This packet has optical, BSE, and SE images of the standard. This allows us to quickly find the standard we want while having all the information easily accessible in hand.

Figure 1. Each of our standard blocks has a name and a duplicate document. This packet has optical, BSE, and SE images of the standard. This allows us to quickly find the standard we want while having all the information easily accessible in hand.

Each of these above items is important. You want to keep both a visual record of your standards, a record of what it is and the condition that it is in, to allow you to track any issues that may pop up (Figure 1). Therefore, having a note section is important. You may find that one of the areas of your standard gives anomalous values and should be avoided. You want to make sure this information is easily accessible to everyone that uses the standard. I suggest scanning and keeping electronic copies in a shared folder on your desktop.

Besides the documentation aspect of care, physical care is just as critical. Most commercial standard blocks come pre-polished and carbon-coated. Over time, both of those will degrade and need to be redone. Usually, the carbon coating damages first, but you also need to check for burn marks and other beam damage done to the standard material itself. When repolishing and recoating, I usually do a solid 10 minute repolish with diamond paste. This removes enough material to eliminate the carbon coating and get new clean, undamaged surfaces while not change the physical appearance all that much. I try my best to avoid using an Al-based polishing material, as they tend to stick around too much and can interfere with my analysis on elements I use. With carbon-based polishing material, it is much easier to see the effects of the carbon. In the end, I do not tend to do quant work on carbon that much, while I often try to quantify aluminum. Whatever you do, document what was done. It can help you both head off and understand issues that may present.

While physically handling your sample, it shouldn’t need to be said, but you should never be touching your sample with ungloved hands. Your oils are bad for both the SEM cleanliness and the sample cleanliness. Avoid any sort of colloidal products with standards, as they do tend to flake with age. When not in use, samples should be held in a desiccator with good desiccant (Figure 3).

A good desiccator should have a rubber molding to help it hold a seal at a minimum. You should try to keep it under vacuum for the best results. While taking this picture, I noticed I should dry my desiccant or replace it. I have seen some users keep a small plastic bag of fresh desiccant in the desiccator as a quick visual reference.

Figure 3. A good desiccator should have a rubber molding to help it hold a seal at a minimum. You should try to keep it under vacuum for the best results. While taking this picture, I noticed I should dry my desiccant or replace it. I have seen some users keep a small plastic bag of fresh desiccant in the desiccator as a quick visual reference.

There are many other tips I can think of sharing, but to wrap it up, standards are valuable in our industry. A good, well cared for standard will last multiple careers while giving consistent results time after time. Take the time to keep your standards in the best condition, and they will repay your time spent on them tenfold.

Cornfields and Characterization: A Story of Failure Analysis

Dr. Jordan Moering, U.S. Eastern Sales Manager, EDAX

It was an icy morning in early November where I found myself, freezing, staring at a chunk of mangled aluminum, carbon fiber, and hickory nestled against mounds of pumpkins in a largely empty cornfield in Sussex County, Delaware. As the sun began to rise over the frosty ground, the carnal wreckage was investigated, pondered over, poked and prodded, touched, and engaged in any other means of characterization at the disposal of the rag-tag cohort of farmers, engineers, enthusiasts, and politicians surrounding me. In hindsight, this scenario seems like something out of a science fiction novel or perhaps a post-apocalyptic memoir, but I can assure you that this is a common sight to behold. Common, at least, at the World Championship Punkin Chunkin.

As it turns out, the twisted composite beam was one of the first instances I experienced in witnessing true engineering failure firsthand. Although the beam failed in some of our early testing, it had previously been attached to a world-class, 7-ton, torsion catapult capable of launching pumpkins over a kilometer at nearly the speed of sound. It could withstand tensile loads exceeding the weight of a Boeing 747 and extended nearly 20 feet in length. All of that impressive performance was a thing of the past as I closely examined the jagged features at the fracture surface, the twists along the flanges of the I-beam, and the shards of carbon fiber shattered amongst the corn husks.

Replacing the broken "Throwing Arm" with a convenient spare that we had brought with us.

Figure 1. Replacing the broken “Throwing Arm” with a convenient spare that we had brought with us.

Although I was just a student at the time, I already recognized the characteristic ductile fracture surface before me. I might have squinted my eyes and imagined some fatigue striations within the metal surface, but sadly this was the only means at my disposal of diagnosing the problem at the time. In a laboratory setting, I would have been able to not only characterize the elemental composition of the beam (it was a gift from a benevolent team sponsor) but also fully describe the crystalline structure with techniques like EBSD, XRD, and EDS. This type of material identification study is routine with modern analytical instruments, but recent advancements have taken this a step further. Had I known then what I know now, the unprecedented capabilities of high-resolution EBSD and ultra-high sensitivity of direct detection could have allowed me to understand and quantify, quite literally, the stressed state of the surrounding metal at the fracture surface.

