Quantitative In Situ Mechanical Testing in Electron Microscopes

M. Legros, D.S. Gianola, and C. Motz about the surface (e.g., optical and SEM) but have some advantages with regard to temporal resolution over 3D probing technologies, which can require long acquisition times to enhance the signal quality. Figure 1 is a three-axis map of the main in situ tools used to investigate deformation mechanisms. The first two axes are the strain resolution and the length scale, which rely on the size of the probe, the incoming wavelength, and the signalto-noise ratios of the sensors and imaging devices. The third axis is the time resolution, which, in part, is dependent on the detectors. The spatial resolution of optical in situ microscopy and Raman is limited by their wavelength and can be improved by looking at ensembles of small objects. X-ray in situ studies require a very intense and focused beam to investigate small deforming volumes in short amounts of time. Such experiments can only be performed at modern synchrotron sources offering high brightness and advanced detectors; the expense of these experiments precludes repeated tests.1,2 SEM and TEM are clearly well adapted for micronand submicron-sized specimens. The lower time limit is often due to the speed of detectors or video frame rates. Recently, this limit has been extended to ultra-fast dynamic imaging by using lasertriggered beams and synchronous detection. In the case of electron microscopy, this very specific tool, for instance, is currently employed to study fast chemical reaction phase transformations3 (for a review, see Reference 4) but will not be discussed here. In this article, we focus our attention on in situ electron microscopy studies where the time intervals are on Introduction In situ transmission electron microscopy (TEM) and scanning electron microscopy (SEM) have long proved to be effective tools to investigate intrinsic deformation mechanisms as they unfold. Slip band formation, dislocation motion and interactions, and crack nucleation and propagation are examples where direct observations gleaned significant insight. With significant technological advances in high-resolution and fast digital imaging, microactuators, and highfidelity sensors, SEM and TEM have clearly opened new horizons toward dynamically acquiring quantitative data during in situ experiments. A parallel driving force for the development of in situ electron microscopy is the miniaturization of functional building blocks (microand nanoelectromechanical systems [MEMS, NEMS], submicron interconnects, nanowires, thin films, micropillars) down to sizes that are similar to TEM and SEM samples. These objects exhibit unusual mechanical properties in comparison to their bulk counterparts, which causes us to question our current understanding of their modes of deformation at and below the micron scale. Acquiring knowledge of the physical basis leading to such structural alterations in addition to quantitative data about these changes has therefore become of paramount importance, scientifically and technologically. Several imaging platforms across a large range of length scales are appealing for in situ mechanical testing. Conventional imaging systems only provide information Abstract This article is devoted to recent progress in the area of in situ electron microscopy (scanning and transmission) and will focus on quantitative aspects of these techniques as applied to the deformation of materials. Selected recent experiments are chosen to illustrate how these techniques have benefited from improvements ranging from sample preparation to digital image acquisition. Known for its ability to capture the underlying phenomena of plastic deformation as they occur, in situ electron microscopy has evolved to a level where fully instrumented microand nanomechanical tests can be performed simultaneously.