Optimized many body potential for fcc metals

A formalism for optimized many body (OMB) potentials to describe the interatomic interactions in fcc metals is described. The OMB approach is based on the Tersoff potential, widely used to describe covalently bonded materials, and is closely related to the charge optimized many body (COMB) potential formalism for oxides. OMB extends to first nearest neighbors only, and employs a third-order Legendre polynomial to distinguish fcc and hcp structures, the strength of which can be adjusted to match the intrinsic stacking fault energy to arbitrary precision. The potential also predicts generalized stacking fault energy curves that are in very close agreement to the values determined from electronic-structure calculations. This potential is thus well-suited to investigating mechanical properties such as plastic deformation at the atomic scale.     

Deposition-rate dependence of granular size distribution in Cu aggregate on liquid substrate studied by atomic force microscopy

In this article, we report an atomic force microscopy study on the microstructure and the deposition-rate dependence of granular size distribution in copper (Cu) ramified aggregates on a liquid substrate. This study shows that the ramified Cu aggregates are composed of Gaussian size distribution granules, which form immediately after the Cu atoms are deposited. The interesting phenomenon is that the mean diameter Φ_m of the granules exponentially decays and approaches a stable value Φ_c with an increase in the deposition rate f. The granular mean diameter Φ_m slightly changes with the time interval Δt during which the film is kept in the vacuum chamber, owing to the large diffusion coefficient of the Cu granules on the liquid substrates. The experimental behavior strongly depends on the properties of the liquid substrate.     

Real-space first-principles electronic structure of edge dislocations: NiAl

The electronic structure of the 100 {010} edge dislocation in NiAl has been calculated using the real-space tight-binding linear muffin-tin orbital recursion method with a self-consistent treatment of electron density redistribution effects in the dislocation core. We demonstrate that quasilocalized states may exist in this metallic system as a result of specific lattice distortions in the dislocation core with 'broken bonds'.     

Vacancy interaction with glissile interstitial clusters in bcc metals

The interaction between a glissile cluster of self-interstitial atoms and a vacancy has been studied in f -Fe by atomistic modelling and elasticity theory. It was found that vacancies can annihilate only with the cluster edge. Vacancies inside the cluster glide prism do not annihilate with interstitials but affect the cluster dynamic properties. Depending on the cluster size and ambient temperature, these vacancies reduce or even prevent cluster motion. Qualitative differences in the results of atomistic and elasticity theory approaches were found, thereby demonstrating the need for the atomistic approach.     

Low-temperature mechanical properties of Ce68Al10Cu20Co2 bulk metallic glass

Although it is well known that traditional metals and alloys will become brittle at low temperatures, the effect of low temperature on the mechanical properties of bulk metallic glasses (BMGs) is yet to be fully understood. In this research, the mechanical properties of Ce₆₈Al₁₀Cu₂₀Co₂ BMG are investigated at low temperatures. It is found that the yield strength of the Ce-based BMG increases significantly with the decrease of temperature. The elastic moduli of the BMG also increase monotonically with the drop of temperature, indicating the continuous stiffening of the BMG, while both Poisson's ratio and global plasticity decline at low temperatures. It is considered that the stiffer atomic bonds of the Ce-based BMG at low temperatures result in the increase of strength, and the higher energy required for nucleation of shear bands also leads to the increase in yield strength.     

Dynamic properties of edge dislocations decorated by interstitial loops in α-iron and copper

Clusters of self-interstitial atoms (SIAs) in the form of parallel crowdions are created directly in high-energy displacement cascades produced in metals by neutron irradiation. They are equivalent to small perfect dislocation loops and, in isolation in pure metals, undergo fast thermally-activated glide in the direction of their Burgers vector. Their strain field and ability to glide allows long-range interaction with other extended defects. Indeed, dislocations decorated by dislocation loops are commonly observed after neutron irradiation. Dislocations gliding under applied stress also encounter these mobile defects. These effects influence mechanical properties and require further investigation. This paper presents results from an atomic-scale study of copper and α-iron at either 0?K or 300?K. Loop drag and breakaway effects are investigated for an edge dislocation under applied stress interacting with a row of SIA loops below its glide plane. The maximum speed at which a loop is dragged is lower in copper than iron, and the applied stress at which this occurs is also lower. These differences in the dynamics of cluster-dislocation interaction are determined by the atomic structure of the defects and cannot be investigated by continuum treatment.     

Characteristics of the interaction of Cu-rich precipitates with irradiation-produced defects in α-Fe

The interaction between copper-rich precipitates in α-iron and either vacancies or self-interstitial atoms and their clusters is studied by atomic-scale modelling. Results are compared with predictions of elasticity theory and interpreted in terms of size misfit of precipitates and defects, and the modulus and cohesive energy differences between iron and copper. Interstitial defects are repelled by precipitates at large distance but, like vacancies, attracted at small distance. Hence, copper precipitates in iron can be sinks for both vacancy and interstitial defects, and can act as strong recombination centres under irradiation conditions. This leads to a tentative explanation for the mixed Cu–Fe structure of precipitates and the absence of precipitate growth under neutron irradiation conditions. More generally, both vacancy and interstitial defects may be strongly bound to precipitates with weaker cohesion than the matrix.     

临床药师问诊与实践体会

问诊是临床医师的基本技能之一.医师通过问诊可以获得病史资料,从而做出临床决策.随着医学模式从传统的生物医学模式向生物心理社会医学模式转变和临床药学的蓬勃发展,药学服务模式从传统的以保障药品供应为中心向以患者为中心模式转变.药师从药房逐步走向临床,参与合理用药和提供药学服务,问诊也应成为临床药师的基本技能之一.本文通过笔者问诊实践,阐述问诊对于临床药师的重要性、药师问诊的内容及其与医生问诊的区别.     

儿童普通感冒治疗中抗菌药物的滥用及原因分析

普通感冒是由多种病原体引起的一种常见的急性呼吸道感染性疾病,无需使用抗菌药物.但在儿童普通感冒治疗的临床实践中抗菌药物滥用情况依然普遍.因此必须了解其滥用原因才能正确合理用药,从而预防药源性疾病的发生,维护儿童健康成长.