Miracle D.B.,Air Force Research Lab |
Laws K.,University of New South Wales |
Senkov O.N.,UES, Inc. |
Wilks G.B.,UTC Inc.
Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science | Year: 2012
A critical analysis of measured partial coordination numbers for binary metallic glasses as a function of composition shows a large scatter of ±1.5 but clear trends. The current work uses two topological models to predict the influence of relative atomic size and concentration on partial coordination numbers. The equations for partial coordination numbers derived from these two models can reproduce measured data within experimental scatter, suggesting that chemical effects on local structure, although present, may be relatively small. Insights gained from these models show that structural site-filling rules are different for glasses with solute atoms that are smaller than solvent atoms and for glasses where solute atoms are larger than solvent atoms. Specifically, solutes may occupy both b and c intercluster sites when the soluteto- solvent radius ratio R is less than 1.26, but only b sites can be occupied by solutes when R>1.26. This distinction gives a simple topological explanation for the observed preference for binary metallic glasses with solutes smaller than solvent atoms. In addition to structure-specific equations, simplified phenomenological equations for partial coordination numbers are given as a convenience. © The Minerals, Metals & Materials Society and ASM International 2011. Source
Lucas M.S.,Air Force Research Lab |
Lucas M.S.,UTC Inc.
TMS Annual Meeting | Year: 2011
The phonon density of states provides the vibrational entropy, making it a key thermodynamic function for understanding alloy phase transformations. The significance of phonon thermodynamics is discussed in the context of recent measurements on binary alloys of Fe with Al, Co, and Cr. For equiatomic B2 ordered FeAl, the vibrational entropy of vacancy formation counteracts the corresponding change in configurational entropy, destabilizing vacancies. Changes in the phonon spectrum upon ordering in equiatomic FeCo are accurately captured using the cluster inversion method for measurements on: random solid solutions. For FeCr alloys, the positive vibrational entropy of mixing helps to stabilize the random solid solution with respect to the unmixed phase. Source
Lucas M.S.,Air Force Research Lab |
Lucas M.S.,UTC Inc. |
Mauger L.,California Institute of Technology |
Muoz J.A.,California Institute of Technology |
And 5 more authors.
Journal of Applied Physics | Year: 2011
The magnetic properties of high-entropy alloys based on equimolar FeCoCrNi were investigated using vibrating sample magnetometry to determine their usefulness in high-temperature magnetic applications. Nuclear resonant inelastic x-ray scattering measurements were performed to evaluate the vibrational entropy of the 57Fe atoms and to infer chemical order. The configurational and vibrational entropy of alloying are discussed as they apply to these high-entropy alloys. © 2011 American Institute of Physics. Source
Lee K.M.,Air Force Research Lab |
Lee K.M.,Azimuth Corporation |
Koerner H.,Air Force Research Lab |
Koerner H.,UTC Inc. |
And 3 more authors.
Soft Matter | Year: 2011
Rapidly reconfigurable, adaptive materials are essential for the realization of "smart", highly engineered technologies sought by aerospace, medicine, and other application areas. Shape memory observed in metal alloys and polymers (SMPs) is a primary example of shape change (adaptation). To date, nearly all shape adaptations in SMPs have been thermally triggered. A desire for isothermal, remotely cued shape adaptations of SMP has motivated examinations of other stimuli, such as light. Only a few reports document so-called light-activated SMP, in both cases exploiting photoinduced adjustments to the crosslink density of a polymer matrix with UV light of 365 nm (crosslinking) and <260 nm (decrosslinking). This work presents a distinctive approach to generating light-activated SMP by employing a glassy liquid crystal polymer network (LCN) material that is capable of rapid photo-fixing with short exposures (<5 min) of eye-safe 442 nm light. Here, linearly polarized 442 nm light is used to photo-fix temporary states in both cantilever and free-standing geometries which are then thermally or optically restored to the permanent shape. The combination of thermal and photo-fixable shape memory presented here yields substantial functionality in a single adaptive material that could reduce part count in applications. As a demonstration of the opportunities afforded by this functional material, the glassy, photoresponsive LCN is thermally fixed as a catapult and subsequently used to transduce light energy into mechanical work, demonstrated here in the "photo-fueled" launching of an object at a rate of 0.3 m s-1. © 2011 The Royal Society of Chemistry. Source
Shenogin S.,Air Force Research Lab |
Shenogin S.,University of Dayton |
Lee J.,Air Force Research Lab |
Lee J.,UTC Inc. |
And 2 more authors.
Journal of Applied Physics | Year: 2014
A multiscale modeling approach to the prediction of electrical conductivity in carbon nanotube (CNT)-polymer composite materials is developed, which takes into account thermally activated molecular mobility of the matrix and the CNTs. On molecular level, a tight-binding density functional theory and non-equilibrium Green's function method are used to calculate the static electron transmission function in the contact between two metallic carbon nanotubes that corresponds to electron transport at 0K. For higher temperatures, the statistical distribution of effective contact resistances is considered that originates from thermal fluctuations of intermolecular distances caused by molecular mobility of carbon nanotube and the polymer matrix. Based on this distribution and using effective medium theory, the temperature dependence of macroscopic electrical resistivity for CNT-polymer composites and CNT mats is calculated. The predicted data indicate that the electrical conductivity of the CNT-polymer composites increases linearly with temperature above 50K, which is in a quantitative agreement with the experiments. Our model predicts a slight nonlinearity in temperature dependence of electric conductivity at low temperatures for percolated composites with small CNT loading. The model also explains the effect of glass transition and other molecular relaxation processes in the polymer matrix on the composite electrical conductivity. The developed multiscale approach integrates the atomistic charge transport mechanisms in percolated CNT-polymer composites with the macroscopic response and thus enables direct comparison of the prediction with the measurements of macroscopic material properties. © 2014 AIP Publishing LLC. Source