What I do at SNL: Part 2

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.

Last time I explained a bit about grain growth.

The Engineering: Pinning

Generally grain growth is undesirable anywhere outside of the manufacturing process. We usually strive for polycrystalline materials to have a fairly small grain size, and for the grains to stay small. This is mostly driven by the inverse relationship between grain size and material strength. There isn't a rigorous model that defines a specific mathematical relationship, but in general, the smaller the grains are, the stronger the material is.

With the goal to slow down or prevent grain growth in mind, engineers noticed that one solution is to pin the boundaries with particles of another phase. Usually the other phase is just something composed of different elements. In steel for instance, the main phase is mostly iron (with small amounts of carbon and other elements). The second phase might be really high in carbon, and only have trace amounts of anything else. The most important thing about the second phase phase particle is that it is inactive, meaning it doesn't move, grow, shrink, etc.

This drawing illustrates how pinning would work:


On the left, the curvature of the blue and red grain boundaries causes them to move as the arrows indicate. This allows them to minimize their length, and therefore is favored by the system (because it lowers the energy). However, if you introduce second phase particles, as in the right image, the boundary can only move a little bit (until it becomes completely straight) before it's pinned. It can't move anymore because any motion would have to increase its length.

In all the simulations I do, pinning is involved. This makes the system behave much differently, and as I'll explain in a future post, may or may not lead to the desired effect.