Direct observations of defect structures

Physical metallurgy has certainly benefited from direct observations of defect structures, as Kacher et al.^{Reference Kacher, Kirchlechner, Michler, Polatidis, Schwaiger, Van Swygenhoven, Taheri and Legros15} show in their article in this issue. They discuss mostly TEM-based observations that led to insights into defect–defect interactions and their impact on mechanical properties. Grain boundaries are one of the most common defects in structural materials, but many details of the complex strain accommodation mechanisms at grain boundaries are still unsolved. In situ experiments have been reported that deal with inherent mechanisms in metals that relax stress at grain boundaries, such as grain-boundary migration and grain rotation. In this respect, automated crystal orientation mapping (ACOM) performed by electron diffraction inside the TEM gives information on the evolution of grain size and grain orientation under load and guides the analysis of different grain-boundary mechanisms. While this is especially important for fine-grained and nanocrystalline materials, dislocation–grain-boundary interactions are another topic highlighted in their overview. In service, engineering materials are exposed to complex stress states and high strain rates. The recent advances of in situ nanomechanical testing covering both of these topics is another central point of their overview.

In situ imaging is most useful in the context of connecting defects and structural features with deformation studies. Due to the multiscale and complex nature of defect structures in deformed materials, correlative microscopy techniques in scanning electron microscopy can provide unique insights. In their article in this issue, Gianola et al.^{Reference Gianola, Britton and Zaefferer16} discuss some of the emerging SEM-based methods for identifying defects, including EBSD, ECCI, and diffraction-contrast STEM. EBSD continues to evolve with faster detectors that enable in situ mechanical testing combined with ECCI and diffraction-contrast STEM, and higher resolution imaging that allow for defect characterization.

Atomic-scale resolved imaging

In their article, Spiecker et al.^{Reference Spiecker, Oh, Shan, Ikuhara and Mao17} carry the discussion to the atomic scale by employing atomic resolved in situ TEM/STEM experiments to study dislocation generation, multiplication, and interaction with hydrogen as well as dislocation–grain-boundary interactions. The strength of these methods is the direct visualization of defects during deformation. An example provided in their article is screw dislocations intersecting a low-angle tilt grain-boundary-inducing kink formation in the transmitted dislocations and jogs in the grain-boundary dislocation array. Such studies are crucial to understand dislocation–grain-boundary transmission stresses and possible resulting strain-rate dependence. Interestingly, functional two-dimensional (2D) materials such as bilayer graphene can also form dislocations as inherent growth defects, which may alter charge transport in devices. Spiecker et al.^{Reference Spiecker, Oh, Shan, Ikuhara and Mao17} show that in situ nanomechanics allows for manipulation of individual dislocations, which in turn leads to mechanical switching of stacking sequences of the 2D material,^{Reference Butz, Dolle, Niekiel, Weber, Waldmann, Weber, Meyer and Spiecker18} a previously unobserved effect.

Local strain measurements

Emerging techniques of measuring local strain around individual defects or groups of defects are discussed by Gammer et al. in their article in this issue.^{Reference Gammer, Richard and Eberl19} Digital image correlation is one of the most readily available methods for strain measurements using light optical or electron microscopy imaging techniques. At the (sub-)nanometer length scale where high-resolution TEM is employed, geometric phase analysis (GPA),^{Reference Hÿtch, Houdellier, Hüe and Snoeck20} dark-field electron holography (DFEH),^{Reference Midgley and Dunin-Borkowski21,Reference Koch, Özdöl and Van Aken22} and four-dimensional (4D) STEM using nanobeam electron diffraction^{Reference Ozdol, Gammer, Jin, Ercius, Ophus, Ciston and Minor23–Reference Pekin, Gammer, Ciston, Ophus and Minor25} have been developed to measure strains. These techniques can also correlate defect structures and mechanical properties. GPA requires an atomically resolved image to map the strains in reciprocal space. DFEH and in-line DFEH take advantage of the phase change of the electron wave traveling through a strained lattice of the sample and need dedicated hardware or computational techniques to acquire the projected strain tensor, and are not yet easily implemented as routine in situ techniques.

In contrast, 4D STEM, where an annular detector is used for dark-field imaging and an ultrafast camera is used to simultaneously record the nanobeam electron diffraction pattern of the rastering beam at every position, is a robust correlative technique. Video camera speeds of several hundred frames per second are required for in situ experiments and are nowadays available by fast complementary metal oxide semiconductor or direct-detection camera systems. In return, the user must be prepared for large data sets to be analyzed. Big data is routine now for synchrotron experiments, where strain mapping uses either monochromated or polychromated x-rays as probes. Laue diffraction with its polychromatic x-rays avoids tedious sample tilting compared to monochromated x-ray techniques such as nano-XRD, as the polychromatic spectrum always contains wavelengths that fulfill the Bragg diffraction condition. Submicron beam sizes and a high sensitivity to probe local orientation gradients make µLaue diffraction a versatile tool.

