Nanotwins require little energy to form in metals, but their impact on strength and ductility is dramatic. New mechanisms of strengthening, strain hardening, ductility, and strain-rate sensitivity ...have been observed in nanowires, films, and bulk materials containing nanoscale twins as the twin-boundary spacing decreases. These mechanisms can act in concert to produce interface-dominated nanomaterials with extreme tensile strength and plastic deformation without breaking. This article reviews recent theoretical and experimental understanding of the physical mechanisms of plasticity in nanotwin-strengthened metals, with a particular focus on the fundamental roles of coherent, incoherent, and defective twin boundaries in plastic deformation of bulk and small-scale cubic systems, and discusses new experimental methods for controlling these deformation mechanisms in nanotwinned metals and alloys.
The shape of loess particles was determined from scanning electron microscope images and biaxial tests were simulated by the discrete element method. The effects of different percentages of angular, ...sub-angular, sub-rounded, rounded and well-rounded particles in the samples on the internal friction angle, coordination number and porosity of the loess are studied, and the evolution of the numerical simulation results during the tests are analysed from the microscopic level. The results show that when the combined roundness and combined ellipticity values decrease, that is, the irregular and flat particle percentage content decreases, the internal friction angle, the average coordination number between particles and the porosity of the sample decrease.
KEY POINTS
Scanning electron microscope images of loess are processed, and the percentage of particles with different shapes in each sample determined.
The microscopic and macroscopic changes of samples during biaxial compression are considered via the discrete element method.
The effects of different particle shape percentage combinations on the internal friction angle, coordination number and porosity of loess are investigated, and the experimental results are explained from the microscopic point of view.
The atmospheric concentrations of gaseous HNO3 , HCl and NH3 and their relative salts have been measured during two field campaigns in the winter and in the summer of 2007 at Beijing (China), as part ...of CAREBEIJING (Campaigns of Air Quality Research in Beijing and Surrounding Region). In this study, annular denuder technique used with integration times of 2 and 24h to collect inorganic and soluble PM2.5 without interferences from gas-particle and particle-particle interactions. The results were discussed from the standpoint of temporal and diurnal variations and meteorological effects. Fine particulate Cl- , NH4+ and SO42- exhibited distinct temporal variations, while fine particulate NO3- did not show much variation with respect to season. Daily mean concentrations of fine particulate NH4+ and SO42- were higher during summer (12.30 μg m-3 and 18.24 μg m-3 , respectively) than during winter (6.51 μg m-3 and 7.50 μg m-3 , respectively). Daily mean concentrations of fine particulate Cl- were higher during winter (2.94 μg m-3 ) than during summer (0.79 μg m-3 ), while fine particulate NO3- showed similar both in winter (8.38 μg m-3 ) and in summer (9.62 μg m-3 ) periods. The presence of large amounts of fine particulate NO3- even in summer are due to higher local and regional concentrations of NH3 in the atmosphere available to neutralize H2 SO4 and HNO3 , which is consistent with the observation that the measured particulate species were neutralized. The composition of fine particulate matter indicated the domination of (NH4 )2 SO4 during winter and summer periods. In addition, the high relative humidity conditions in summer period seemed to dissolve a significant fraction of HNO3 and NH3 enhancing fine particulate NO3- and NH4+ in the atmosphere. All measured particulate species showed diurnal similar patterns during the winter and summer periods with higher peaks in the early morning, especially in summer, when humid and stable atmospheric conditions occurred. These diurnal variations were affected by wind direction suggesting regional and local source influences. The fine particulate species were correlated with NOx and PM2.5 , supporting the hypothesis that traffic may be also an important source of secondary particles.
Simultaneous increase of the ductility and strength of a bulk nanostructured Al alloy with an average grain size of ∼ 100 nm (curve NS) has been achieved by precipitating Guinier–Preston (G–P) zones, ...η′, and η particles (curve NS+P). Bulk nanostructured materials usually have high strength, but disappointingly low ductility. All previous attempts to enhance the ductility of nanostructured materials have sacrificed their yield strengths. This strategy is applicable to many nanostructured alloys and composites.
This article systematically overviews the grain size effect on deformation twinning and detwinning in face-centered cubic (fcc) metals. With decreasing grain size, coarse-grained fcc metals become ...more difficult to deform by twinning, whereas nanocrystalline (nc) fcc metals first become easier to deform by twinning and then become more difficult, exhibiting an optimum grain size for twinning. The transition in twinning behavior from coarse-grained to nc fcc metals is caused by the change in deformation mechanisms. An analytical model based on observed deformation physics in nc metals, i.e., grain boundary emission of dislocations, provides an explanation of the observed optimum grain size for twinning in nc fcc metals. The detwinning process is caused by the interaction between dislocations and twin boundaries. Under a certain deformation condition, there exists a grain size range where the twinning process dominates over the detwinning process to produce the highest density of twins.
