There has been relatively little study on time-dependent mechanical properties of nanowires, in spite of their importance for the design, fabrication and operation of nanoscale devices. Here we report a dislocation-mediated, time-dependent and fully reversible plastic behaviour in penta-twinned silver nanowires. In situ tensile experiments inside scanning and transmission electron microscopes show that penta-twinned silver nanowires undergo stress relaxation on loading and complete plastic strain recovery on unloading, while the same experiments on single-crystalline silver nanowires do not exhibit such a behaviour. Molecular dynamics simulations reveal that the observed behaviour in penta-twinned nanowires originates from the surface nucleation, propagation and retraction of partial dislocations. More specifically, vacancies reduce dislocation nucleation barrier, facilitating stress relaxation, while the twin boundaries and their intrinsic stress field promote retraction of partial dislocations, resulting in full strain recovery.

This paper reports quantitative mechanical characterization of silicon carbide (SiC) nanowires (NWs) via in situ tensile tests inside scanning electron microscopy using a microelectromechanical system. The NWs are synthesized using the vapor-liquid-solid process with growth direction of ⟨111⟩. They consist of three types of structures, pure face-centered cubic (3C) structure, 3C structure with an inclined stacking fault (SF), and highly defective structure, in a periodic fashion along the NW length. The SiC NWs are found to deform linear elastically until brittle fracture. Their fracture origin is identified in the 3C structures with inclined SFs, rather than the highly defective structures. The fracture strength increases as the NW diameter decreases from 45 to 17 nm, approaching the theoretical strength of 3C SiC. The size effect on fracture strength of SiC NWs is attributed to the size-dependent defect density rather than the surface effect that is dominant for single crystalline NWs.

In spite of numerous studies on mechanical behaviors of nanowires (NWs) focusing on the surface effect, there is still a general lack of understanding on how the internal microstructure of NWs influences their deformation mechanisms. Here, using quantitative in situ transmission electron microscopy based nanomechanical testing and molecular dynamics simulations, we report a transition of the deformation mechanism from localized dislocation slip to delocalized plasticity via an anomalous tensile detwinning mechanism in bitwinned metallic NWs with a single twin boundary (TB) running parallel to the NW length. The anomalous tensile detwinning starts with the detwinning of a segment of the preexisting TB under no resolved shear stress, followed by the propagation of a pair of newly formed TB and grain boundary leading to a large plastic deformation. An energy-based criterion is proposed to describe this transition of the deformation mechanism, which depends on the volume ratio between the two twin variants and the cross-sectional aspect ratio.

Penta-twinned metallic NWs have recently received much attention due to their excellent mechanical properties. However, their elasticity size effect remains not well understood due to the large discrepancy in the reported experimental and simulation results. This paper reports an experimental effort to address the discrepancy about the size-dependent Young's modulus of penta-twinned Ag NWs. Two independent experiments on the same NW, in-situ resonance test and tensile test in a scanning electron microscope, were used to measure the Young's moduli. The cross-sectional shape of the Ag NWs was found to transit from pentagon to circle with decreasing NW diameter, which can modify the Young's modulus as much as 8% (for resonance test) and 19% (for tensile test) for the tested diameter range. This work confirmed that the Young's modulus of penta-twinned Ag NWs increases with decreasing NW diameter.

A plethora of nanostructures with outstanding properties have emerged over the past decades. Measuring their mechanical properties and understanding their deformation mechanisms is of paramount importance for many of their device applications. To address this need innovative experimental techniques have been developed, among which a promising one is based upon microelectromechanical systems (MEMS). This article reviews the recent advances in MEMS platforms for the mechanical characterization of one-dimensional (1D) nanostructures over the past decade. A large number of MEMS platforms and related nanomechanics studies are presented to demonstrate the unprecedented capabilities of MEMS for nanoscale mechanical characterization. Focusing on key design considerations, this article aims to provide useful guidelines for developing MEMS platforms. Finally, some of the challenges and future directions in the area of MEMS-enabled nanomechanical characterization are discussed.