[Image above] Credit: Justin Kern; Flickr CC BY-NC-ND 2.0
If you’re not a materials scientist, “controlling grain growth” sounds like lingo you might hear on the farm.
But if you are familiar with materials science and engineering, you know that controlling grains has another important meaning, too.
In fact, controlling grain growth is critical when it comes to engineering ceramic materials. That’s because grain boundaries, the interfaces between individual crystals (grains) that make up a polycrystalline material, determine a material’s properties and performance.
So, if I told you that a team led by researchers at the University of California, Davis has published new research that will shift the paradigm of grain growth control, that would be pretty big news, right?
You can bet the farm it’s big news.
According to Ricardo Castro, associate professor of materials science and engineering at UC Davis and senior author of the new work, “Growth has always been considered a thermodynamically favorable process, and grain growth control is performed by inducing drag forces to stop diffusion (mobility of the boundary) from happening.”
But the team’s new work shows that diffusion isn’t the only consideration when it comes to controlling grain growth. In fact, sometimes, the scientists say, controlling diffusion doesn’t even matter.
The researchers examined one material, gadolinium-doped yttria-stablized zirconia (YSZ), very closely. Their kinetic studies and calorimetery, spectroscopy, and microscopy measurements of gadolinium-doped YSZ showed that the dopant, which segregates to grain boundaries, decreases grain boundary energy within the material.
Less grain boundary energy means there is less of a push on the boundary to move.
Gadolinium doesn’t freeze the grain boundaries, however—they still have the ability to move, although they don’t. “Although the boundaries still have mobility, they follow a random walk, with no net movement,” according to Castro.
It’s like a large crowd at a concert: If there a lot of people jammed together, moving very energetically and frenetically (high grain boundary energy)—say, a concert featuring the Celtic punk band Flogging Molly—people at the edges of the crowd are going to have a hard time staying in place.
But if the crowd is more calm (low grain boundary energy)—say, at an Enya concert—then the people at the edges might shuffle their feet back and forth, but with negligible net movement in the end.
“The take home message is simple—ceramics with no grain growth can and shall be designed by targeting a reduction in the grain boundary energy,” Castro writes in an email. “This work proved that although the atoms in the system can move around at that temperature, they move like a drunk person—with lots of steps but no net movement. Therefore, the boundary doesn’t migrate, and no grain growth is observed.”
These results are surprising because researchers previously assumed that thermodynamics have little effect on grain growth control. Instead, this work provides “striking evidence” that thermodynamics have a dominant effect on grain growth over grain mobility and diffusion, according to the paper.
“The results break an old paradigm that grain growth should be controlled by pinning the movement of the grain boundary by creating obstacles for its movement—so-called drag-forces,” Castro explains via email. “This old paradigm considers the thermodynamics of the system always favorable—grains always want to grow and will grow if they have mobility. We prove this is not true, and grains may not grow if the thermodynamics allow a metastable state.”
Ultimately, because grain boundaries control a material’s properties, being able to tailor the formation of those boundaries could open avenues to better design of future materials. Plus, the researchers think their results extend beyond gadolinium-doped YSZ as well.
“Although the proof of concept is for zirconia, we have additional recently published results that corroborate these observations for other systems,” Castro writes via email. “We believe this combination proves materials professionals have a new way to control grain growth, and they should be considering this for other systems and revisiting previously reported data with new optics.”
“We believe this is so big, textbooks will need to be amended,” Castro adds.
The paper, published in Acta Materialia, is “Thermodynamics versus kinetics of grain growth control in nanocrystalline zirconia” (DOI: 10.1016/j.actamat.2017.07.005).