Choreography of atoms during the bainite transformation

The term choreography refers to the design of orderly, almost predictable and often graceful movements of physical bodies, used often in the context of dance. However, dance can be associated with random movements, for example when a flash mob assembles in a public place such as a busy railway station.

These two kinds of motions have analogies in solid-state transformations where atoms can move in a disciplined or uncoordinated manner while achieving a structural change. In steels, there is a further complication in that carbon atoms are much smaller than iron atoms, with dramatic differences in their atomic mobilities. It is then possible that the slower iron atoms are forced to move in a choreographed sequence whereas the carbon atoms follows more random trajectories.

I will demonstrate in this lecture that the nature of atomic motion during the bainite transformation has profound consequences on the development of structure at scales ranging from atoms to engineering structures. Furthermore, there is a sufficiently clear picture of the mechanisms involved to use the theory quantitatively in designing steels (here be the role of a choreographer). This will be illustrated with examples including the world’s first bulk nanostructured metal, the rail steel that has no carbides and yet has served without maintenance for more than one billion metric tonnes of traffic, a new phase diagram for Fe-C that is firmly founded on both high-resolution and synchrotron evidence, the sound of bainite, etc.