(V) KEYNOTE: The Connection Between Bulk and Interfacial Phases in a Titanium Twin Boundary

The notion of interfacial phases and phase transitions, recognized already by Gibbs many years ago, and more recently referred to as interfacial complexions, has become an active area of research. In this talk we will discuss experimental and computational investigations of interfacial phases and phase transitions for the case of a deformation twin in hexagonal-close-packed titanium. The observations establish a correspondence between the interfacial structure and a metastable polymorph of titanium that is coherently strained in the interfacial region. The presence of this complexion is linked to large magnitudes of the interfacial excess volume and stress, which underlie first-order and solid-state wetting types of interfacial phase transitions induced by tensile and compressive lattice strain, respectively. The results are discussed in the context of solid-state phase transformations, and more generally the possibility of exploiting metastable phases to tune interfacial properties relevant to materials properties.