The most frequently used deconstruction and characterization device we had at our disposal - an angle grinder.

Figure 2. The most frequently used deconstruction and characterization device we had at our disposal – an angle grinder.

While my first foire into failure analysis lacked the sophistication of modern analytical capabilities, it did spark an intense curiosity into this critical line of work. The modern electron microscopist, lab technician, or researcher has a wealth of opportunity at his/her disposal for understanding how materials fail. Sometimes these failures originate at some inclusion or material defect that could have previously been detected by methods like micro-XRF or EDS elemental analysis. Other times, inherent weaknesses in the system concentrate stress in ways that might not be apparent to the naked eye. Techniques like high-resolution EBSD and X-ray diffraction might be used to prevent these calamities. The list goes on and on.

I’ve only been working at EDAX for several months now, but every day I wake up and get to work with individuals who face scenarios, not unlike my previous encounter with twisted beams and flying pumpkins. Although a researcher at semiconductor foundry might not be surrounded by farmers in the middle of a cornfield, they certainly may find themselves staring at an improperly functioning device, wondering where things went wrong. In this capacity and many others, I find myself relating to our customers. I empathize with their challenges, and I am excited to help them uncover solutions to some problems that they previously were not aware of.

Because if there is one thing I have learned from Punkin Chunkin and Advanced EM Characterization, it is that you never know what you will find under the surface of your material.

The video below is of the beam in operation prior to its demise: https://www.youtube.com/watch?v=aYKGaLq3xPM.

Sunshine of My Life

Lin Nan, Regional Sales Manager, EDAX

EDAX is in a scientific business, exploring the unknown by looking at small materials. As EDAX employees, this makes us proud because we know that small things matter and our products and services help people discover scientific breakthroughs that make the world a better place.

But even for people like us, at least it never occurred to me, that a small virus, just nanometers in size, can change our lives so significantly. It reflects how little we know and how insignificant we are compared to the massive nature of the unknown. Science and human civilization still have a long way to go. We should remain respectful and humble about the world and nature.

COVID-19 has changed our lives in a way that no one expected, and maybe no one ever wanted with suffering and loss of life. Furthermore, the impact on our perception of society and the world may have been changed forever.

For the past six years at EDAX, like lots of our colleagues, I have always performed the majority of my job on the road. In my position as a Sales Manager, I promote and manage EDAX business and help our customers explore unknowns in small scale samples, hoping that it contributes to science. Airports, hotel breakfast, and complaints from my wife have become routine to me. Luckily, with “known” science and technology, the FaceTime and video calls do make it much easier for me to stay connected with my family while I’m traveling.

But for the past six months, my routine has changed completely. I have been sitting still within my apartment, like most people around the world. Ironically, instead of using video calls to connect with my family, I am now using video conferences and other internet resources to conduct business remotely and keep in touch with our customers. I have become the family man that I never dreamed I would become over the past 10 years, and it is a dream come true.

Lin Nan and his family spending time during the pandemic.

(left to right): Yuanna, Rong Xu, and Lin Nan enjoying time together during the pandemic.

Face-to-face communication is certainly always the most effective form of real-time interaction, but it requires close proximity to others. But when this is impossible, we realize the advantages of online meetings, including time, convenience, and economic impacts.

For us, we can almost respond and interact immediately, without trying to squeeze another customer site visit into our busy travel schedules, which could take another few weeks or even months. Without physically traveling, we actually get more support from our colleagues in applications and R&D, even from different time zones, which provides more expertise and profound knowledge to our customers that salespeople normally cannot deliver. In the scientific field, this is really valued by customers.

On the other hand, for customers, it is easier to set up and participate, since meetings can be attended from anywhere and at any time with a smart device and internet access. Especially when a meeting is presented by an application scientist, rather than a salesperson.

This may become the new norm for communication.

This type of lifestyle change probably only comes once in a lifetime. It can be depressing and frustrating, but at the same time, it is valuable and enjoyable to me that I could spend more time with family, something that I never did before, and I can make up for lost time with them.

Especially with the birth of my new baby daughter this July, I have been fully involved and able to foster her growth without being away, missing moments that I missed with my two and half year old son. There is no escape from waking up in the middle of the night, changing diapers, and bottle feeding. Like it or not, that is all part of our life.

I named my son “煦” pronounced “XU” and my daughter “熙” pronounced “XI”.

Not only do both characters look similar, but the spelling is similar as well. The only difference is the last letter, which is “U” and “I”.

