Science as Art: Strain Field Images

For our next image in the “Science as Art” series, Michelle put together a complete blog for us to explain her images.  I will turn it over to Michelle now!

Have you ever been sitting in your car in the parking lot of the grocery store, minding your own business, when suddenly you see a rogue shopping cart go rolling at full speed, only to be abruptly stopped when it runs into some poor unsuspecting stranger’s car door?  For most normal people, their first thought is, “Oooo.  Bummer.  That’s gonna leave a mark!”  For me, my second thought is typically, “Hmm.  I wonder what the strain field would look like around that ding.”  (But then again, I’ve never claimed to be “normal”.)  😉

The majority of the work I do here at GE Global Research focuses on measuring “plastic strain” (as in…how much plastic strain did the grocery cart just put in the car door?).  Although I’ve never actually looked at a car door dent in my microscope, the concept is pretty much the same – I use a technique called Electron Backscatter Diffraction (EBSD) to look at various metals that have undergone some type of deformation process, then get a general idea of how much plastic strain or damage was introduced into the metal.  (For anyone that read my previous blog entries, these are the same type of measurements I made on the Space Shuttle bolts .)  Being able to measure plastic strain helps our understanding of how metals behave under certain conditions, and can also help us predict when a metal may fail.  I doubt that anyone has ever had their car door fall apart as a result of a shopping cart hit, but for the types of materials we work on here at GE, predicting the effects of plastic strain on things like aircraft engines and nuclear power plants is a pretty big deal!

Last year, my group held a contest to submit some of our work for a 2012 calendar, and I was lucky enough to have two of my images selected!  The January image is a “misorientation map” of a material in which we’re able to see fields of plastic strain produced by a second phase particle in a stainless steel.  What this means in plain English is the following:

I was looking at a stainless steel material when I noticed some weird, linear shaped particles in the matrix of the stainless steel.  (This was kind of a “Sesame Street” moment for me.  Remember the “One of these things is not like the other…” song?  Who knew that something I learned in elementary school would be useful when doing research?!?)  I knew that stainless steels have these particles, so I decided to have some fun and take a closer look at them (Figure 1.)

After getting a better look, I found that these particles had a different crystal structure than the stainless steel matrix, so I decided to collect a map of the area (Figure 2.)  Although the phase map is neat to look at, it doesn’t give me any real information about the plastic strain in the sample.  If I take the same data set and process it with a “misorientation” algorithm (developed here at GE Global Research), I get a different result – one that gives us insight into what the strain fields actually look like!  (Figure 3.)  So what exactly is this mysterious “misorientation” thing?  The term “misorientation” refers to a concept in crystallography where you measure the angles between two crystals.  Going back to our shopping cart and door dent example:  the car door had a certain orientation when it was sitting innocently in the parking lot (let’s call this “Orientation 1”).  Then along came the evil shopping cart and put a big dent in the door (this one will be “Orientation 2”).  The door now has a certain amount of distortion to it – a different shape than it originally was.  If we want to figure out how much the door has been deformed, we can calculate the difference between before and after the door was hit (or the difference between Orientation 1 and Orientation 2).

Misorientation is a little more complicated than this (and it occurs at a MUCH smaller scale!), but hopefully you get the general idea…it’s just a tool for us to measure strain.  In Figure 3 below, an area in red has high deformation (~10° or more), and an area with low deformation is blue.  What we learned from the measurements I made were that 1) we didn’t have a pure stainless steel matrix – we also had delta ferrite.  2)  In addition to having particles in the matrix, we can see from the misorientation map that the delta ferrite particles have a visible “plume” of strain coming off of them.  Ultimately, we learned that this material isn’t the composition that we thought it was, the delta ferrite particles were causing additional strain in the material.

Figure 1 (above): Backscatter electron images of particles.
Figure 1 (above): Backscatter electron images of particles.
Figure 2 (above): Phase map of delta ferrite particles (red) in a stainless steel matrix (blue).  This map is showing that the red and blue areas are crystallographically different.
Figure 2 (above): Phase map of delta ferrite particles (red) in a stainless steel matrix (blue). This map is showing that the red and blue areas are crystallographically different.
Figure 3 (above):  Misorientation map of the same particles.  Note the red “plumes” of strain coming off of the delta ferrite particles.
Figure 3 (above): Misorientation map of the same particles. Note the red “plumes” of strain coming off of the delta ferrite particles.

Using the same technique, (EBSD for misorientation mapping) I measured a different material for the 2012 calendar’s July image (Figure 4).  This material is made from a cobalt alloy and during testing, the material ultimately failed.  (Cobalt alloys are typically used in high temperature corrosive environments .)  From the map below, we can see that there are multiple crack regions, plus there are areas of localized high strain (as indicated by the areas in red).

In the previous example, we were studying the material to prevent failure.  In this example, the material had already failed, but we needed to understand the mechanisms that lead to the failure.  In either situation, it makes for an interesting and colorful map.  (And I’ll bet that you never look at a grocery cart in a parking lot the same way now!)

Figure 4 (above):  EBSD misorientation map of a cobalt alloy.  Areas in black indicate regions where the material cracked, and ultimately failed.  Areas in red indicate localized regions of high strain.
Figure 4 (above): EBSD misorientation map of a cobalt alloy. Areas in black indicate regions where the material cracked, and ultimately failed. Areas in red indicate localized regions of high strain.

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