Overview of U.S. Foreign Language Education Policy

Foreign language education policy has been a topic of concern in the USA since the time of the founding fathers. A paradox of American society is that it is a nation formed from a “melting pot” of immigrants, while the pressure to assimilate to a uniform American culture has resulted in a dominantly monolingual population. With changes in immigration rates and shifts in the national origins of immigrants, the attitudes and perspectives of the general public toward foreigners and foreign languages have also adjusted over time. This literature review will give an overview of how current events and public opinion have shaped foreign language education policy by mapping how languages have been taught over the past century

Elucidating the dual role of grain boundaries as dislocation sources and obstacles and its impact on toughness and brittle-to-ductile transition

In this paper, we resolve the role of grain boundaries on toughness and the brittle-to-ductile transition. On the one hand, grain boundaries are obstacles for dislocation glide. On the other hand, the intersection points of grain boundaries with the crack front are assumed to be preferred dislocation nucleation sites. Here, we will show that the single contributions of grain boundaries (obstacles vs. source) on toughness and the brittle-to-ductile transition are contradicting, and we will weight the single contributions by performing carefully designed numerical experiments by means of two-dimensional discrete dislocation dynamics modelling. In our parameter studies, we vary the following parameters: (i) the mean free path for dislocation glide, δ, combined with (ii) the (obstacle) force of the grain boundary, ϕ, and (iii) the dislocation source spacing along the crack front, λ. Our results show that for materials or microstructures for which the mean distance of the intersection points of grain boundaries with the crack front is the relevant measure for λ, a decrease of grain size results in an increase of toughness. The positive impact of grain boundaries outweighs the negative consequences of dislocation blocking. Furthermore, our results explain the evolving anisotropy of toughness in cold-worked metals and give further insight into the question of why the grain-size-dependent fracture toughness passes through a minimum (and the brittle-to-ductile transition temperature passes through a maximum) at an intermediate grain size. Finally, a relation of the grain-size-dependence of fracture toughness in the form of K(dδ, dλ) = KIC + kdδ0.5/dλ is deduced

Free energy function of dislocation densities by large scale atomistic simulations

The energy of complex dislocation microstructures is a fundamental property of continuum plasticity on the nanoscale. The question how the energy depends on the characteristic of a dislocation network is still not fully answered, although various - and often contradicting - models have been proposed in the literature. In this talk, this question is addressed using large scale Molecular Dynamics simulations of nanoindentation, which have been conducted to gain insight into the relationship between dislocation microstructures and the associated free energy from an atomistic level. Several single crystalline samples of aluminum are indented using varying tip radii to study possible size effects. In the largest sample, a 24nm tip is used to indent into a volume of 150³nm³ that consists of about 2×108 atoms. Thus, these atomistic simulations are reaching a size that is comparable to experiments. Dislocation microstructures are directly identified from the atomistic data, providing the mean to measure both the total and geometrically necessary dislocation densities in the volume and further related them to the energy which is obtained from the simulations as well. Using this approach, an atomistically informed free energy function for dislocation densities is derived from nanomechanical simulations, without the need to account for theoretical or phenomenological arguments commonly used in modeling crystal plasticity. Furthermore, several size effects are clearly observed in the conducted series of simulations with varying tip radii and sample volumes. Whereas for small indenter tips only plastic deformation by dislocations is observed, twinning and subgrains formation additionally occur in the samples underneath the indenter tips having a radius of 16nm or higher. This mechanism is having a significant influence on the measured geometrically necessary dislocation densities

The brittle-to-ductile transition in cold-rolled tungsten sheets: the rate-limiting mechanism of plasticity controlling the BDT in ultrafine-grained tungsten