Both characters mean warm and harmonious sunshine, which my son and daughter bring to my life. It reflects my faith as well, there is always a bright side, and everything is at its best arrangement.

Among the world of unrest we are experiencing now, a little sunshine is particularly important to keep the faith within.

Tomorrow will be fine!

“Don’t Sweat the Small Stuff” vs. “It’s the Little Things that Matter Most”

Dr. Stuart Wright, Senior Scientist, EDAX

A few weeks ago, my colleague at EDAX, Shawn Wallace, posed a question that has stayed with me, and so I thought my turn at the EDAX blog would be a good place to address it. Shawn was building an EBSD structure file for a new phase and encountered the following dialog for adding an atom to the unit cell.

Dialog box for building a new structure file for a new phase.

Shawn asked how important it was to get the Ion Type correct for the structure he was working with. I realized I had implemented this capability several years ago for kinematical calculations of structure factors but had never really explored it’s impact on the calculations. I guessed that it would not have much of an impact, but I wasn’t entirely sure that was the case. The choice of ion type affects the atomic scattering factor used in structure factor calculations. I looked through our phase structure database for a binary compound containing Fe and decided to use a simple Al-Fe structure to check out the effect of the ion type selection on the structure factor calculation.

Structure factors for the Fe and Fe+3 selections in the dialog box.

I calculated structure factors for the Fe and Fe+3 selections in the dialog box above. As shown, the difference in the structure calculation results is imperceptible in the kinematically simulated patterns. The maximum difference between the two patterns is only a 1.6% difference in the relative intensity of the {100} bands.

Kinematically simulated patterns for Fe and Fe+3.

Here is a table showing that the structure factors are quite similar, confirming my initial guess. I haven’t tried any other structures, so it is not a complete study, but I suspect other structures will follow the trend shown by the simple Al-Fe structure. Thus, my conclusion is, Don’t Sweat the Small Stuff.

(hkl) FFe FFe+3
110 4.773 4.826
100 1.291 1.738
200 3.256 3.252
211 2.566 2.562
111 1.098 1.104
220 2.189 2.186
222 1.633 1.632
210 0.913 0.908
310 0.858 1.856
321 1.462 1.461

With that little study wrapped up, I turned my attention to choosing the Debye-Waller factor used in the dynamical simulation. In the dialog above, it says the default Debye-Waller factor for iron is “0.003106 for bcc, 0.533 for fcc”. Does the choice of Debye-Waller factor matter? Here are dynamically simulated patterns for these values.

Dynamically simulated patterns using the Debye Wall factor.

The two patterns are quite different. To correctly use the new simulation tools, I need to expend some effort to learn more about Debye-Waller factors. Clearly, It’s the Little Things that Matter Most.

Want a Free Set of Microanalysis Standards?

Dr. Shangshang Mu, Applications Engineer, EDAX

Modern EDS systems are capable of quantitative analysis with or without standards. Unlike standard-less analysis, the k-ratio is either calculated in the software or based on internal standards. For analysis with standards, it is measured from a reference sample with known composition under the same conditions as the unknown sample. As an applications engineer, sometimes users ask me where to order these standards. Usually, I point them to the vendors that manufacture and distribute reference standards where you can order either off-the-shelf or customized standard blocks. In addition to these commercial mounts, I always tell them that they can request a set of mineral, glass, and rare earth element phosphate standards from the National Museum of Natural History free of charge! These are very useful standards that I’ve seen widely used in not only the geoscience world but also in various manufacturing industries. These free standards are also great for those graduate students with limited budgets and ideal for practicing sample preparation (yes, I was one of them).

This set of standards is officially called the Smithsonian Microbeam Standards and includes 29 minerals, 12 types of glass, and 16 REE phosphates. You can find out more information about these standards and submit a request form by clicking on the link below:
https://naturalhistory.si.edu/research/mineral-sciences/collections-overview/reference-materials/smithsonian-microbeam-standards

I mentioned sample preparation earlier. Yes, you read that right. These standards come in pill capsules containing from many tiny grains to a few larger ones and you need to mount them on your own (Figure 1).

Grains in a pill capsule.

Figure 1. Grains in a pill capsule.

Since you can get the information such as the composition, locality, and references for each standard from the website, what I want to discuss in this blog post is how to prepare them properly for X-ray analysis. The first tricky thing is to get them out of the capsules. The grains in Figure 1 are almost the largest in this set and you won’t get too many of this size. Some of the grains are even too tiny to be seen at first glance. For the majority that are really tiny, you need to tap the capsule a couple of times to release the grains that get stick to the capsule wall, then you can open the capsule very carefully and let the grains slide out with a little tapping.