It has been long understood that fretting differs from sliding wear in that the relative displacement between the bodies is generally smaller than the size of the contact between them, with debris ...ejection from the contact thus playing an important role in the behaviour of the contact in fretting. Whilst these ideas were clearly articulated more than 30 years ago via Godet's third-body approach and Berthier's concept of the tribology circuit, calculation of wear rates in fretting have continued to employ Archard's wear equation (or approaches directly derived from it), despite this approach assuming that the rate of wear is controlled by the rate of generation of wear debris (as opposed to the rate of its ejection from the contact). It has been shown recently that when debris ejection is the rate-determining-process in fretting, the instantaneous rate of wear is inversely proportional to a characteristic dimension of the wear scar. When non-conforming specimen pair geometries (such as cylinder-on-flat) are employed in fretting testing, the wear scar size increases as wear proceeds, and thus the instantaneous rate of wear decreases. In this paper, wear equations have been derived for three commonly employed non-conforming pair specimen geometries, which all take the form Vw=KRn−1Edn (Vw is the wear scar volume, R is the radius of the non-plane specimen(s) in the pair and Ed is the frictional energy dissipated) where n varies between 0.67 and 0.8 depending upon the geometry and assumptions made regarding the governing equation. It is argued that the assumptions upon which the analysis is based are most valid for the cylinder-on-flat contact configuration with fretting perpendicular to the cylinder axis where the length of the line contact is large compared to the wear scar width. It is demonstrated that, despite the often apparently good fit of experimental data to an Archard-type equation, it is not appropriate to employ such Archard-type approaches to the analysis of fretting data in situations where debris ejection is the rate-determining-process. The equations derived in this paper relating wear scar size to some measure of the duration of the test should be used for such analysis instead of the linear relationships generally employed in previous work.
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•Fretting wear depends upon the rate of debris ejection from the contact.•Debris expulsion rate falls as the critical dimension of wear scar increases.•In non-conforming contact geometries, wear rate falls as a test proceeds.•Archard-type wear equations are not always appropriate for fretting wear.•New equations are presented for commonly employed fretting test configurations.
The molecular mechanism underlying gastric cancer (GC) invasion and metastasis is still poorly understood. In this study, we tried to investigate the roles of CXCR4 and CXCR2 signalings in gastric ...cancer metastasis. A highly invasive gastric cancer cell model was established. Chemokines receptors were profiled to search for the accountable ones. Then the underlying molecular mechanism was investigated using both in vitro and in vivo techniques, and the clinical relevance of CXCR4 and CXCR2 expression was studied in gastric cancer samples. CXCR4 and CXCR2 were highly expressed in a high invasive gastric cancer cell model and in gastric cancer tissues. Overexpression of CXCR4 and CXCR2 was associated with more advanced tumor stage and poorer survival for GC patients. CXCR4 and CXCR2 expression strongly correlated with each other in the way that CXCR2 expression changed accordingly with the activity of CXCR4 signaling and CXCR4 expression also changed in agreement with CXCR2 activity. Further studies demonstrated CXCR4 and CXCR2 can both activated NF-κB and STAT3 signaling, while NF-κBp65 can then transcriptionally activate CXCR4 and STAT3 can activate CXCR2 expression. This crosstalk between CXCR4 and CXCR2 contributed to EMT, migration and invasion of gastric cancer. Finally, Co-inhibition of CXCR4 and CXCR2 is more effective in reducing gastric cancer metastasis. Our results demonstrated that CXCR4 and CXCR2 cross-activate each other to promote the metastasis of gastric cancer.
The CoFeB sandwiched by Ta and MgO layers enables a perpendicular magnetic anisotropy (PMA) and provides a pathway for such application. In this paper, we reported the origin of PMA in CoFeB using ...the anomalous Hall effect (AHE) and polarized neutron reflectometry (PNR). From PNR experiments, we obtained the details of the magnetic and structural depth profiles inside the film. It is found that the PMA properties of CoFeB layers deposited above and under MgO layer are different and PNR measurements confirmed that a large PMA in the CoFeB above MgO layer is related to its low magnetization. Based on this PMA mechanism, we obtain a high sensitivity of AHE in the perpendicular CoFeB, which opens a new avenue to detect ultralow magnetic field.
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