Month: March 2014

Fun with Rhenium

Josh Kacher,  Post doc – UC Berkeley

One of the perks of having done my undergraduate degree at Brigham Young University was that I got to know the people at EDAX pretty well. Over the years since I’ve moved out of Utah, I’ve been able to collaborate with them on a number of different projects, which has been a great benefit to my work. Recently, I had the opportunity to spend a week with them working on a particularly challenging material, rhenium.

To give some background of the material since it’s one that the majority of people aren’t familiar with, rhenium is a high density refractory metal with a HCP crystal lattice. It maintains good ductility over a wide range of temperatures, making it an attractive material for applications in extreme environments. Unfortunately, it’s expensive, rare, and we’re running out of it. My work is focused on obtaining a fundamental understanding of the material in hopes of being able to dilute or replace it in some of its current applications. Unfortunately for me, it also tends to have a difficult to characterize microstructure due to difficulties in sample preparation and multiscale twinning mechanisms active during deformation.

The samples I looked at were slices from a compressed rhenium pillar (4 mm tall, 3 mm diameter). Since any mechanical grinding tends to induce twinning in the material, all sample preparation was done using a twin-jet electropolishing setup using a perchloric solution as the electrolyte. Differential etching rates dependent on crystal orientations resulted in quite a bit of topography on the polished sample. To get an idea of the feature sizes I was interested in, we first made use of the new PRIAS imaging feature, which made for some fun 3D-looking maps of the surface. Below are a few of the PRIAS images, along with the region from the EBSD pattern used to form the image:

Karcher_image1_blog

The ridges in the images mark where the grain or twin boundaries are located. As can be seen, the microstructure consists of both larger twins, on the scale of hundreds of nanometers, as well as small crisscrossing twins, on the scale of tens of nanometers.

With this information, we were ready to get started doing full EBSD scans. We settled on a 100 nm step size over a 70×70 micron area. Although we knew some of the twins would be missed, a step size small enough to capture them all would have been prohibitively slow.

Karcher_image2_blog

As can be seen, virtually every grain in the scan area had some level of twinning, some even developed multiple twinning systems. All the twins were identified as {111}<100> type and had an average twin width of ~700 nm. At this point, I already had most of the information I was looking for, but something interesting stood out to me in the orientation map. There seems to be many locations where the twins transmitted directly across the boundary. Although I had seen similar behavior in TEM characterization, it always involved nanotwins, not twins at the scale seen in this scan. To try to discover what was going on, six different areas were chosen for more in depth analysis.

Karcher_image3

Of these six interactions, four show twin transmission and two show the twins stopped at the boundary. The easiest factor to take into account was the misorientation across the boundary. It turns out that this was also the best as those boundaries with misorientation angles below 23° were found to allow twin transmission while those with higher misorientation angles acted as barriers. The next thing to check was the alignment of the twin planes and shear vectors associated with the twinning on either side of the boundary. This was done using what’s known as the m’ factor, which is calculated by:

m’=cos θ cos Ψ

where θ  is the angle between twin plane normals on either side of the boundary and ψ is the angle between shear vectors. Calculating m’ for all six twinning events gave:

Twin system

1

2*

3

4

5

6

Boundary misorientation

15°

23°

72°

13°

18°

36°

m’

0.94

0.91

0.91

0.19

0.95

0.95

0.97

Transmits?

Y

Y

Y

N

Y

Y

N

* Two twinning systems interacting at the boundary

The maximum m’ value possible is 1, so all in all of the cases, only twins with a high m’ value developed at the boundary. So, not only was it possible to predict whether a twin would transmit across a boundary, we could also predict which twinning system would nucleate if given the incoming twin and boundary angle. Twin 6 presents a particularly interesting case. Even though the interaction has a high m’ factor, or in other words, has a closely aligned available twin plane on the other side of the boundary, the twin still did not transmit. That is, the misorientation angle seems to be a better indication of the barrier strength of the boundary than the m’ factor.