For mounting, the easiest way is to mount the standards in epoxy using a mounting cup and let it cure. I did this in a fancy way to make it look like a commercial mount (Figure 2). I ordered a 30 mm diameter circular retainer with 37 holes used by commercial mount manufacturers (Figure 3) and filled the holes with standards on my own. I must admit that the retainer is not cheap, but you can machine the mount by yourself or have a machine shop do it for you. In addition to looking pretty, the retainer ensures a good layout so you can quickly locate the standards you need during microanalysis, and you can mount the same type of standards on one block and get rid of the hassle of frequently venting and pumping the SEM chamber to switch standard blocks.

Examples of commercial mounts.

Figure 2. Examples of commercial mounts.

 

30 mm diameter circular retainer with 37 holes.

Figure 3. 30 mm diameter circular retainer with 37 holes.

To prevent the tiny grains from moving and floating up when pouring the epoxy mix, I placed the retainer upside down and pressed it onto a piece of sticky tape (Figure 4a) and positioned the grains on the sticky surface of the tape within the holes. When tapping the capsule to let the grains slide out and fall into the hole, the other holes were covered to prevent contamination (Figure 4b). These holes are small in diameter and pouring the epoxy mix directly will trap air bubbles in the hole to separate the grains from the epoxy mix. To overcome this problem, I filled up the hole by letting the epoxy mix drip down very slowly along the inner surface of the hole.

Positioning grains within the holes of the retainer.

Figure 4. Positioning grains within the holes of the retainer.

For general grinding, I start with wet 240 grit SiC sandpaper with subsequent use of 320, 400, 600, 800, and 1,200 grit wet SiC sandpapers. But coarser grits can grind off tiny grains in this case, so I would recommend starting with a relatively fine grit based on the sizes of the grains you receive and always use a light microscope or magnifier to check the grinding. For polishing abrasive, I used 1 micron and 0.3 micron alumina suspensions on a polishing cloth. For the grains used as standards or quantification in general, the surface needs to be perfectly flat. However, the napped polishing cloth tends to abrade epoxy and the grains at different rates, creating surface relief and edge rounding, especially on tiny grains. To mitigate this effect, the polishing should be checked under a light microscope constantly and stopped as soon as the scratches are removed. A vibratory final polishing with colloidal silica is optional. Followed by ultrasonic cleaning and carbon coating, the standard mount is ready to use.

Note that commercial mount manufacturers may prepare standards individually (especially for metal standards) and insert them into the holes from the back of the retainer and fasten them with retaining rings (Figure 5a). A benefit of this approach is that the standards on the mount are changeable, so you can load all the standards you need on one mount before microanalysis. I used to make several individual mounted standards that can fit into the retainer (Figure 5b) but this process is very time consuming and much trickier to keep the small surface flat during grinding and polishing.

a) The back of a commercial metal standard mount. b) A tiny cylindrical mount that can fit into the retainer holes.

Figure 5. a) The back of a commercial metal standard mount. b) A tiny cylindrical mount that can fit into the retainer holes.

This is definitely a good set of standards to keep in your lab. With EDAX EDS software, in addition to quantification with these standards, you can also use them to create a library and explore the Spectrum Matching feature. The next time you want to quickly determine the specific type of a mineral, you can simply collect a quick spectrum and click the “Match” button, and the software will compare the unknowns to the library you just created.

Between the Lines

Dr. René de Kloe, Applications Specialist, EDAX

While I am testing new hardware and software versions, I use it as an opportunity to collect some data on unique materials. Testing detector speed or general software functionality is easiest on a simple material like an undeformed Ni or Fe alloy. But, I think it is a shame to perform longer duration tests on materials I have already seen many times before. For such occasions, I look through my collection of materials for something nice to map. During testing of the upcoming APEX™ 2.0 EBSD software, I collected a few larger scans on fossils that I had found during geological fieldwork and family holidays. This included large single-field scans and a Montage map, where we combine beam scans with stage movements for a large mosaic map.

Cross-section through a fossil crinoid stem and IPF on PRIAS™ center map of the fossil crinoid stem sample collected from the indicated area.

Figure 1. a) Cross-section through a fossil crinoid stem. b) IPF on PRIAS™ center map of the fossil crinoid stem sample collected from the indicated area.

For example, Figure 1a shows a cross-section through a fossil crinoid stem. At the edge, the lighter areas represent the structure of the organism, while the darker areas are later sedimentary infill.

This is beautifully visible in the 2.1 x 1.7 mm IPF on PRIAS™ center map, where the biomineral structure appears smooth and fine-grained. In contrast, the infill is more equiaxed and shows topography due to compositional differences (Figure 1b).

