We present differences in the mechanical behavior of nanoscale gold and molybdenum single crystals. A significant strength increase is observed as the size is reduced to 100 nm. Both nanocrystals exhibit discrete strain bursts during plastic deformation. We postulate that they arise from significant differences in the dislocation behavior. Dislocation starvation is the predominant mechanism of plasticity in nanoscale fcc crystals, while junction formation and hardening characterize bcc plasticity. A statistical analysis of strain bursts is performed as a function of size and compared with stochastic models. DOI: 10.1103/PhysRevLett.100.155502 PACS numbers: 62.25.ÿg, 61.46.Hk, 81.07.ÿb, 81.16.Rf The mechanical behavior of crystals is dictated by dislocation motion in response to applied force. While it is difficult to observe the motion of individual dislocations, several correlations can be made between the microscopic stress-strain behavior and dislocation activity. In bulk, plasticity in metals occurs by the motion of dislocations, which multiply in the course of plastic deformation causing strain hardening. Although this fundamental concept is often assumed to be applicable to crystals of any dimensions, numerous recent studies have shown that conventional plasticity breaks down at the submicron scale. Recently, many experimental and computational investigations of fcc crystals (Au, Al, Cu, Ni) have demonstrated a pronounced size effect, whose main premise is ''smaller is stronger '' [1-11]. In this work we investigate flow stress as a function of diameter in gold (fcc) and molybdenum (bcc) single crystal nanopillars subjected to uniaxial microcompression. The results that follow suggest that fcc and bcc crystals have fundamentally different plasticity mechanisms when reduced to nanoscale with significant strain hardening present in the latter and virtually none in the former. In a striking deviation from classical mechanics, there is a significant increase in strength as crystal size is reduced to 100 nm; however, in gold crystals (fcc) the highest strength achieved represents 44% of its theoretical strength, while in molybdenum crystals (bcc) it is only 7%. This suggests that plasticity in Au is likely controlled by nucleation of new dislocations rather than by interactions of the preexisting ones. On the contrary, the smallest molybdenum nanopillar achieves only 7% of its theoretical strength, implying that plasticity is likely driven by the intricate motion and interactions of dislocations inside the pillar rather than by nucleation events. These remarkable differences in mechanical response of fcc and bcc crystals to uniaxial microcompression challenge the applicability of conventional strain hardening to nanoscale crystals. Single crystal nanopillars described in this work were fabricated via the focused ion beam system and subsequently uniaxially compressed along the h001i direction in the nanoindenter with a flat punch tip of 30 m diameter. The specifics of fabrication and testing conditions are b...
Cold atmospheric-pressure plasmas are currently in use in medicine as surgical tools and are being evaluated for new applications, including wound treatment and cosmetic care. The disinfecting properties of plasmas are of particular interest, given the threat of antibiotic resistance to modern medicine. Plasma effluents comprise (V)UV photons and various reactive particles, such as accelerated ions and radicals, that modify biomolecules; however, a full understanding of the molecular mechanisms that underlie plasma-based disinfection has been lacking. Here, we investigate the antibacterial mechanisms of plasma, including the separate, additive and synergistic effects of plasma-generated (V)UV photons and particles at the cellular and molecular levels. Using scanning electron microscopy, we show that plasma-emitted particles cause physical damage to the cell envelope, whereas UV radiation does not. The lethal effects of the plasma effluent exceed the zone of physical damage. We demonstrate that both plasma-generated particles and (V)UV photons modify DNA nucleobases. The particles also induce breaks in the DNA backbone. The plasma effluent, and particularly the plasma-generated particles, also rapidly inactivate proteins in the cellular milieu. Thus, in addition to physical damage to the cellular envelope, modifications to DNA and proteins contribute to the bactericidal properties of cold atmospheric-pressure plasma.
Existing and emerging methods in computational mechanics are rarely validated against problems with an unknown outcome. For this reason, Sandia National Laboratories, in partnership with US National Science Foundation and Naval Surface Warfare Center Carderock Division, launched a computational challenge in mid-summer, 2012. Researchers and engineers were invited to predict crack initiation and propagation in a simple but novel geometry fabricated from a common off-the-shelf commercial engineering alloy. The goal of this international Sandia Fracture Challenge was to benchmark the capabilities for the prediction of deformation and damage evolution associated with ductile tearing in structural metals, including physics models, computational methods, and numerical implementations currently available in the computational fracture community. Thirteen teams participated, reporting blind predictions for the outcome of the Challenge. The simulations and experiments were performed independently and kept confidential. The methElectronic supplementary material The online version of this article (doi:10.1007/s10704-013-9904-6) contains supplementary material, which is available to authorized users.Sandia National Laboratories, Albuquerque, NM, USA e-mail: blboyce@sandia.gov ods for fracture prediction taken by the thirteen teams ranged from very simple engineering calculations to complicated multiscale simulations. The wide variation in modeling results showed a striking lack of consistency across research groups in addressing problems of ductile fracture. While some methods were more successful than others, it is clear that the problem of ductile fracture prediction continues to be challenging. Specific areas of deficiency have been identified through this effort. Also, the effort has underscored the need for additional blind prediction-based assessments.
We investigate phase separation including elastic coherency effects in the bulk and at surfaces and find a reduction of the solubility limit in the presence of free surfaces. This mechanism favours phase separation near free surfaces even in the absence of external stresses. We apply the theory to hydride formation in nickel, iron and niobium and obtain a reduction of the solubility limit by up to two orders of magnitude at room temperature in the presence of free surfaces. These effects are concisely expressed through a solubility modification factor, which transparently expresses the long-ranged elastic effects in a terminology accessible e.g. to ab initio calculations.
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