Plastic deformation was observed in damascene Cu interconnect test structures during an in situ electromigration experiment and before the onset of visible microstructural damage (voiding, hillock formation). We show here, using a synchrotron technique of white beam x-ray microdiffraction, that the extent of this electromigration-induced plasticity is dependent on the linewidth. In wide lines, plastic deformation manifests itself as grain bending and the formation of subgrain structures, while only grain rotation is observed in the narrower lines. The deformation geometry leads us to conclude that dislocations introduced by plastic flow lie predominantly in the direction of electron flow and may provide additional easy paths for the transport of point defects. Since these findings occur long before any observable voids or hillocks are formed, they may have direct bearing on the final failure stages of electromigration.

When crystalline materials are mechanically deformed in small volumes, higher stresses are needed for plastic flow. This has been called the "Smaller is Stronger" phenomenon and has been widely observed. Various size-dependent strengthening mechanisms have been proposed to account for such effects, often involving strain gradients. Here we report on a search for strain gradients as a possible source of strength for single-crystal submicron pillars of gold subjected to uniform compression, using a submicron white-beam (Laue) x-ray diffraction technique. We have found both before and after uniaxial compression, no evidence of either significant lattice curvature or sub-grain structure. This is true even after 35% strain and a high flow stress of 300 MPa were achieved during deformation. These observations suggest that plasticity here is not controlled by strain gradients or sub-structure hardening, but rather by dislocation source starvation, wherein smaller volumes are stronger because fewer sources of dislocations are available. Text only2

Strong and tough structural materials occurring in nature have long fueled the search for an advanced class of strong biomimetic synthetic structural materials (SSMs). One such example in recent years is the rapid progress associated with the naturally occurring helicoidal composite structures inspiring the development of the tough helicoidally architectured synthetic structural composites (HA-SSCs). Most of the tough HA-SSCs have been fabricated using conventional composite resins and fibers or modern three-dimensional (3D) printing materials. Only recently, we ventured further into expanding the possibilities of imitating the helicoidal architecture in smaller scales (microns and tens of microns) while exploring the possibilities of utilization of various polymer-based composite formulas. In our previous research articles, we had established near-field electrospinning as an additive manufacturing methodology to construct tough and resilient 3D helicoidal membranes with micron-sized polymer fibers. In this article, we advance to the next level of developing fully composite structures from electrospun helicoidally assembled microribbons. A key aspect explored in this article is the importance of surface treatment. The physical, thermal, and mechanical experiments indicated superior adhesion between the components of composites, which led to enhanced toughness and impact properties. We further affirmed that reducing the helicoidal angular orientation of ribbons in the HA-SSCs can help further enhance the specific toughness and impact resistance of these composites. Furthermore, we found that the mechanical and physical properties of these composites can be tuned via several architectural features (helicoidal rotational angles, interface characteristics between layers and between ribbons) to suit the different strain-rate needs for various applications (combat army vest, sports gears such as helmets, flexible piezoelectric sensors, or polycarbonate-based solar photovoltaics (PV) modules). The experimental findings here suggest the importance of interfacial characteristics (between layers and between ribbons), the ribbon-to-matrix ratio, and angular arrangement of the ribbons as design guidelines for achieving next-generation HA-SSC materials, which are not only strong but also extremely tough and impact-resistant.

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The indentation size effect (ISE) has been observed in numerous nanoindentation studies on crystalline materials; it is found that the hardness increases dramatically with decreasing indentation size -a "smaller is stronger" phenomenon. Some have attributed the ISE to the existence of strain gradients and the geometrically necessary dislocations (GNDs). Since the GND density is directly related to the local lattice curvature, the Scanning X-ray Microdiffraction (µSXRD) technique, which can quantitatively measure relative lattice rotations through the streaking of Laue diffractions, can used to study the a) Present address: Division of Engineering, Brown University, Providence, RI 02912; electronic mail: gang_feng@brown.edu 2 strain gradients. The synchrotron µSXRD technique we use -which was developed at the Advanced Light Source (ALS), Berkeley Lab -allows for probing the local plastic behavior of crystals with sub-micrometer resolution. Using this technique, we studied the local plasticity for indentations of different depths in a Cu single crystal. Broadening of Laue diffractions (streaking) was observed, showing local crystal lattice rotation due to the indentation-induced plastic deformation. A quantitative analysis of the streaking allows us to estimate the average GND density in the indentation plastic zones. The size dependence of the hardness, as found by nanoindentation, will be described, and its correlation to the observed lattice rotations will be discussed.3

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In this study, we demonstrate the use of parallel plate far field electrospinning (pp-FFES) based manufacturing system for the fabrication of polyacrylonitrile (PAN) fiber reinforced polyvinyl alcohol (PVA) strong polymer thin films (PVA SPTF). Parallel plate far field electrospinning (also known as the gap electrospinning) is generally used to produce uniaxially aligned fibers between the two parallel collector plates. In the first step, a disc containing PVA/H2O solution/bath (matrix material) was placed in between the two parallel plate collectors. Next, a layer of uniaxially aligned sub-micron PAN fibers (filler material) produced by pp-FFES was directly collected/embedded in the PVA/H2O solution by bringing the fibers in contact with the matrix. Next, the disc containing the matrix solution was rotated at 45∘ angular offset and then the next layer of the uniaxial fibers was collected/stacked on top of the previous layer with now 45∘ rotation between the two layers. This process was continued progressively by stacking the layers of uniaxially aligned arrays of fibers at 45∘ angular offsets, until a periodic pattern was achieved. In total, 13 such layers were laid within the matrix solution to make a helicoidal geometry with three pitches. The results demonstrate that embedding the helicoidal PAN fibers within the PVA enables efficient load transfer during high rate loading such as impact. The fabricated PVA strong polymer thin films with helicoidally arranged PAN fiber reinforcement (PVA SPTF-HA) show specific tensile strength 5 MPa · cm3· g−1 and can sustain specific impact energy (8 ± 0.9) mJ · cm3· g−1, which is superior to that of the pure PVA thin film (PVA TF) and PVA SPTF with randomly oriented PAN fiber reinforcement (PVA SPTF-RO). The novel fabrication methodology enables the further capability to produce even further smaller fibers (sub-micron down to even nanometer scales) and by the virtue of its layer-by-layer processing (in the manner of an additive manufacturing methodology) allowing further modulation of interfacial and inter-fiber adherence with the matrix materials. These parameters allow greater control and tunability of impact performances of the synthetic materials for various applications from army combat wear to sports and biomedical/wearable applications.

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