In situ nanomechanical testing provides critical insight into the fundamental processes that lead to deformation phenomena in materials. Often, in situ tests are performed in relevant conditions such as high or low temperatures, tribological contact, gas environments, or under radiation exposure. Modern diffraction and imaging methods of materials under load provide high spatial resolution and enable extraction of quantitative mechanical data from local microstructure components or nano-sized objects. The articles in this issue cover recent advances in different types of in situ nanomechanical testing methods, spanning from dedicated nanomechanical testing platforms and microelectromechanical systems devices to deformation analyses via in situ diffraction and imaging methods. This includes scanning electron microscopy, advanced scanning transmission electron microscopy, electron diffraction, x-ray diffraction, and synchrotron techniques. Emerging areas such as in situ tribology enable novel insights into the origin of deformation mechanisms, while the evolution of microelectromechanical systems for controlled in situ testing provide opportunities for advanced control and loading strategies. Discussion on the current state of the art for in situ nanomechanical testing and future opportunities in imaging, strain sensing, and testing environments are also addressed.

In situ nanomechanical testing in (scanning) transmission electron microscopy provides unique opportunities for studying fundamental deformation processes in materials. New insights have been gained by combining advanced imaging techniques with novel preparation methods and controlled loading scenarios. For instance, by applying in situ high-resolution imaging during tensile deformation of metallic nanostructures, the interplay of dislocation slip and surface diffusion has been identified as the key enabler of superplasticity. Evidence for dislocation pinning by hydrogen defect complexes has been provided by in situ imaging under cyclic pillar compression in a tunable gas environment. And, for the very first time, individual dislocations have been moved around in situ in two-dimensional materials by combining micromanipulation and imaging in a scanning electron microscope.

Defects in crystalline solids control the properties of engineered and natural materials, and their characterization focuses our strategies to optimize performance. Electron microscopy has served as the backbone of our understanding of defect structure and their interactions, owing to beneficial spatial resolution and contrast mechanisms that enable direct imaging of defects. These defects reside in complex microstructures and chemical environments, demanding a combination of experimental approaches for full defect characterization. In this article, we describe recent progress and trends in methods for examining defects using scanning electron microscopy platforms. Several emerging approaches offer attractive benefits, for instance, in correlative microscopy across length scales and in in situ studies of defect dynamics.

Functional and mechanical properties of modern devices are directly controlled by the stress and strain state acting on the materials within. For manufacturers, elastic strain engineering of complex materials systems throughout processing and utilization is crucial. This requires methodologies with ever-increasing spatial and temporal resolutions. On the other hand, the nanoscale elastic strain field around individual defects fundamentally controls the deformation of crystalline materials. To date, a variety of techniques are available for measuring elastic strain, including transmission electron microscopy, electron backscatter diffraction, and x-ray diffraction. Recent advances in instrumentation have dramatically improved speed and resolution, enabling direct elastic strain mapping during in situ deformation at the nanoscale. In addition, plastic strain can be determined during deformation using digital image correlation. Current techniques are surveyed here to accurately quantify complex strain fields at the nanoscale and their potential to resolve scientific challenges in materials science.

The mechanical response of modern alloys results from a complex interplay between existing microstructure and its evolution with time under stress. To unravel these processes, in situ approaches intrinsically have a critical advantage to explore the basic mechanisms involving dislocations, grain boundaries (GBs), and their interactions in real time. In this article, we discuss recent findings using in situ nanomechanical testing techniques and refined crystallographic analysis tools. Advancements in in situ nanomechanics not only include multiaxial loading conditions, which bring us closer to real-world applications, but also high strain-rate testing, which is critical to compare experiments and simulations. In particular, unraveling the details of GB-based mechanisms and related microstructural changes will facilitate significant breakthroughs in our understanding of the behavior of materials on macroscopic length scales.

The high precision offered by small-scale mechanical testing has allowed the relationships between mechanical behavior and specific microstructural features to be determined to an unprecedented degree. However, of most interest to scientists and engineers is often the behavior of materials under service conditions in an extreme environment, such as high/low temperatures, high strain rates, hydrogen atmosphere, or radiation. In this article, we detail progress made to adapt nanomechanical testing systems and techniques to observe materials behavior in situ in extreme environments.

Tribology—the study of contacting, sliding surfaces—seeks to explain the fundamental mechanisms underlying friction, adhesion, lubrication, and wear, and to apply this knowledge to technologies ranging from transportation and manufacturing to biomedicine and energy. Investigating the contact and sliding of materials is complicated by the fact that the interface is buried from view, inaccessible to conventional experimental tools. In situ investigations are thus critical in visualizing and identifying the underlying physical processes. This article presents key recent advances in the understanding of tribological phenomena made possible by in situ experiments at the nanoscale. Specifically, progress in three key areas is highlighted: (1) direct observation of physical processes in the sliding contact; (2) quantitative analysis of the synergistic action of sliding and chemical reactions (known as tribochemistry) that drives material removal; and (3) understanding the surface and subsurface deformations occurring during sliding of metals. The article also outlines emerging areas where in situ nanoscale investigations can answer critical tribological questions in the future.

The field of in situ nanomechanics is greatly benefiting from microelectromechanical systems (MEMS) technology and integrated microscale testing machines that can measure a wide range of mechanical properties at nanometer scales, while characterizing the damage or microstructure evolution in electron microscopes. This article focuses on the latest advances in MEMS-based nanomechanical testing techniques that go beyond stress and strain measurements under typical monotonic loadings. Specifically, recent advances in MEMS testing machines now enable probing key mechanical properties of nanomaterials related to fracture, fatigue, and wear. Tensile properties can be measured without instabilities or at high strain rates, and signature parameters such as activation volume can be obtained. Opportunities for environmental in situ nanomechanics enabled by MEMS technology are also discussed.

Three-dimensional printing (3DP) is becoming a standard manufacturing practice for a variety of biomaterials and biomedical devices. This layer-by-layer methodology provides the ability to fabricate parts from computer-aided design files without the need for part-specific tooling. Three-dimensional printed medical components have transformed the field of medicine through on-demand patient care with specialized treatment such as local, strategically timed drug delivery, and replacement of once-functioning body parts. Not only can 3DP technology provide individualized components, it also allows for advanced medical care, including surgical planning models to aid in training and provide temporary guides during surgical procedures for reinforced clinical success. Despite the advancement in 3DP technology, many challenges remain for forward progress, including sterilization concerns, reliability, and reproducibility. This article offers an overview of biomaterials and biomedical devices derived from metals, ceramics, polymers, and composites that can be three-dimensionally printed, as well as other techniques related to 3DP in medicine, including surgical planning, bioprinting, and drug delivery.