840 research outputs found
Relationship between solidification microstructure and hot cracking susceptibility for continuous casting of low-carbon and high-strength low-alloyed steels: A phase-field study
© The Minerals, Metals & Materials Society and ASM International 2013Hot cracking is one of the major defects in continuous casting of steels, frequently limiting the productivity. To understand the factors leading to this defect, microstructure formation is simulated for a low-carbon and two high-strength low-alloyed steels. 2D simulation of the initial stage of solidification is performed in a moving slice of the slab using proprietary multiphase-field software and taking into account all elements which are expected to have a relevant effect on the mechanical properties and structure formation during solidification. To account for the correct thermodynamic and kinetic properties of the multicomponent alloy grades, the simulation software is online coupled to commercial thermodynamic and mobility databases. A moving-frame boundary condition allows traveling through the entire solidification history starting from the slab surface, and tracking the morphology changes during growth of the shell. From the simulation results, significant microstructure differences between the steel grades are quantitatively evaluated and correlated with their hot cracking behavior according to the Rappaz-Drezet-Gremaud (RDG) hot cracking criterion. The possible role of the microalloying elements in hot cracking, in particular of traces of Ti, is analyzed. With the assumption that TiN precipitates trigger coalescence of the primary dendrites, quantitative evaluation of the critical strain rates leads to a full agreement with the observed hot cracking behavior. © 2013 The Minerals, Metals & Materials Society and ASM International
Doping, density of states and conductivity in polypyrrole and poly(p-phenylene vinylene)
The evolution of the density of states (DOS) and conductivity as function of
well controlled doping levels in OC_1C_10-poly(p-phenylene vinylene)
[OC_1C_10-PPV] doped by FeCl_3 and PF_6, and PF_6 doped polypyrrole (PPy-PF_6
have been investigated. At a doping level as high as 0.2 holes per monomer, the
former one remains non-metallic, while the latter crosses the metal-insulator
transition. In both systems a similar almost linear increase in DOS as function
of charges per unit volume c* has been observed from the electrochemical gated
transistor data. In PPy-PF_6, when compared to doped OC_1C_10-PPV, the energy
states filled at low doping are closer to the vacuum level; by the higher c* at
high doping more energy states are available, which apparently enables the
conduction to change to metallic. Although both systems on the insulating side
show log(sigma) proportional to T^-1/4 as in variable range hopping, for highly
doped PPy-PF_6 the usual interpretation of the hopping parameters leads to
seemingly too high values for the density of states.Comment: 4 pages (incl. 6 figures) in Phys. Rev.
Water mites from stagnant waters of Paraguay
Near to the Paraguayan town Concepción water mites were sampled in one shallow lake and four ponds, so-called Tajamares. These waters are characterized briefly in their abiotic aspects. The six net catches had as a result 29 species out of 13 genera and 5 families. Six species are new to science. Just a single species is found in 7 out of 13 genera. The genus Koenikea is the one with the largest species number, namely 7. Only a few species show greater abundances. Many species have markedly inferior abundances: of 10 species only a single individual could be caught. Not a single species occurred in all five waters
Taxonomic diversity and identification problems of oncaeid microcopepods in the Mediterranean Sea
The species diversity of the pelagic microcopepod
family Oncaeidae collected with nets of 0.1-mm mesh
size was studied at 6 stations along a west-to-east transect
in the Mediterranean Sea down to a maximum depth of
1,000 m. A total of 27 species and two form variants have
been identified, including three new records for the
Mediterranean. In addition, about 20, as yet undescribed,
new morphospecies were found (mainly from the genera
Epicalymma and Triconia) which need to be examined
further. The total number of identified oncaeid species was
similar in the Western and Eastern Basins, but for some cooccurring
sibling species, the estimated numerical dominance
changed. The deep-sea fauna of Oncaeidae, studied
at selected depth layers between 400 m and the near-bottom
layer at >4,200 m depth in the eastern Mediterranean
(Levantine Sea), showed rather constant species numbers
down to ∼3,000 m depth. In the near-bottom layers, the
diversity of oncaeids declined and species of Epicalymma
strongly increased in numerical importance. The taxonomic
status of all oncaeid species recorded earlier in the
Mediterranean Sea is evaluated: 19 out of the 46 known
valid oncaeid species are insufficiently described, and most
of the taxonomically unresolved species (13 species) have
originally been described from this area (type locality). The
deficiencies in the species identification of oncaeids cast
into doubt the allegedly cosmopolitan distribution of some
species, in particular those of Mediterranean origin. The
existing identification problems even of well-described
oncaeid species are exemplified for the Oncaea mediacomplex,
including O. media Giesbrecht, O. scottodicarloi
Heron & Bradford-Grieve, and O. waldemari Bersano &
Boxshall, which are often erroneously identified as a single
species (O. media). The inadequacy in the species identification
of Oncaeidae, in particular those from the Atlantic
and Mediterranean, is mainly due to the lack of reliable
identification keys for Oncaeidae in warm-temperate and/or