Conventionally produced tungsten (W) sheets are brittle at room temperature. In contrast to that, severe deformation by cold rolling transforms W into a material exhibiting room-temperature ductility with a brittle-to-ductile transition (BDT) temperature far below room temperature. For such ultrafine-grained (UFG) and dislocation-rich materials, the mechanism controlling the BDT is still the subject of ongoing debates. In order to identify the mechanism controlling the BDT in room-temperature ductile W sheets with UFG microstructure, we conducted campaigns of fracture toughness tests accompanied by a thermodynamic analysis deducing Arrhenius BDT activation energies. Here, we show that plastic deformation induced by rolling reduces the BDT temperature and also the BDT activation energy. A comparison of BDT activation energies with the trend of Gibbs energy of kink-pair formation revealed a strong correlation between both quantities. This demonstrates that out of the three basic processes, nucleation, glide, and annihilation, crack tip plasticity in UFG W is still controlled by the glide of dislocations. The glide is dictated by the mobility of the screw segments and therefore by the underlying process of kink-pair formation. Reflecting this result, a change of the rate-limiting mechanism for plasticity of UFG W seems unlikely, even at deformation temperatures well below room temperature. As a result, kink-pair formation controls the BDT in W over a wide range of microstructural length scales, from single crystals and coarse-grained specimens down to UFG microstructures

Who's driving anyway? Herculean efforts to identify the drivers of breast cancer

The continuing advancement of sequencing technologies has made the systematic identification of all driving somatic events in cancer a possibility. In the June 2012 issue of Nature, five papers show some significant headway in this endeavor, each a herculean effort of genome sequencing, and transcriptome and copy number analysis resulting in data on thousands of breast cancers. Integrating these massive datasets, the authors were able to further subdivide breast cancer and identify a number of novel driver genes. While the studies represent a leap forward in describing the genomics of breast cancer, and clearly highlight the tremendous diversity between tumors, the studies only scrape the surface of molecular changes in breast tumors, with more granularity to come from the study of epigenomics, single cell sequencing, and so on. The immediate importance of the data to clinical care is currently unknown, and will depend upon detailed identification and functional analysis of driver mutations. © 2012 BioMed Central Ltd

Techniques to accelerate convergence of stress-controlled molecular dynamics simulations of dislocation motion

Dislocation mobility —the relation between applied stress and dislocation velocity—is an important property to model the mechanical behavior of structural materials. These mobilities reflect the interaction between the dislocation core and the host lattice and, thus, atomistic resolution is required to capture its details. Because the mobility function is multiparametric, its computation is often highly demanding in terms of computational requirements. Optimizing how tractions are applied can be greatly advantageous in accelerating convergence and reducing the overall computational cost of the simulations. In this paper we perform molecular dynamics simulations of ½ 〈1 1 1〉 screw dislocation motion in tungsten using step and linear time functions for applying external stress. We find that linear functions over time scales of the order of 10–20 ps reduce fluctuations and speed up convergence to the steady-state velocity value by up to a factor of two

MD simulation study on defect evolution and doping efficiency of p-type doping of 3C-SiC by Al ion implantation with subsequent annealing

We use molecular dynamics (MD) simulation with numerical characterisation and statistical analysis to study the mechanisms of damage evolution and p-type doping efficiency by aluminum (Al) ion implantation into 3C silicon carbide (SiC) with subsequent annealing. By incorporating the electronic stopping power for implantation, a more accurate description of the atomic-scale mechanisms of damage evolution and distribution in SiC can be obtained. The simulation results show a novel observation that the recrystallization process occurs in the region below the subsurface layer, and develops from amorphous-crystalline interface to the damage center region, which is a new insight into previously published studies. During surface recrystallization, significant compressive stress concentration occurs, and more structural phase transition atoms and dislocations formed at the damage-rich-crystalline interface. Another point of interest is that for low-dose implantation, more implantation-induced defects hamper the doping efficiency. Correspondingly, the correlation between lattice damage and doping efficiency becomes weaker as the implant dose increases under the same annealing conditions. Our simulation also predicts that annealing after high temperature (HT) implantation is more likely to lead to the formation of carbon vacancies (V-C).Peer reviewe