Another beautiful scan was collected while I was trying out the new APEX™ 2.0 EBSD Montage map wizard. This wizard allows easy pre-imaging of the entire scan field to set the actual scan area. With the wizard, setting up such a large, 18 million point, 30-field Montage map over a 1.3 x 7 mm area can be done in a few minutes.

Calcite rock sample with fossils and EBSD Montage map of one of the nummulite fossils.

Figure 2. a) Calcite rock sample with fossils. b) EBSD Montage map of one of the nummulite fossils.

We collected these two scans on calcite rocks for which you can simply load the appropriate crystal structure. But collecting data is not always that easy, especially if you are not sure what phase(s) you have in your sample. And ultimately, EBSD data collection is based on pattern analysis and then matching the detected bands against a lookup table. In most cases, you can just search the included EDAX structure file database that contains close to 500 phases and covers most commonly studied materials, such as the calcite used for the scans above.

But where do these files come from? Partly, they are a result of our combined legacy. Over the years, we have seen many materials and often painstakingly identified which bands to select to get reliable indexing results. Nowadays, you can create phase files directly using atomic and crystallographic information. However, you can continue to extract the majority of “new” phase files from XRD databases, such as the AMCS, ICSD, or ICDD PDF databases. These databases contain 10’s to sometimes 100’s of thousands of phase descriptions that are based on XRD measurements. The XRD data shows which lattice planes are effective X-ray diffractors, and are also useful to construct a structure file for electron diffraction patterns.

Indexed olivine EBSD pattern.

Figure 3. Indexed olivine EBSD pattern.

And there the fun starts. Often there are multiple possibilities for phases or minerals (e.g., solid solution series) available in the database. Which one to select? And in many cases, there is no single-phase file that matches the pattern exactly. There are always bands that do not get labeled or are shown in the overlay that are not visible in the real pattern. This is due to the differences between X-ray and electron diffraction intensities or simply incomplete database entries. Time for some human intervention. The APEX™ EBSD software contains advanced tools to modify and optimize the reflector tables of imported or calculated structure files. First, the color-coding itself. All bands are labeled with a color that corresponds to the IPF color triangle, so equivalent lattice planes get identical colors. This allows a visual inspection if bands that are designated with the same color also appear identical.

IPF color triangle.

Figure 4. IPF color triangle.

Then there is a band ID tool to help identify non-labeled bands in the diffraction patterns. When a pattern appears correctly indexed, but a number of bands are not labeled, the user can draw a line on the missing band. The software then shows which lattice plane corresponds to that band and also indicates all crystallographic equivalent planes. When it is still difficult to identify the correct indexing solution, it can be beneficial to bypass the Hough band detection and instead manually draw the bands for indexing. A good trick for low symmetry crystals is only to select the thinnest bands. These correspond to the lattice planes with the largest d-spacings and should be the important low-index crystallographic planes. By excluding the (often) large number of bands with similar bandwidths, you reduce the number of options and more quickly land at the best matching orientation or phase.

Manual Band Selection tool.

Figure 5. Manual Band Selection tool.

When a solution is found that matches the thin bands, you can start drawing in the other ones. When drawing a band, the software automatically shows where all the crystallographic equivalent planes should be. If a line is drawn where no band is present, you have the wrong candidate, and you need to look further. If all the indicated bands match in appearance and width, you can enable the reflector. This way, it is easy to interactively generate a matching phase file. Just keep in mind that when you have optimized a structure file to a pattern, it is a good idea to find some more patterns from that phase (if necessary, just rotate the sample to get a different orientation) and verify that all the bands in the other patterns are also properly identified. This is especially important for low symmetry materials where few lattice planes are equivalent.

Band optimization sequence on an EBSD pattern from W2C. The initial reflector table (a) misses a number of strong bands. Manually selecting a band (b) shows which reflector this is and where the crystallographic equivalent bands should be. This can be repeated (c) until all clear bands have been labeled.

Figure 6. Band optimization sequence on an EBSD pattern from W2C. The initial reflector table (a) misses a number of strong bands. Manually selecting a band (b) shows which reflector this is and where the crystallographic equivalent bands should be. This can be repeated (c) until all clear bands have been labeled.

Although it can be rewarding to identify a new phase and optimize the structure file to allow for EBSD mapping of a new and interesting material, I would like to end with a word of warning. When you are working with a good pattern and successfully identify the phase and orientation, it is very tempting to keep looking for bands and completely fill the pattern with everything you can see. But that is often a bad idea, as the weaker bands will typically not get selected by the Hough transformation on the poorer patterns that are used during indexing. Enjoy playing with the materials and structure files, but don’t overdo it.

Diffraction pattern with all visible bands enabled for indexing.

Figure 7. Diffraction pattern with all visible bands enabled for indexing.

