In situ deformation of thin films on substrates

High stress twinning in a compositionally complex steel of very high stacking fault energy

Deformation twinning is rarely found in bulk face-centered cubic (FCC) alloys with very high stacking fault energy (SFE) under standard loading conditions. Here, based on results from bulk quasi-static tensile experiments, we report deformation twinning in a micrometer grain-sized compositionally complex steel (CCS) with a very high SFE of ~79 mJ/m², far above the SFE regime for twinning (<~50 mJ/m²) reported for FCC steels. The dual-nanoprecipitation, enabled by the compositional degrees of freedom, contributes to an ultrahigh true tensile stress up to 1.9 GPa in our CCS. The strengthening effect enhances the flow stress to reach the high critical value for the onset of mechanical twinning. The formation of nanotwins in turn enables further strain hardening and toughening mechanisms that enhance the mechanical performance. The high stress twinning effect introduces a so far untapped strengthening and toughening mechanism, for enabling the design of high SFEs alloys with improved mechanical properties.

Mechanical twinning is difficult to trigger in face centered cubic alloys with high stacking fault energies (SFEs) under standard tensile loading. Here, the authors report high stress twinning in a bulk compositionally complex steel of very high SFE, enhancing the material’s mechanical performance.

In Situ TEM Observations of Discharging/Charging of Solid-State Lithium-Sulfur Batteries at High Temperatures

Understanding the structural evolution of Li₂ S upon operation of lithium-sulfur (Li-S) batteries is inadequate and a complete decomposition of Li₂ S during charge is difficult. Whether it is the low electronic conductivity or the low ionic conductivity of Li₂ S that inhibits its decomposition is under debate. Furthermore, the decomposition pathway of Li₂ S is also unclear. Herein, an in situ transmission electron microscopy (TEM) technique implemented with a microelectromechanical systems (MEMS) heating device is used to study the precipitation and decomposition of Li₂ S at high temperatures. It is revealed that Li₂ S transformed from an amorphous/nanocrystalline to polycrystalline state with proceeding of the electrochemical lithiation at room temperature (RT), and the precipitation of Li₂ S is more complete at elevated temperatures than at RT. Moreover, the decomposition of Li₂ S that is difficult to achieve at RT becomes facile with increased Li⁺ ion conduction at high temperatures. These results indicate that Li⁺ ion diffusion in Li₂ S dominates its reversibility in the solid-state Li-S batteries. This work not only demonstrates the powerful capabilities of combining in situ TEM with a MEMS heating device to explore the basic science in energy storage materials at high temperatures but also introduces the factor of temperature to boost battery performance.

In situ thermomechanical testing methods for micro/nano-scale materials

The advance of micro/nanotechnology in energy-harvesting, micropower, electronic devices, and transducers for automobile and aerospace applications has led to the need for accurate thermomechanical characterization of micro/nano-scale materials to ensure their reliability and performance. This persistent need has driven various efforts to develop innovative experimental techniques that overcome the critical challenges associated with precise mechanical and thermal control of micro/nano-scale specimens during material characterization. Here we review recent progress in the development of thermomechanical testing methods from miniaturized versions of conventional macroscopic test systems to the current state of the art of in situ uniaxial testing capabilities in electron microscopes utilizing either indentation-based microcompression or integrated microsystems. We discuss the major advantages/disadvantages of these methods with respect to specimen size, range of temperature control, ease of experimentation and resolution of the measurements. We also identify key challenges in each method. Finally, we summarize some of the important discoveries that have been made using in situ thermomechanical testing and the exciting research opportunities still to come in micro/nano-scale materials.

In situ nanomechanical testing in focused ion beam and scanning electron microscopes

The recent interest in size-dependent deformation of micro- and nanoscale materials has paralleled both technological miniaturization and advancements in imaging and small-scale mechanical testing methods. Here we describe a quantitative in situ nanomechanical testing approach adapted to a dual-beam focused ion beam and scanning electron microscope. A transducer based on a three-plate capacitor system is used for high-fidelity force and displacement measurements. Specimen manipulation, transfer, and alignment are performed using a manipulator, independently controlled positioners, and the focused ion beam. Gripping of specimens is achieved using electron-beam assisted Pt-organic deposition. Local strain measurements are obtained using digital image correlation of electron images taken during testing. Examples showing results for tensile testing of single-crystalline metallic nanowires and compression of nanoporous Au pillars will be presented in the context of size effects on mechanical behavior and highlight some of the challenges of conducting nanomechanical testing in vacuum environments.