While deviatoric strains are straightforward to extract, hydrostatic strain remains undetected as the positions of the diffraction spots remain unchanged.^{Reference Barabash, Ice and Walker26} The full strain tensor requires knowledge of the corresponding diffracting wavelength by an energy scan or an energy-sensitive detector. µLaue has been successfully applied for in situ compression, tension, and low-cycle fatigue tests.^{Reference Maaß, Van Petegem, Grolimund, Van Swygenhoven, Kiener and Dehm27–Reference Cornelius and Thomas29} New developments using monochromated x-rays encompass fast scanning x-ray microscopy and full-field diffraction x-ray microscopy combining imaging and diffraction as well a coherent Bragg diffraction.^{Reference Cornelius and Thomas29} While strains are measured with unprecedented accuracy, experimental challenges and tedious data analysis currently prevent routine application of these methods.

Environmental in situ nanomechanical testing

The impact of environment is a critical area of in situ nanomechanical testing that has expanded in recent years. In their article, Barnoush et al.^{Reference Barnoush, Hosemann, Molina-Aldareguia and Wheeler30} report on recent progress in extending in situ nanomechanics to low (–140°C) and elevated temperatures (1000°C) and the challenges that need to be considered. Similarly, recent advances in strain-rate-dependent in situ nano- and micromechanical experiments ranging from creep at 10^–6 s^–1 to impact testing at 10⁶ s^–1 are summarized. Barnoush et al.^{Reference Barnoush, Hosemann, Molina-Aldareguia and Wheeler30} report longstanding challenges in understanding the complex mechanisms of hydrogen embrittlement. This gained new momentum by in situ nanomechanical testing. In situ nanomechanical testing allows individual microstructure components (grain boundaries, phases, and grain interior) to be separately addressed.

Another topic reported in their article revolves around in situ nanomechanical testing of materials exposed to radiation. For harsh environments, in situ nanomechanics are key in discriminating individual deformation mechanisms for complex engineering materials and may in future present a tool to monitor materials by extracting only small volumes. Advances in mechanical testing instrumentation and testing methods under such special environments are discussed in the Barnoush et al. article.^{Reference Barnoush, Hosemann, Molina-Aldareguia and Wheeler30}

Nanotribology

Insights into nanotribology from in situ experiments have enabled identification of physical processes occurring at and below a wear surface. In their article in this issue, Jacobs et al.^{Reference Jacobs, Greiner, Wahl and Carpick31} describe key advances in in situ nanotribology that have enabled direct observations of processes in the sliding contact, including tribochemistry and subsurface deformation. For example, in situ TEM nanotribology has revealed a number of material transformations in layered materials typically used as lubricants such as MoS₂³² and graphite.^{Reference Merkle, Erdemir, Eryilmaz, Johnson and Marks33} Studies of corrosive environments have benefitted from quasi-in situ AFM-based probing investigations coupled with ex situ analysis. Subsurface dislocation processes in metal contacts during in situ nanotribology demonstrate the impact of in situ testing, and surface adhesion phenomena are readily observed with in situ TEM. As Jacobs et al.^{Reference Jacobs, Greiner, Wahl and Carpick31} discuss, breakthroughs in in situ nanotribological characterization have the potential to impact a wide array of applications and technologies.

Integrated micro-/nanofabrication

Bhowmick et al.^{Reference Bhowmick, Espinosa, Jungjohann, Pardoen and Pierron34} describe how integrated micro-/nanofabrication can lead to incredible control and opportunities for complex testing schemes. Lab-on-chip testing provides material testing schemes with accurate mechanical testing and precise measurements of load and displacement. For example, it is possible to investigate rate effects^{Reference Ramachandramoorthy, Gao, Bernal and Espinosa35} or combine multimodal testing capabilities such as simultaneous electrical and mechanical measurements.^{Reference Zeng, Gammer, Ozdol, Nordqvist, Nygård, Krogstrup, Minor, Jäger and Olsson36} The flexibility of microelectromechanical systems devices leads to adaptation for high cycle fatigue, in situ wear and environmental testing. Future designs integrating control of temperature, liquids, and electrochemistry will provide insight into operando conditions of materials deformation.