How Many Electrons Do You Need For An EBSD Pattern?

Matt Nowell, EBSD Product Manager, EDAX

I always liked the commercial that asked,” How many licks does it take to get to the center of a Tootsie Pop?”. I like contests where you estimate the number of M&Ms in a jar. Taking the concept away from delicious treats and moving towards something more technical, I’ve also enjoyed looking at the number of grains we need to measure with EBSD to get a good idea of the texture of a material.

Recently I’ve been working with our new Clarity™ Direct Electron Detector for EBSD. It’s the first commercial EBSD direct detector and will be launching soon. Traditionally, EBSD patterns are captured when the diffracted electrons strike a phosphor screen, where energy is converted into light photons, which are focused through a lens onto an imaging sensor, where the light photons are then converted back to electrons. However, a direct electron detector is just that, it captures the diffracted electrons directly. This allows us to count the electrons in an EBSD pattern directly.

EBSD pattern collected with Clarity™ with an average of 5,000 electrons per pixel.

Figure 1. EBSD pattern collected with Clarity™ with an average of 5,000 electrons per pixel.

Take the EBSD pattern collected from a nickel superalloy using the Clarity™ shown in Figure 1. For an EBSD pattern like this, remember that it has been background corrected to flat-field the image and improve the contrast. This is because the actual live EBSD pattern does not have a uniform intensity across the sensor, as shown in Figure 2. In this example, a background collected while imaging many grains was collected and subtracted from the live signal to produce the image in Figure 1. The background image has the spatial information for a specific orientation removed, while retaining the overall intensity gradient that is a function of the material of interest and the sample geometry. Note that the Clarity™ uses four direct electron detectors that are coupled together. The cross-hair image visible in Figure 2 shows the location of the seams between the detectors. These can be masked out of the image if desired but are quickly minimized with this background correction.

EBSD pattern from Figure 1 prior to background correction.

Figure 2. EBSD pattern from Figure 1 prior to background correction.

For Figure 1, a pixel at the center of the signal intensity contained approximately 10,000 electrons, and the average counts for all pixels was approximately 5,000 electrons. After background subtraction, I drew a line across the image, and the intensity profile across this line is shown in Figure 3. This profile shows that the final processed EBSD pattern has a dynamic range of about 1,700 electrons.

Line profile across the EBSD pattern in Figure 1 showing the dynamic range of the EBSD signal.

Figure 3. Line profile across the EBSD pattern in Figure 1 showing the dynamic range of the EBSD signal.

EBSD pattern with an average of 10 electrons per pixel.

Figure 4. EBSD pattern with an average of 10 electrons per pixel.

Now seeing that I could count the number of electrons in an EBSD pattern, I wanted to know how many I needed to get a usable EBSD pattern. I could decrease the exposure time, decrease the beam current, or do both. In this case, I continually decreased the exposure time to find where the EBSD pattern indexing started to fail. Figure 4 shows an EBSD pattern where the maximum number of electrons is 20 and the average number of electrons is 10. Even with this small amount of a signal, I was still able to index it with a confidence index of 0.92 and a fit of 0.6°, which indicates a good orientation solution. Talk about doing a lot with a little. This performance is enabled by the single electron sensitivity and zero readout noise of the detector, which makes this camera very exciting for low beam dose applications for beam-sensitive materials. I look forward to sharing more later.

Indexing solution for the pattern in Figure 4 with a confidence index of 0.92.

Figure 5. Indexing solution for the pattern in Figure 4 with a confidence index of 0.92.

How to Get a Good Answer in a Timely Manner

Shawn Wallace, Applications Engineer, EDAX

One of the joys of my job is troubleshooting issues and ensuring you acquire the best results to advance your research. Sometimes, it requires additional education to help users understand a concept. Other times, it requires an exchange of numerous emails. At the end of the day, our goal is not just to help you, but to ensure you get the right information in a timely manner.

For any sort of EDS related question, we almost always want to look at a spectrum file. Why? There is so much information hidden in the spectrum that we can quickly point out any possible issues. With a single spectrum, we can quickly see if something was charging, tilted, or shadowed (Figure 1). We can even see weird things like beam deceleration caused by a certain imaging mode (Figure 2). With most of these kinds of issues, it is common to run into major quant related problems. Any quant problems should always start with a spectrum.

Figure 1. The teal spectrum shows a strange background versus what a normal spectrum (red) should look like for a material.

Figure 1. The teal spectrum shows a strange background versus what a normal spectrum (red) should look like for a material.

This background information tells us that the sample was most likely shadowed and that rotating the sample to face towards the detector may give better results.

