477 research outputs found
Crystal Growth in Fluid Flow: Nonlinear Response Effects
We investigate crystal-growth kinetics in the presence of strong shear flow
in the liquid, using molecular-dynamics simulations of a binary-alloy model.
Close to the equilibrium melting point, shear flow always suppresses the growth
of the crystal-liquid interface. For lower temperatures, we find that the
growth velocity of the crystal depends non-monotonically on the shear rate.
Slow enough flow enhances the crystal growth, due to an increased particle
mobility in the liquid. Stronger flow causes a growth regime that is nearly
temperature-independent, in striking contrast to what one expects from the
thermodynamic and equilibrium kinetic properties of the system, which both
depend strongly on temperature. We rationalize these effects of flow on crystal
growth as resulting from the nonlinear response of the fluid to strong shearing
forces.Comment: to appear in Phys. Rev. Material
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Nucleation and phase selection in undercooled melts: Magnetic alloys of industrial relevance (MAGNEPHAS)
Studies of phase selection and microstructure evolution in high-performance magnetic materials are an urgent need for optimization of production routes. Containerless solidification experiments by electromagnetic levitation and drop tube solidification were conducted in undercooled melts of Fe-Co, Fe-Ni soft magnetic, and Nd-Fe-B hard magnetic alloys. Melt undercooling under microgravity was achieved in the TEMPUS facility during parabolic flight campaigns. For Fe-Co and Fe-Ni alloys significant effects of microgravity on metastable phase formation were discovered. Microstructure modifications as well as metastable phase formation as function of undercooling and melt flow were elucidated in Nd-Fe-B. Modeling of solidification processes, fluid flow and heat transfer provide predictive tools for microstructure engineering from the melt. They were developed as a link between undercooling experiments under terrestrial and microgravity conditions and the production routes of magnetic materials
Microstructure and Phase Formation in a Rapidly Solidified Laser-Deposited Ni-Cr-B-Si-C Hardfacing Alloy
In this study, microstructural evolutions and phase selection phenomena during laser deposition of a hardfacing Ni-Cr-B-Si-C alloy at different processing conditions are experimentally investigated. The results show that even minor variations in the thermal conditions during solidification can modify the type and morphology of the phases. Higher undercoolings obtained at faster cooling rates suppressed the primary borides and encouraged floret-shape mixtures of Ni and Cr5B3 via a metastable reaction. Variations in the boride phases are discussed in terms of nucleation-and growth-controlled phase selection mechanisms. These selection processes also influenced the nature and proportion of the Ni-B-Si eutectics by changing the amount of the boron available for the final eutectic reactions. The results of this work emphasize the importance of controlling the cooling rate during deposition of these industrially important alloys using laser beam or other rapid solidification techniques. (C) The Minerals, Metals & Materials Society and ASM International 201
Superconducting properties and Fermi-surface topology of the quasi-two-dimensional organic superconductor -(BETS)GaCl
The Fermi surface topology of the organic superconductor \lbets has been
determined using the Shubnikov-de Haas and magnetic breakdown effects and
angle-dependent magnetoresistance oscillations. The former experiments were
carried out in pulsed fields of up to 60 T, whereas the latter employed
quasistatic fields of up to 30 T. All of these data show that the Fermi-surface
topology of \lbets is very similar to that of the most heavily-studied organic
superconductor, \cuscn, except in one important respect; the interplane
transfer integral in \lbets is a factor larger than that in \cuscn .
The increased three-dimensionality of \lbets is manifested in radiofrequency
penetration-depth measurements, which show a clear dimensional crossover in the
behaviour of . The radiofrequency measurements have also been used
to extract the Labusch parameter determining the fluxoid interactions as a
function of temperature, and to map the flux-lattice melting curve.Comment: 24 pages 10 figure
Magnetotransport in a pseudomorphic GaAs/GaInAs/GaAlAs heterostructure with a Si delta-doping layer
Magnetotransport properties of a pseudomorphic
GaAs/Ga0.8In0.2As/Ga0.75Al0.25As heterostructure are investigated in pulsed
magnetic fields up to 50 T and at temperatures of T=1.4 K and 4.2 K. The
structure studied consists of a Si delta-layer parallel to a Ga0.8In0.2As
quantum well (QW). The dark electron density of the structure is n_e=1.67x
10^16 m^-2. By illumination the density can be increased up to a factor of 4;
this way the second subband in the Ga0.8In0.2As QW can become populated as well
as the Si delta-layer. The presence of electrons in the delta-layer results in
drastic changes in the transport data, especially at magnetic fields beyond 30
T. The phenomena observed are interpreted as: 1) magnetic freeze-out of
carriers in the delta-layer when a low density of electrons is present in the
delta-layer, and 2) quantization of the electron motion in the two dimensional
electron gases in both the Ga0.8In0.2As QW and the Si delta-layer in the case
of high densities. These conclusions are corroborated by the numerical results
of our theoretical model. We obtain a satisfactory agreement between model and
experiment.Comment: 23 pages, RevTex, 11 Postscript figures (accepted for Phys. Rev. B
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