(V) From mixed mode transformations to chemical boundary creation in Fe-C-Mn steels

This symposium in honour of prof John Agren (KTH, Sweden) being awarded the J. Willard Gibbs Phase Equilibria Award correctly highlights the importance of Phase Equilibria. Knowing the final equilibrium is indeed of crucial importance in all metallurgical processing but understanding the routes to get there are of equal significance and as challenging. In this presentation I will address the mixed-mode character of the austenite-ferrite and ferrite-austenite phase transformation and demonstrate the significance of the starting microstructure, both from a topological perspective as well as a chemical perspective. I will then show how understanding the element partitioning at the moving interface and in particular the rapid decoupling of the interface from the chemical heterogeneity can lead to new microstructure, the chemical boundary concept, which can be used to create nanostructured steels with outstanding strength and ductility combinations.