Figure 2. Many microscopes can decelerate the beam to help with imaging. This deceleration is great for imaging but can cause EDS quant issues. Therefore, we recommend reviewing the spectrum up front to reduce the number of emails to troubleshoot this issue.

Figure 2. Many microscopes can decelerate the beam to help with imaging. This deceleration is great for imaging but can cause EDS quant issues. Therefore, we recommend reviewing the spectrum up front to reduce the number of emails to troubleshoot this issue.

To save the spectrum, right-click in the spectrum window, then click on Save (Figure 3). From there, save the file with a descriptive name, and send it off to the applications group. These spectrum files also include other metadata, such as amp time, working distance, and parameters that give us so many clues to get to the bottom of possible issues.

Figure 3. Saving a spectrum in APEX™ is intuitive. Right-click in the area and a pop-up menu will allow you to save the spectrum wherever you want quickly.

Figure 3. Saving a spectrum in APEX™ is intuitive. Right-click in the area and a pop-up menu will allow you to save the spectrum wherever you want quickly.

For information on EDS backgrounds and the information they hold, I suggest watching Dr. Jens Rafaelsen’s Background Modeling and Non-Ideal Sample Analysis webinar.

The actual image file can also help us confirm most of the above.

Troubleshooting EBSD can be tricky since the issue could be from sample prep, indexing, or other issues. To begin, it’s important to rule out any variances associated with sample preparation. Useful information to share includes a description of the sample, as well as the step-by-step instructions used to prepare the sample. This includes things like the length of time, pressure, cloth material, polishing compound material, and even the direction of travel. The more details, the better!

Now, how do I know it is a sample prep problem? If the pattern quality is low at long exposure times (Figure 4) or the sample looks very rough, it is probably related to sample preparation (Figure 4). That being said, there could be non-sample prep related issues too.

Figure 4. This pattern is probably not indexable on its own. Better preparation of the sample surface is necessary to index and map this sample correctly.

Figure 4. This pattern is probably not indexable on its own. Better preparation of the sample surface is necessary to index and map this sample correctly.

For general sample prep guidelines, I would highly suggest Matt Nowell’s Learn How I Prepare Samples for EBSD Analysis webinar.

Indexing problems can be challenging to troubleshoot without a full data set. How do I know my main issues could be related to indexing? If indexing is the source, a map often appears to be very speckled or just black due to no indexing results. For this kind of issue, full data sets are the way to go. By full, I mean patterns and OSC files. These files can be exported out of TEAM™/APEX™. They are often quite large, but there are ways available to move the data quickly.

For the basics of indexing knowledge, I suggest checking out my latest webinar, Understanding and Troubleshooting the EDAX Indexing Routine and the Hough Parameters. During this webinar, we highlight attributes that indicate there is an issue with the data set, then dive into the best practices for troubleshooting them.

As for camera set up, this is a dance between the microscope settings, operator’s requirements, and the camera settings. In general, more electrons (higher current) allow the experiment to go faster and cover more area. With older CCD based cameras, understanding this interaction was key to good results. With the newer Velocity™ cameras based on CMOS technology, the dance is much simpler. If you are having difficulty while trying to optimize an older camera, the Understanding and Optimizing EBSD Camera Settings webinar can help.

So how do you get your questions answered fast? Bury us with information. More information lets us dive deeper into the data to find the root cause in the first email, and avoids a lengthy back and forth exchange of emails. If possible, educate yourself using the resources we have made available, be it webinars or training courses. And always, feel free to reach out to my colleagues and me at edax.applications@ametek.com!

Shelf Life

Dr. Bruce Scruggs, XRF Product Manager, EDAX

Recently, we had a customer request to see a demonstration on the Orbis micro-XRF system. As we talked about what they would like to see, he mentioned that he had made some test XRF measurements on table salt, and he couldn’t measure the iodine content. I agreed to measure the iodine content in table salt. Initially, I thought this would be a very straightforward exercise, as table salt is just NaCl with some iodine added, but this was anything but straightforward.

The iodization of salt in the United States began about a century ago. Iodine is an important micronutrient for thyroid gland health. Certain portions of the American population had diets deficient in iodine and the iodization of table salt was chosen as a method to increase the level of iodine in the average American diet. The salt iodization process was inexpensive; salt does not spoil and estimates of table salt consumption were available.

Some weeks before the customer demo, I bought some iodized table salt from the local grocery store. The ingredients list showed iodine in the form of potassium iodide at about 45 ppm iodine. This concentration was consistent with my web searches. I pressed a pile of salt grains onto a piece of carbon tape and measured it with the Orbis system using a 2 mm spot size (the system was equipped to measure down to a 30 μm spot size, small enough for individual grains, but I wanted to avoid any potential issues with grain to grain variations). It was easy enough and I could measure the I(L) lines with I(Lα) at 3.937 keV (Figure 1).

