The mechanical properties of any materials are highly dependent on defects and defect interactions. To improve the mechanical performance and design better materials, it is critical to understand the way defects influence the mechanical properties fundamentally. Transmission electron microscope (TEM) is a powerful tool for the characterization of defects, and thus there is a long history of studying defects using TEM. In situ TEM straining stages were first developed in 1950s, for example, to enable direct observations of dislocations and their interactions with other defects such as twins, grain boundaries (GBs), and materials interfaces. With the recent development of load sensors, in situ TEM mechanical testing combines the power of TEM imaging and diffraction with quantitative load and displacement measurements to provide quantitative understanding of the deformation and fracture mechanisms in various materials. Here, we used in situ TEM to study the deformation and fracture mechanisms of ceramics and alloys.Firstly, we demonstrated a novel method to evaluate the conditional fracture toughness of thin films and to correlate with in-situ study of fracture mechanisms. Nanocrystalline TiN thin films were investigated using this method. In-situ TEM bright field imaging reveals three crack propagation pathways, namely bridging, intergranular fracture and a mixed mode of transgranular and intergranular fracture. Our methodology is universal and can be applied to other ceramic material systems to evaluate the fracture toughness and study the deformation and failure mechanisms. To further understand the deformation mechanisms of nanocrystalline ceramics, we conducted in situ TEM compression testing on nanocrystalline TiN nanopillars. Grain rotation is detected during the deformation of nanocrystalline ceramics, which effectively alleviates the lattice strain.Next, we studied the deformation mechanisms of a new type of alloy, high-entropy alloys (HEAs), with the help of in situ TEM and focused ion beam (FIB) fabrication. The deformation mechanism of HEA nanopillar is revealed by simultaneous measurement of mechanical response and dislocation dynamics. By observing dislocation activities leading to dislocation slip on a single slip plane in HEA nanopillars using in-situ TEM, a series of yielding events are revealed, including activation/deactivation of dislocation sources, intermittent propagation of dislocation arrays, collective dislocation jumps, and finally slip avalanches with large stress drops. The experimentally-obtained stress-dependent slip-size distributions and the spatial properties of the slips in the HEA nanopillars agree with the MFT-model predictions. We obtained a scaling collapse of the slip-avalanche size distributions as function of applied stress and dislocation activities that confirm MFT-scaling predictions and indicate that the applied stress is a critical tuning parameter.Lastly, we studied the soliton-like dislocation waves in HEA nanopillars. The waves propagate initially smoothly with rise and falls in the wave width, followed by intermittent jumps. We show that the waves were formed by the operation of multiple Frank-Read dislocation sources. The propagation of dislocation waves is accompanied by intermittent bursts of dislocation activities over a large area of the nanopillars. Thus, the correlation study of mesoscopic mechanic testing and nm-scale dislocation imaging here provides unprecedented insights into the less observable dislocation processes during the quiescent periods between large avalanches and collective dislocation dynamics.
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In situ TEM investigation of deformation and fracture mechanisms of ceramics and alloys
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