tropical seas. Future efforts should be directed to the
construction of identification keys that can be updated
according to the latest taxonomic findings, which can be
used by the non-expert as well as by the specialist. The
adequate consideration of the numerous, as yet undescribed,
microcopepod species in the world oceans, in
particular the Oncaeidae, is a challenge for the study of the
structure and function of plankton communities as well as
for global biodiversity estimates
Ac hopping conduction at extreme disorder takes place on the percolating cluster
Simulations of the random barrier model show that ac currents at extreme
disorder are carried almost entirely by the percolating cluster slightly above
threshold; thus contradicting traditional theories contributions from isolated
low-activation-energy clusters are negligible. The effective medium
approximation in conjunction with the Alexander-Orbach conjecture leads to an
excellent analytical fit to the universal ac conductivity with no nontrivial
fitting parameters
Negative Magnetoresistance in the Nearest-neighbor Hopping Conduction
We propose a size effect which leads to the negative magnetoresistance in
granular metal-insulator materials in which the hopping between two nearest
neighbor clusters is the main transport mechanism. We show that the hopping
probability increases with magnetic field. This is originated from the level
crossing in a few-electron cluster. Thus, the overlap of electronic states of
two neighboring clusters increases, and the negative magnetoresistance is
resulted.Comment: Latex file, no figur
Experimental identification of a surface integrity model for turning of AISI4140
In this work an experimental study of the turning of AISI4140 is presented. The scope is the understanding of the workpiece microstructure and hardness-depth-profiles which result from different cutting conditions and thus thermomechanical surface loads. The regarded input parameters are the cutting velocity (vc = 100, 300 m/min), feed rate (f = 0.1, 0.3 mm), cutting depth (ap = 0.3, 1.2 mm) and the heat treatment of the workpiece (tempering temperatures 300, 450 and 600°C). The experimental data is interpreted in terms of machining mechanisms and material phenomena, e.g. the generation of white layers, which influence the surface hardness. Hereby the process forces are analyzed as well. The gained knowledge is the prerequisite of a workpiece focused process control
Phase-field modelling of ternary eutetic solidification in hot dip galvanization
Continuous hot dip galvanizing is one of the commercially most important process techniques used for protecting steel sheets against corrosion. Preheated steel sheets are con-tinuously drawn through a pot with a molten zinc alloy. After passing a gas jet that controls the layer thickness multiple cooling units act to cool down the sheet. During passing these aggregates nucleation, dendritic primary solidification and the formation of a binary and ter-nary eutectic occurs. In the present paper, the coupled modelling of macroscopic heat flow, multiphase thermodynamics and crystal growth during solidification of a Zn-2.5 wt.% Al-1.5 wt.% Mg alloy is presented. The heat flow problem requires a numerical domain in the order of meters, growth of primary Zn-dendrites in the order of several hundred micrometers, and the interdendritic eutectic in the order of several nanometers. For technical alloys like the ternary system considered here, a thermodynamic database has been online linked to a phase-field model to describe phase transformations including all occurring solid/liquid or sol-id/solid interfaces. Process simulations have been used for getting appropriate thermal bound-ary conditions for 3D phase field simulations which were performed at three different length-scales. For modelling primary dendritic Zn-a seed density model was used for predicting the grain structure within the Zn layer. At a smaller length-scale, a small part of a Zn-dendrite surface was taken as starting point for simulating the transition between primary binary eutec-tic and ternary eutectic coupled growth of Zn-rich, Al-rich, and MgZn2-phases. Finally, the morphology of the ternary eutectic has been evaluated at the smallest length scale. The com-parison with real solidification microstructure reveals encouraging agreements
On dispersive energy transport and relaxation in the hopping regime
A new method for investigating relaxation phenomena for charge carriers
hopping between localized tail states has been developed. It allows us to
consider both charge and energy {\it dispersive} transport. The method is based
on the idea of quasi-elasticity: the typical energy loss during a hop is much
less than all other characteristic energies. We have investigated two models
with different density of states energy dependencies with our method. In
general, we have found that the motion of a packet in energy space is affected
by two competing tendencies. First, there is a packet broadening, i.e. the
dispersive energy transport. Second, there is a narrowing of the packet, if the
density of states is depleting with decreasing energy. It is the interplay of
these two tendencies that determines the overall evolution. If the density of
states is constant, only broadening exists. In this case a packet in energy
space evolves into Gaussian one, moving with constant drift velocity and mean
square deviation increasing linearly in time. If the density of states depletes
exponentially with decreasing energy, the motion of the packet tremendously
slows down with time. For large times the mean square deviation of the packet
becomes constant, so that the motion of the packet is ``soliton-like''.Comment: 26 pages, RevTeX, 10 EPS figures, submitted to Phys. Rev.
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