Expanding time–temperature-transformation (TTT) diagrams to interfaces: A new approach for grain boundary engineering

Abstract Bulk phase transformation kinetics were not well understood before Davenport and Bain developed time–temperature-transformation (TTT) diagrams for steel alloys in the 1930s. These powerful diagrams revolutionized the heat treatment of steel and other alloys. Grain boundaries and internal interfaces are now known to behave in a phase-like manner, referred to as ‘complexions’, and their transitions can be represented on TTT diagrams. We present experimental grain boundary complexion TTT diagrams for polycrystalline Al2O3 and Y2O3. Grain boundary mobility discontinuities in Y2O3 occur at different temperatures for different annealing times, an unusual pattern that becomes understandable when viewed on a complexion TTT diagram. Similarly, the anisotropy of Al2O3 complexion kinetics can be visualized with these diagrams. Complexion TTT diagrams are a graphical tool to control interface-related phenomena such as diffusion, creep, oxidation, and microstructure evolution. They could explain why two-step sintering produces dense nanocrystalline ceramics and offer insight into other processes as well.