To sum up, what began as a relatively routine effort to characterize the deformed microstructure of rhenium turned out to be an interesting study on twin/grain boundary interactions in HCP materials. There’s still significant work left to do to really understand what’s going on, but the EBSD data provides a great starting point. A big thanks to the people at EDAX for letting me come spend such an enjoyable and productive week with them!

Microstructural Treasure Hunting

René de Kloe, Applications Specialist EDS, EBSD

My work is all about answering microanalysis related questions from potential customers about what the EDAX system can do to questions from existing users who want to know how to do something specific. These questions may range from the simple “sure we can” type to in-depth scientific discussions on parts of PhD research projects. Because of this variation, working at the forefront of microanalysis development is never boring.

However, the most difficult questions are not asked by customers or fellow electron microscopists. The hardest question actually comes from family, friends, and immigration officials at airports. That question is: “so what is it that you actually do?” Try answering that question to someone who has never seen an electron microscope before or knows what a crystal is! My standard answer is that I work for a company that makes laboratory equipment but unfortunately that does not really explain anything as the next question is often something like “in which hospital do you work?” or “why is that important?”

Then it becomes time for some examples. An easy one is doing a quick analysis of some jewellery to check if there is any Ni in it which may set off an allergic reaction or to verify the gold or silver content that an item should have according to the stamp. But more often I like to take a look at an everyday item or something that everybody has heard of. So if I find anything that might be interesting, I try to take a look at it in a spare moment.

To me the key of looking at any new sample is keeping my eyes open for hidden treasures without too much knowledge of what supposedly happened to a material. Just like geocaching, the world-wide treasure hunting game where people put a cache somewhere to guide you to special places that you never would have found without the cache that is hidden there.

Recently I had such a case. My wife is a veterinary surgeon and in the practice where she works a patient came in who suffered from bladder stones. After the initial treatment, dogs are typically prescribed a specific diet that prevents further formation of these stones. But you need to know what crystals these stones are made of to select the proper diet. So in addition to sending a sample for the routine chemical analysis we decided to take a look ourselves with the SEM and EDS.

Fig 1 Smooth bladder stone
Fig 2 Flaky surface of the bladder stone.
Figure 1: Smooth bladder stone Figure 2: Flaky surface of the bladder stone

Initial SEM imaging showed a nice smooth surface on a bladder stone that was flaking off a little with sharp euhedral crystals underneath. EDS showed a clear N signal from the stones, which is indicative of the presence of ammonium-compounds. However when we split the stone things looked very different.

Fig 3 N (red) and P (green) EDS maps superimposed on fractured stone
Fig 4 Ca (blue) superimposed on fractured stone
Figure 3: N (red) and  P (green) EDS maps
superimposed on fractured stone
Figure 4: Ca (blue) superimposed on fractured stone

The N was only present in the flaky surface layer. Underneath was a mantle that was rich in Ca and O, with grains whose shape are indicative for Ca-oxalate CaC2O4 and the stone had a core of CaPO4. Unfortunately I could not extract EBSD patterns from the crystals, but I will certainly keep them around for a chance to polish the stone or prepare a FIB lift-out specimen. The different compositions were of interest as oxalate and phosphates require different treatment. Treatment for oxalates requires a low protein diet and slightly increased pH while treatment for the phosphate needs the pH of the urine to become more acidic. 

Fig 5 Euhedral Ca-oxalate crystals
Fig 6 CaPO4 needles in the core of the stone
Figure 5: Euhedral Ca-oxalate crystals Figure 6: CaPO4 needles in the core of the stone

On the outside the stone did not look like something special and certainly the patient will not be all too pleased with their presence. But after cracking it open to take a closer look, a formation sequence of the stone could be recognised that helped in formulating the proper medical treatment. And in the middle of the stone was my final reward, a beautiful microstructure with fine calcium phosphate needles.