442 research outputs found
Fracture toughness of two Cr2Hf+Cr intermetallic composites as a function temperature
Journal ArticleFracture toughness as a function of temperature was evaluated for two Cr2Hf+Cr intermetallic composites, each in two different microstructural conditions. The proeutectic microstructures based on Cr-6.5Hf (at%) showed a significant increase in fracture toughness with an increase from room temperature to 600°C
The chemically ordered glass: the limiting composition for chemical order in amorphous packings of hard-sphere mixtures
We consider the limits of chemical order (i.e. avoidance of solute-solute contact) in amorphous close-packed mixtures of hard spheres of two different sizes. The upper bound on the solute concentration, beyond which solute-solute contact cannot be avoided, is determined for a range of particle size ratios. Scaling the composition by this limiting value is found to collapse plots of the solute coordination and mixture energies as a function of composition for different values of size ratios onto a single master curve. A number of features of the behaviour of amorphous alloys in the vicinity of this limiting concentration are discussed.Fil: Harrowell, Peter. University of Sydney; AustraliaFil: Fernandez, Julian Roberto. Consejo Nacional de Investigaciones CientÃficas y Técnicas; Argentina. Comisión Nacional de EnergÃa Atómica; ArgentinaFil: Miracle, D. B.. Wright-Patterson Air Force Base; Estados Unido
Crucial role of sidewalls in velocity distributions in quasi-2D granular gases
Our experiments and three-dimensional molecular dynamics simulations of
particles confined to a vertical monolayer by closely spaced frictional walls
(sidewalls) yield velocity distributions with non-Gaussian tails and a peak
near zero velocity. Simulations with frictionless sidewalls are not peaked.
Thus interactions between particles and their container are an important
determinant of the shape of the distribution and should be considered when
evaluating experiments on a tightly constrained monolayer of particles.Comment: 4 pages, 4 figures, Added reference, model explanation charified,
other minor change
In situ visualization of Ni-Nb bulk metallic glasses phase transition
We report the results of the Ni-based bulk metallic glass structural
evolution and crystallization behavior in situ investigation. The X-ray
diffraction (XRD), transmission electron microscopy (TEM), nano-beam
diffraction (NBD), differential scanning calorimetry (DSC), radial distribution
function (RDF) and scanning probe microscopy/spectroscopy (STM/STS) techniques
were applied to analyze the structure and electronic properties of Ni63.5Nb36.5
glasses before and after crystallization. It was proved that partial surface
crystallization of Ni63.5Nb36.5 can occur at the temperature lower than for the
full sample crystallization. According to our STM measurements the primary
crystallization is originally starting with the Ni3Nb phase formation. It was
shown that surface crystallization drastically differs from the bulk
crystallization due to the possible surface reconstruction. The mechanism of
Ni63.5Nb36.5 glass alloy 2D-crystallization was suggested, which corresponds to
the local metastable (3x3)-Ni(111) surface phase formation. The possibility of
different surface nano-structures development by the annealing of the
originally glassy alloy in ultra high vacuum at the temperature lower, than the
crystallization temperature was shown. The increase of mean square surface
roughness parameter Rq while moving from glassy to fully crystallized state can
be caused by concurrent growth of Ni3Nb and Ni6Nb7 bulk phases. The simple
empirical model for the estimation of Ni63.5Nb36.5 cluster size was suggested,
and the obtained values (7.64 A, 8.08 A) are in good agreement with STM
measurements data (8 A-10 A)
Development of high temperature refractory-based multi-principle-component alloys by thermodynamic calculations and rapid alloy prototyping
Recently, new refractory-based high entropy alloys (HEAs) have been investigated for potential use as high temperature structural alloys, and some alloys exhibit excellent high temperature strength and ductility. While the high entropy alloy community is generally concerned with obtaining single phase solid-solution phases, secondary strengthening phases are usually required to achieve an adequate balance of mechanical and physical properties for structural applications. This contribution will report on new Mo,Nb-based alloys that have been developed using HEA design guidelines, as well as new tools that enable thermodynamic property predictions and rapid alloy prototyping and assessment.
