Understanding interfacial behavior of materials via in situ micromechanical characterization

We present here a series of examples demonstrating how in situ electron microscopy micromechanical characterization can be employed to understand the behavior of individual interfaces that often dictate macroscale mechanical performance. We highlight how careful experimental design enables characterization of targeted interfaces under application-relevant loading conditions, thereby revealing critical mechanical properties and failure mechanisms. To achieve this, we show exemplary testing geometries for studying various deformation and fracture modes in materials relevant to nuclear, aerospace, and energy storage applications. We also propose testing and characterization strategies that may advance the understanding of interfacial failure in functional devices. Taken together, we hope to inspire wider application of these in situ micromechanical approaches as well as the development of novel methodologies for improved scientific insights. Impact statement Interfaces, the boundaries between dissimilar domains within materials, often control damage initiation and propagation during mechanical failure. The usually complex microstructure of engineering materials means that mechanical testing of the material as a whole provides only the overall mechanical response but makes it difficult to determine the contributions of individual interfaces. Motivated by this, we present a range of examples to illustrate how in situ micromechanical characterization within scanning and transmission electron microscopes (SEM and TEM) can address this knowledge gap. This approach involves mechanically testing micron- or submicron-scale specimens, typically fabricated by focused ion beam milling, that contain only one or a few interfaces, while directly observing how the tested interface(s) fail and measuring their micromechanical properties. Using in situ approaches, we can achieve well-defined loading conditions that facilitate data interpretation and access the dynamic evolution of the failure mechanisms. Our examples span diverse material systems and failure modes relevant to real-world applications, which include ductile deformation, brittle fracture, and environmental-assisted failure of individual interfaces. We also discuss how in situ micromechanical characterisation may be further developed to enable broader applications and deeper insights.