What I do at SNL: Part 1

People are always asking about what I actually do at my job at Sandia National Laboratories. I could claim that my work is too sensitive to discuss because it's related to national security or something, but (1) that's just not the case, (2) my research will be published in a public journal, and (3) that would stop me from talking about it (which isn't much fun). This series of posts will attempt to explain the basics of my work, but I can't promise it will be interesting or easily understood.

Introduction: Grain Growth

Almost everything that's made out of metal has a polycrystalline structure. This simply means (as you might have guessed) that it's made of many separate crystalline areas. These areas' atoms are arranged in a perfect lattice, and the difference between any two areas is just the orientation of the lattice. The term grain is used to describe one crystalline area, and grain boundaries seperate them. This picture shows a few grains, and how the atoms' arrangement gives rise to the structure:

There is no difference between the red, blue, and green atoms, but it's easier to see that they are actually oriented differently when they are colored. This structure has three unique grains and three grain boundaries.

You can probably see that there is extra space between the atoms at the boundaries, and you can imagine that there would be missing bonds at the boundaries also. Because of these reasons, this structure isn't the most stable. It would be more stable if the grain boundaries disappeared and all the atoms were arranged along the same lattice. This would eliminate the extra space and maximize the number of bonds (which are favored).

Grain growth is the process that materials go through to minimize grain boundary area (or length in the 2D example above). If red atoms along the red/blue boundary were to move slightly so that they lined up with the blue atoms, they would effectively become blue atoms, and that boundary would move. Eventually all the red atoms would align with the blue atoms, the red grain would disappear, and the grain boundary itself would also disappear. (If you stop to think about it, grain growth is a misnomer: for any grain to grow, one or more other grains need to shrink. However, the average grain size increases, so the name makes sense from a statistical point of view.)

Because the atoms have to move, grain growth usually only occurs at high temperatures (meaning close to the material's melting temperature); therefore it doesn't happen in bridge metal, or a pop can. It can happen in places like a jet engine, and is used during many manufacturing processes.

At the most basic level, my job is simulating grain growth. Next time I'll explain pinning, which is a way that grown growth can can be slowed, or even stopped.