An elemental palette of Mo-Nb-Hf-Ta-Ti-V-W-Zr was chosen in order to promote the formation of a single body-centered cubic (BCC) solid-solution phase upon solidification, which facilitates homogenization heat treatments. Al, Cr, and Si were also included to promote secondary phase formation. These 11 elements were then used to calculate the phases present and their reaction temperatures of 3-, 4-, 5-, and 6-component alloy compositions from all of the available PandatTM databases. Mo and Nb were required to be present in each alloy composition in order to maintain modest alloy costs and densities.
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Systematic first-principles study of impurity hybridization in NiAl
We have performed a systematic first-principles computational study of the
effects of impurity atoms (boron, carbon, nitrogen, oxygen, silicon, phosporus,
and sulfur) on the orbital hybridization and bonding properties in the
intermetallic alloy NiAl using a full-potential linear muffin-tin orbital
method. The matrix elements in momentum space were used to calculate real-space
properties: onsite parameters, partial densities of states, and local charges.
In impurity atoms that are empirically known to be embrittler (N and O) we
found that the 2s orbital is bound to the impurity and therefore does not
participate in the covalent bonding. In contrast, the corresponding 2s orbital
is found to be delocalized in the cohesion enhancers (B and C). Each of these
impurity atoms is found to acquire a net negative local charge in NiAl
irrespective of whether they sit in the Ni or Al site. The embrittler therefore
reduces the total number of electrons available for covalent bonding by
removing some of the electrons from the neighboring Ni or Al atoms and
localizing them at the impurity site. We show that these correlations also hold
for silicon, phosporus, and sulfur.Comment: Revtex, 8 pages, 7 eps figures, to appear in Phys. Rev.
Response of macroinvertebrates to experimental nutrient and fish additions in European localities at different latitudes
The sedimentary and plant-associated macroinvertebrate communities have been examined in a series of mesocosm experiments carried out in five locations, in Finland, Sweden, UK, Northwest Spain and Mediterranean Spain (the International Mesocosm Experiment). The experiments were designed to investigate how shallow lake ecosystems respond to the addition of nutrients and planktivorous fish along a climate gradient. Nutrient additions increased sedimentary benthos densities and total biomass to a plateau at intermediate levels of nutrients. The effect was greater in the warmest localities. Beyond the saturation point a further increase in nutrient loading had different effects depending on locality. Only in the southernmost site (in Mediterranean Spain), did it induce a shift to a turbid phase and macrophyte disappearance. Thus there are breakpoints in the relationship with nutrients, leading to alternative sediment communities subsequent to the water alternative states. Presence of fish increased the densities and biomass of sediment macroinvertebrates, more clearly in the more southern locality, where the fish Gambusia did not prey in the bottom animals. A stabilising role of macrophytes was observed by cutting them, samples taken three weeks after cutting indicated a higher effect of fish on the sediment community when macrophytes were not present. Nutrients tended also to increase densities of plant associated macroinvertebrates at low levels with a flattening of response at intermediate levels and a decline at high levels. At high fish level the numbers and biomass of plant-associated macroinvertebrates were lowered in all cases, because of increased predation. However fish addition at low level in most cases had the opposite effect, because their predation pressure was not high and regeneration of nutrients through their activities might increase food availability and control invertebrate predators. Fish influenced especially community composition, because they prey selectively. Addition of fish at low densities increased macroinvertebrate diversity. A diel migratory cycle was observed in plant-associated macroinvertebrates, but only in the southerly locations. Latitudinal influences on the structure of the macroinvertebrate communities occur but that they operate indirectly. There was a prominent effect of nutrients on these communities so that starting trophy is very important. Fish effects are layered on these and depend to a large extent on the particular range of diet of the species involved.Se ha estudiado la comunidad