Microstructural Black Swans

Abstract Black swans are a metaphor for rare events with extreme consequences. In the domain of structural materials, black swans represent features in the microstructure that lead to catastrophic failure; as a result of their rarity, they are difficult to observe and often overlooked. These unusual weakest-link features are described variously as incipient, emergent, or anomalous. They give rise to localization, percolation, or avalanche events such as fracture, ductile rupture, dielectric breakdown, corrosion pit nucleation, and fatigue-crack initiation; as such, they are limiting cases in the concept of a representative volume. In this perspective, three examples are given of rare microstructural features and how they limit the mechanical reliability of structural metals. After taking stock of these examples, a future outlook considers the need for high-throughput testing and non-destructive characterization as well as detection algorithms and materials modelling strategies, including accelerated machine learning methods, that can capture anomalous events.