Brazil nut effect in a rectangular plate under horizontal vibration

An intruder to a group of identical small beads enclosed in a rectangular plate will gradually migrate to either the center or one side of the plate when the plate is subjected to a horizontal vibration. By considering probabilities for a bead to move into and off the space between the intruder and the near side of the plate, we predict that the size ratio and the mass ratio of the intruder to small bead have equal but opposite effects in determining the direction of migration. The prediction is confirmed by a molecular dynamics simulation. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Cyclic-loading-induced accumulation of geometrically necessary dislocations near grain boundaries in an Ni-based superalloy

In this study, the fatigue-induced microstructure produced in a nickel-based polycrystalline superalloy that was subjected to cyclic loading was characterized by polychromatic x-ray microdiffraction (PXM) together with in-situ neutron diffraction and transmission-electron microscopy (TEM). In-situ neutron-diffraction measurements reveal two distinct stages of the fatigue damage: cyclic hardening followed by cyclic softening. Three-dimensional spatially resolved PXM micro-Laue measurements find an increase in the density of geometrically necessary dislocations near the grain boundaries, which is accompanied by lattice rotations and grain subdivisions. The PXM results are in agreement with the in-situ neutron-diffraction and TEM results.     

Study of the intrinsic ductile to brittle transition mechanism of metallic glasses

We provide a quantitative account of the shear-banding behavior in Mg-based bulk metallic glasses. Microcompression experiments showed that shear bands propagated in an intermittent manner at the micron and submicron scales. The shear-banding kinetics involves competition between shear-induced softening and subsequent recovery mechanisms; the former depends strongly on the elastic energy release rate at the onset of shear-banding, whereas the latter is correlated with the shear-band offset and scales with the characteristic dimension of the microsamples. With the increase in the sample size, more elastic energy is released into the shear band, which results in larger fluctuations in the serrated plastic flows and the propensity for unstable shear-band propagation. The outcome of the current study reveals the origin of the intrinsic ductile to brittle transition mechanism in metallic glasses.     

A study on the surface severe plastic deformation behavior of a Zr-based bulk metallic glass (BMG)

A surface treatment process, which can generate the severe plastic deformation in the near-surface layer of crystalline materials, is successfully applied on the Zr₅₀Cu₄₀Al₁₀ bulk metallic glass (BMG). The experiment is implemented using 20 WC/Co balls with a velocity of about 5 m/s to bombard the surface of the samples in a purified argon atmosphere. The plastic-flow deformation in the unconstrained sample edge was observed, which exhibits the good intrinsic ductility of this material under the experimental condition. In the sub-surface layer, the bombardment-induced shear-band operations generate the extrusion and intrusion marks on the side face. Differential scanning calorimetry (DSC) shows that the free volumes of the deformed BMG have increased, and possible crystallization may occur during the process. X-ray diffraction (XRD) and synchrotron high-energy X-ray diffraction techniques were used to inspect the possible crystal phase. A nanoindentation test shows that on the side surface, the hardness increases and, then, decreases with the distance from the processed surface. Four-point-bending-fatigue behavior has been studied and related to the modified surface structure and the compressive residual stress introduced by the surface treatment.     

In situ observation of pitting corrosion of the Zr50Cu40Al10 bulk metallic glass

The polarization behavior of a Zr₅₀Cu₄₀Al₁₀ (atomic percent) bulk metallic glass was investigated in a 0.6 M NaCl electrolyte while being observed using optical microscopy (OM). In situ anodic-polarization experiments revealed a tendency towards a diffusion-controlled mechanism other than passivity. In situ OM images illustrated pits initiated at the corrosion potential (that is, open-circuit conditions). Scanning electron microscopy (SEM) was utilized to examine the region where the corrosion pits initiated. SEM images showed that the area of pit initiation was surrounded by an oxide that formed on the surface due to the exposure to the NaCl electrolyte.     

Investigating students’ perceived satisfaction,behavioral intention,and effectiveness of e-learning: A case study of the Blackboard system

Although the benefits of e-learning have been discussed in various previous studies; it is a critical issue of better understanding the reasons why some learners are dissatisfied with the e-learning experience. Therefore, this research investigates learners’ satisfaction, behavioral intentions, and the effectiveness of the Blackboard e-learning system. A total of 424 university students were surveyed using a standard questionnaire. The results showed that perceived self-efficacy is a critical factor that influences learners’ satisfaction with the Blackboard e-learning system. Perceived usefulness and perceived satisfaction both contribute to the learners’ behavioral intention to use the e-learning system. Furthermore, e-learning effectiveness can be influenced by multimedia instruction, interactive learning activities, and e-learning system quality. This research proposes a conceptual model for understanding learners’ satisfaction, behavioral intention, and effectiveness of using the e-learning system.