de macroinvertebrados bentónicos del sedimento asà como la de los asociados a los macrófitos en una serie de mesocosmos concebidos para un experimento múltiple simultáneo llevado a cabo en cinco localidades europeas: Finlandia, Suecia, Reino Unido (UK), Noroeste de España y Levante Español. El experimento fue diseñado para investigar como responden los ecosistemas de lagos someros a los aportes de nutrientes y a la abundancia de peces planctÃvoros a lo largo de un gradiente climático. La adición de nutrientes incrementó la densidad del bentos del sedimento y su biomasa hasta estabilizarse a niveles intermedios de nutrientes, siendo este efecto mayor en las latitudes más bajas y cálidas. Más allá del punto de saturación, un aumento adicional de la carga de nutrientes tuvo efectos diferentes dependiendo de la localidad y solo en el lugar de menor latitud, (en el Levante Español) indujo el cambio hacia una fase turbia, con desaparición de los macrófitos, esto es, hay puntos de ruptura relacionados con la carga de nutrientes que conducen a comunidades alternativas en el sedimento, subsiguientes a los mencionados estados alternativos en las aguas someras. La presencia de peces aumentó la densidad y biomasa de los macroinvertebrados del sedimento, ello también más claramente en la localidad situada más al Sur, ya que el pez planctÃvoro ensayado (Gambusia) por lo general no depreda en los animales del sedimento. Se observó también el papel estabilizador de los macrófitos comparando las muestras tomadas antes y después de que estos fueran cortados, por ejemplo se observó un efecto mayor de los peces en las comunidades del sedimento cuando los macrófitos no estaban presentes. Los nutrientes tendieron también a incrementar las densidades de los macroinvertebrados asociados a los macrófitos cuando se aportan a bajos niveles, con una estabilización de la respuesta para los valores intermedios y un declive para los valores más altos. Cuando se adicionaron densidades altas de peces, el número y biomasa de los macroinvertebrados asociados a los macrófitos descendieron en todos los casos, como resultado del aumento de la depredación. Sin embargo, las adiciones a baja densidad de peces, en la mayorÃa de los casos, tuvo el efecto contrario, ya que su presión de depredación no es alta y pueden contribuir a la regeneración de los nutrientes a través de la vÃa detrÃtica que incrementa la disponibilidad de alimento, asà como al control de otros invertebrados depredadores. La presencia de peces influenció especialmente la composición de la comunidad, ya que su depredación es selectiva. La adición de peces en bajas densidades aumentó la diversidad de los macroinvertebrados. También fue observado que el ciclo diario de migración en los macroinvertebrados asociados a los macrófitos, es más importante en las localidades ubicadas en las latitudes de más al Sur. Las influencias latitudinales en la estructura de las comunidades de macroinvertebrados, si bien sucede, opera indirectamente. Hay un efecto dominante de los nutrientes en estas comunidades, y el nivel trófico de las condiciones iniciales es muy importante. Los efectos de los peces se subordinan al de los nutrientes y dependen en gran medida de la dieta de las especies involucradas
The repulsive lattice gas, the independent-set polynomial, and the Lov\'asz local lemma
We elucidate the close connection between the repulsive lattice gas in
equilibrium statistical mechanics and the Lovasz local lemma in probabilistic
combinatorics. We show that the conclusion of the Lovasz local lemma holds for
dependency graph G and probabilities {p_x} if and only if the independent-set
polynomial for G is nonvanishing in the polydisc of radii {p_x}. Furthermore,
we show that the usual proof of the Lovasz local lemma -- which provides a
sufficient condition for this to occur -- corresponds to a simple inductive
argument for the nonvanishing of the independent-set polynomial in a polydisc,
which was discovered implicitly by Shearer and explicitly by Dobrushin. We also
present some refinements and extensions of both arguments, including a
generalization of the Lovasz local lemma that allows for "soft" dependencies.
In addition, we prove some general properties of the partition function of a
repulsive lattice gas, most of which are consequences of the alternating-sign
property for the Mayer coefficients. We conclude with a brief discussion of the
repulsive lattice gas on countably infinite graphs.Comment: LaTex2e, 97 pages. Version 2 makes slight changes to improve clarity.
To be published in J. Stat. Phy
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