RESUMO
Understanding the interactions between planar defects and complex dislocation structures in a material is of great significance to simplify its design. In this paper, we show that, from an atomistic perspective, by using molecular dynamics simulations on nanoindentations, a prismatic dislocation loop in Ni3Al appears in pairs with a butterfly-like shape. The planar defects in Ni3Al can effectively block the movement of the prismatic dislocation loop pairs and play a hardening role. Among the impediment factors, twinning boundaries are the strongest and antiphase boundaries are the weakest. Superlattice intrinsic and complex stacking faults have basically the same blocking effect. Furthermore, we systematically elucidate the hardening effects and interaction mechanisms between the prismatic dislocation loop pairs and planar defects. These findings provide novel insights into the nanostructured design of materials with excellent mechanical properties.
RESUMO
There is a long standing contradiction on the tensile response of zinc oxide nanowires between theoretical prediction and experimental observations. Although it is proposed that there is a ductile behavior dominated by phase transformation, only an elastic deformation and brittle fracture was witnessed in experiments. Using molecular dynamics simulations, we clarified that, as the lateral dimension of zinc oxide nanowires increases to a critical value, an unambiguous ductile-to-brittle transition occurs. The critical value increases with decreasing the strain rate. Factors including planar defects and surface contamination induce brittle fracture prior to the initiation of phase transformation. These findings are consistent with previous atomistic standpoints and experimental results.
RESUMO
The tensile behaviours of [111]-oriented SiC nanowires with various microstructures are investigated by using molecular dynamics simulations. The results revealed the influence of microstructures on the brittleness and plasticity of SiC nanowires. Plastic deformation is mainly induced by the anti-parallel sliding of 3C grains along an intergranular amorphous film parallel to the plane and inclined at an angle of 19.47° with respect to the nanowire axis. Our study suggests that the wide dispersion of mechanical properties of SiC nanowires observed in experiments might be attributed to their diverse microstructures.
RESUMO
Severe acute respiratory syndrome (SARS) is a serious disease with many puzzling features. We present a simple, dynamic model to assess the epidemic potential of SARS and the effectiveness of control measures. With this model, we analysed the SARS epidemic data in Beijing. The data fitting gives the basic case reproduction number of 2.16 leading to the outbreak, and the variation of the effective reproduction number reflecting the control effect. Noticeably, our study shows that the response time and the strength of control measures have significant effects on the scale of the outbreak and the lasting time of the epidemic.
RESUMO
A rubber-like pseudoelastic behavior is discovered in single-crystalline face-centered-cubic (FCC) Cu nanowires in atomistic simulations. Nonexistent in bulk Cu, this phenomenon is associated primarily with a reversible crystallographic lattice reorientation driven by the high surface-stress-induced internal stresses due to high surface-to-volume ratios at the nanoscale level. The temperature-dependence of this behavior leads to a shape memory effect (SME). Under tensile loading and unloading, the nanowires exhibit recoverable strains up to over 50%, well beyond the typical recoverable strains of 5-8% for most bulk shape memory alloys (SMAs). This behavior is well-defined for wires between 1.76 and 3.39 nm in size over the temperature range of 100-900 K.