Non-clasical Nucleation in Supercooled Nickel

The dynamics of homogeneous nucleation and growth of crystalline nickel from the super-cooled melt is examined during rapid quenching using molecular dynamics and a modified embedded atom method potential. The character of the critical nuclei of the crystallization transition is examined using common neighbor analysis and visualization. At nucleation the saddle point droplet consists of randomly stacked planar structures with an in plane triangular order. These results are consistent with previous theoretical results that predict that the nucleation process in some metals is non-classical due to the presence of long-range forces and a spinodal.Comment: 4 pages, 5 figure

Use of the Richtmyer-Meshkov Instability to Infer Yield Stress at High-Energy Densities

We use the Richtmyer-Meshkov instability (RMI) at a metal-gas interface to infer the metal’s yield stress (Y) under shock loading and release. We first model how Y stabilizes the RMI using hydrodynamics simulations with a perfectly plastic constitutive relation for copper (Cu). The model is then tested with molecular dynamics (MD) of crystalline Cu by comparing the inferred Y from RMI simulations with direct stress-strain calculations, both with MD at the same conditions. Finally, new RMI experiments with solid Cu validate our simulation-based model and infer Y~0.47 GPa for a 36 GPa shock

Calculations of the structure and properties of rapidly quenched NI/ZR alloys.

Using molecular dynamics and a modified embedded atom potential developed by our group we studied the diffusivity and viscosity of molten Nil-XZrX alloys as a function of composition, temperature, and cooling rate. Previous results indicate that these potentials represent the Ni-Zr system quite well . Liquid alloys were quenched at rates of 5 x 10{sup 11} and 10{sup 12} K/s. For x < 0.04 the solidified alloys were crystalline . For higher x values, the solidified alloys were amorphous . For the amorphous alloys, the composition dependence of the calculated glass transition temperature Tg follows the general trend of experimental Tg values . The calculated viscosity and diffusivity show systematic variation with composition . For the undercooled Ni-6 at .% Zr melt the calculated viscosity shows the Vogel-Fulcher-Tamman (VFT) behavior characteristic of a 'fragile' glass

Experimental and computational analysis of abnormal grain growth

© 2015 Institute of Materials, Minerals and Mining. The mechanisms involved in the abnormal grain growth of the iron based oxide dispersion strengthened alloys are analysed in the present work. Its microstructural evolution takes place at high temperatures (0.9Tm) and is characterised by an initial submicrometre size microstructure and a strong || rolling direction (RD) texture that evolves into a few extremely coarse grains (mm sizes) with ||RD orientation. The analysis of the observed grain boundaries has been completed by molecular dynamics simulations. Microstructure evolution consists of an extended recovery process, followed by an abnormal grain growth stage, consequence of the orientation pinning mechanism and the proximity to a symmetric tilt boundary family between the ||RD and ||RD grains.PM 2000 is a trademark of Plansee GmbH Inc. CC and GP acknowledge the Ministry of Economy and Competitiveness (MINECO) for the financial support through the National project no. ENE2009-13766-C04-01. GP acknowledges the MINECO for supporting her research under a FPI Grant (grant no. BES-2010-032747). The use of the computational facilities provided by CTI (Trueno cluster) is also gratefully acknowledged.Peer Reviewe

Flight Mechanics of the Wright Aircraft 1903-1912

Perhaps the most curious aspect of the Wright Brothers' program to invent and commercialize the airplane is their decision in 1900 to use their novel canard configuration, and to persist with that geometry until 1910 despite the known deficiency that the aircraft were unstable in pitch. The reasons for their initial choice are well-known. Several studies in the part twenty years have proven beyond doubt that the Wrights did not intentionally make their canards unstable. The pitch instability of their machine was an unwitting byproduct of their design chosen partly out of fear of the conventional design and partly (they reasoned) for more positive control. With their great emphasis on control, the Wrights were able to develop a successful aircraft, albeit difficult to fly additionally because the 1903 aircraft also possessed a fast spiral instability. A canard design is not necessarily unstable, but owing chiefly to their airfoil, and an unfortunate fore-and-aft mass distribution, the Wright canards were all unstable. Though easier to fly, their 1909 aircraft was more unstable than the famous 1903 FZper and the Brothers did not have a stable design until they finally adopted a conventional aft horizontal tail in 1910. Successful control of the canard aircraft depended heavily on large damping-in-pitch. The purpose of this paper is to apply modern analysis of flight mechanics to trace the detailed flying characteristics of their powered aircraft from 1903 to 1910 when they finally gave up the canard. Its a story in which technology, stubborness and commercialization are intimately mingled; we are concerned here only with the technology. © by 2003

Dynamical and transport properties of liquid gallium at high pressures

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