Modelling of dislocation generation and interaction during high-speed deformation of metals

Recent experiments by Kiritani et al. have revealed a surprisingly high rate of vacancy production during high-speed deformation of thin foils of fcc metals. Virtually no dislocations are seen after the deformation. This is interpreted as evidence for a dislocation-free deformation mechanism at very high strain rates. We have used molecular-dynamics simulations to investigate high-speed deformation of copper crystals. Even though no pre-existing dislocation sources are present in the initial system, dislocations are quickly nucleated and a very high dislocation density is reached during the deformation. Due to the high density of dislocations, many inelastic interactions occur between dislocations, resulting in the generation of vacancies. After the deformation, a very high density of vacancies is observed, in agreement with the experimental observations. The processes responsible for the generation of vacancies are investigated. The main process is found to be incomplete annihilation of segments of edge dislocations on adjacent slip planes. The dislocations are also seen to be participating in complicated dislocation reactions, where sessile dislocation segments are constantly formed and destroyed.Comment: 8 pages, LaTeX2e + PS figures. Presented at the Third Workshop on High-speed Plastic Deformation, Hiroshima, August 200

Dislocation nucleation and vacancy formation during high-speed deformation of fcc metals

Recently, a dislocation free deformation mechanism was proposed by Kiritani et al., based on a series of experiments where thin foils of fcc metals were deformed at very high strain rates. In the experimental study, they observed a large density of stacking fault tetrahedra, but very low dislocation densities in the foils after deformation. This was interpreted as evidence for a new dislocation-free deformation mechanism, resulting in a very high vacancy production rate. In this paper we investigate this proposition using large-scale computer simulations of bulk and thin films of copper. To favour such a dislocation-free deformation mechanism, we have made dislocation nucleation very difficult by not introducing any potential dislocation sources in the initial configuration. Nevertheless, we observe the nucleation of dislocation loops, and the deformation is carried by dislocations. The dislocations are nucleated as single Shockley partials. The large stresses required before dislocations are nucleated result in a very high dislocation density, and therefore in many inelastic interactions between the dislocations. These interactions create vacancies, and a very large vacancy concentration is quickly reached.Comment: LaTeX2e, 8 pages, PostScript figures included. Minor modifications only. Final version, to appear in Philos. Mag. Let

A library of ab initio Raman spectra for automated identification of 2D materials

Raman spectroscopy is frequently used to identify composition, structure and layer thickness of 2D materials. Here, we describe an efficient first-principles workflow for calculating resonant first-order Raman spectra of solids within third-order perturbation theory employing a localized atomic orbital basis set. The method is used to obtain the Raman spectra of 733 different monolayers selected from the computational 2D materials database (C2DB). We benchmark the computational scheme against available experimental data for 15 known monolayers. Furthermore, we propose an automatic procedure for identifying a material based on an input experimental Raman spectrum and illustrate it for the cases of MoS$_2$ (H-phase) and WTe$_2$ (T$^\prime$-phase). The Raman spectra of all materials at different excitation frequencies and polarization configurations are freely available from the C2DB. Our comprehensive and easily accessible library of \textit{ab initio} Raman spectra should be valuable for both theoreticians and experimentalists in the field of 2D materialsComment: 17 pages, 7 figure

The Columbia City Trailhead: A Collaborative Construction Engineering Technology Capstone Experience

In 2010, a collaborative effort between a nonprofit trail advocacy organization, a small rural Indiana city, trade unions, grantmaking organizations, materials suppliers, contractors, and the Construction Engineering Technology program at Indiana University – Purdue University Fort Wayne (IPFW) led to the successful construction of a trailhead building in a city park. Multidisciplinary collaboration began with a design charrette in January, bringing together architects, brickmasons, carpenters, electricians, engineers, greenbuilding experts, landscapers, professors, and students. Starting with a site plan by a local architect and a construction blueprint from another trailhead elsewhere in the state, charrette participants improved the design and site location. Students completed the design, obtained approval from the customer (the nonprofit trail group), and obtained approval from local and state governments. Because this was a capstone course, students were required to demonstrate knowledge and skills they acquired during their four-year degree program. As such, they created the blueprints of the new design, estimated costs and materials, scheduled the construction, and fulfilled the role of project manager. Construction professionals mentored the students as they built the trailhead restroom. The magic words “student project” led to substantial donations of money, labor, excavating, and materials from many sources. As a result, the project was completed at one-quarter the price bid by a private contractor. Assessment of student learning was conducted by the instructor, departmental colleagues, and working construction professionals