Basic concepts of grain-boundary structure and phase behavior: From theory and experiments to material properties

Understanding and controlling structure-processing-properties-performance relationships form the central pillar of materials science and engineering.Formation of phases and evolution of material imperfections (defects) provides the two primary features of a system that enables control of these relationships.Although the impact of imperfections such as dislocations or grain boundaries on material properties has been explored quite deeply, little is known about the thermodynamic phases of the defects themselves.In recent decades, a growing appreciation for the occurrence of phase transformations of surfaces and grain boundaries has emerged.This concept of grain-boundary phase transformation and its impact on properties is at the core of this issue and introductory article.The thermodynamic fundamentals will be explained, experimental and theoretical tools to uncover grain-boundary phases and related property changes are discussed and applied to different material systems.In addition, we also want to look beyond and introduce the readers to novel findings on phase transformations of other defects, such as dislocations.In several cases, phase transformations of defects have been demonstrated to dramatically affect their properties and in turn, the overall properties of the bulk materials containing them.The additional ability to control materials properties and performance by tailoring both defect distributions and their thermodynamic phase state motivate ongoing theoretical, computational, and experimental efforts to understand and control defect phase behavior.