(A): Salt spectrum with peak deconvolution, not including I(L) series; Fig 1(B): The same salt spectrum as in (A) with peak deconvolution including I(L) series.

Figure 1. (A) Salt spectrum with peak deconvolution, not including I(L) series. (B) The same salt spectrum as in (A) with peak deconvolution including I(L) series.

Some weeks later, during the actual customer demonstration, we measured a variety of customer supplied samples and the customer asked to measure table salt near the end of the demo. I put my table salt sample into the Orbis and was astonished to find that the iodine signal disappeared (Figure 2). Peak fitting and quantification results showed no detectable iodine. After a discussion with the customer, I began to suspect that the salt iodization level was not stable, given that solid I2 is known to undergo sublimation at room temperature. I spoke to the customer again and in his previous attempts, he measured table salt (from shakers) in the company cafeteria. I often wonder how long that salt has been in the shaker!

The same salt sample, as Figure 1, measured on the Orbis a few weeks later without the presence of iodine.

Figure 2. The same salt sample, as Figure 1, measured on the Orbis a few weeks later without the presence of iodine.

Further web searches indicated that indeed, the iodization level of salt has a certain shelf life depending on many factors, including temperature, humidity, impurities in the salt, the chemical form of the iodine bearing additives, and product packaging. For example, potassium iodide is oxidized by contact with oxygen and atmospheric moisture and the resulting iodine then undergoes sublimation. In various regions of the world, iodized table salt is formulated to improve its shelf life with regard to iodine retention based on the characteristics of the table salt and the general environment, e.g., desert, tropical. Based on this loss mechanism, I suspect that there must also be a significant loss of iodine during cooking depending on whether salt is added while cooking or directly applied before consuming.

In my case, the iodine level had dropped below detectable limits in about three weeks of being left out on the table. The grains of salt ranged in size from about 100 – 500 μm in characteristic dimensions, and I was curious to what characteristic depth XRF was measuring. Was there possibly any iodine left in the largest crystals? This depth can be estimated based on the fluorescent signal energy as the exciting X-ray energy always has to be greater than the fluoresced photons (The physics are a bit different for electron excitation where the answer is determined by electron penetration depth into the sample).

XRF measurement depth can be estimated from the Beer-Lambert equation for the absorption and transmission of light:

Equation 1

Equation 1.

The mass absorption coefficient (MAC) describes how readily the I(Lα) signal line at 3.937 keV will be absorbed by the NaCl matrix. It can be described as follows:

Equation 2

Equation 2.

For NaCl, we have two MACs describing how Na and Cl each absorb the 3.937 keV photon. The easiest way to get the full matrix MAC is to back-calculate it from the Beer-Lambert equation and any web-based calculator describing X-ray absorption/transmission characteristics modeling the fluoresced photon traversing the sample matrix to the detector. I prefer the website, http://henke.lbl.gov/optical_constants/filter2.html. By inputting the sample matrix formula (including trace elements if desired), and an arbitrary path length, one can get the calculated result for I/Io and then rearrange Equation 1 to solve for the NaCl matrix MAC by inputting the previously used path length and the known density of table salt. The result is: μNaCl(3.937 keV) ~ 540 cm2/g.

Rearranging Equation 1, one can solve for the signal path length through the sample traversed by the fluoresced photon to the detector as a function of I/Io:

Equation 3

Equation 3.

The XRF Emission Depth, D, would typically be defined as normal to the sample surface, and you should also consider the take-off angle (TOA) of the detector defined from the sample surface, as shown in Equation 4.

Equation 4

Equation 4.

Table 1 shows the XRF Emission Depth as a function I/Io with a nominal detector TOA of 50ᵒ.

I/Io [%] Path Length, x [μm] Emission Depth, D [μm]
10 20 15
1 39 30
0.1 59 45

Table 1. XRF Emission Depth as a function of the signal transmission ratio, I/Io.

The definition of the characteristic XRF path length and emission depth is somewhat arbitrary, as it depends on the value assigned to the signal transmission ratio, I/Io. Typically, the characteristic path length is defined as the length over which 99% of the signal is absorbed. Hence:

Equation 5

Equation 5.

It is interesting to note from Table 1, that at 50% of the critical emission depth, the XRF signal is undergoing 90% absorption.

Coming back to the original analysis, it is possible that iodine was still present at the core of the larger 500 μm grains of salt. Further analyses could be done on cross-sectioned grains or pulverized grains to make that determination. It would be possible to measure cross-sectioned grains of NaCl using the 30 μm spot size on the Orbis to study how readily iodine is lost as a function of depth into the NaCl grain, but that is a study for another day.