134 research outputs found
Phase field simulations of coupled phase transformations in ferroelastic-ferroelastic nanocomposites
We use phase field simulations to study composites made of two different
ferroelastics (e.g., two types of martensite). The deformation of one material
due to a phase transformation can elastically affect the other constituent and
induce it to transform as well. We show that the phase transformation can then
occur above its normal critical temperature and even higher above this
temperature in nanocomposites than in bulk composites. Microstructures depend
on temperature, on the thickness of the layers, and on the crystal structure of
the two constituents -- certain nanocomposites exhibit a great diversity of
microstructures not found in bulk composites. Also, the periodicity of the
martensite twins may vary over 1 order of magnitude based on geometry.
keywords: Ginzburg-Landau, martensitic transformation, multi-ferroics,
nanostructure, shape-memory alloyComment: 8 pages, 15 figure
Effect of lattice mismatch-induced strains on coupled diffusive and displacive phase transformations
Materials which can undergo slow diffusive transformations as well as fast
displacive transformations are studied using the phase-field method. The model
captures the essential features of the time-temperature-transformation (TTT)
diagrams, continuous cooling transformation (CCT) diagrams, and microstructure
formation of these alloys. In some materials systems there can exist an
intrinsic volume change associated with these transformations. We show that
these coherency strains can stabilize mixed microstructures (such as retained
austenite-martensite and pearlite-martensite mixtures) by an interplay between
diffusive and displacive mechanisms, which can alter TTT and CCT diagrams.
Depending on the conditions there can be competitive or cooperative nucleation
of the two kinds of phases. The model also shows that small differences in
volume changes can have noticeable effects on the early stages of martensite
formation and on the resulting microstructures.
-- Long version of cond-mat/0605577
-- Keywords: Ginzburg-Landau, martensite, pearlite, spinodal decomposition,
shape memory, microstructures, TTT diagram, CCT diagram, elastic compatibilityComment: 10 pages, 13 figures, long version of cond-mat/0605577. Physical
Review B, to appear in volume 75 (2007
Grain-boundary grooving and agglomeration of alloy thin films with a slow-diffusing species
We present a general phase-field model for grain-boundary grooving and
agglomeration of polycrystalline alloy thin films. In particular, we study the
effects of slow-diffusing species on grooving rate. As the groove grows, the
slow species becomes concentrated near the groove tip so that further grooving
is limited by the rate at which it diffuses away from the tip. At early times
the dominant diffusion path is along the boundary, while at late times it is
parallel to the substrate. This change in path strongly affects the
time-dependence of grain boundary grooving and increases the time to
agglomeration. The present model provides a tool for agglomeration-resistant
thin film alloy design. keywords: phase-field, thermal grooving, diffusion,
kinetics, metal silicidesComment: 4 pages, 6 figure
Is diversity good?
Prominent ethical and policy issues such as affirmative action and female
enrollment in science and engineering revolve around the idea that diversity is
good. However, even though diversity is an ambiguous concept, a precise
definition is seldom provided. We show that diversity may be construed as a
factual description, a craving for symmetry, an intrinsic good, an instrumental
good, a symptom, or a side effect. These acceptions differ vastly in their
nature and properties. The first one cannot lead to any action and the second
one is mistaken. Diversity as intrinsic good is a mere opinion, which cannot be
concretely applied; moreover, the most commonly invoked forms of diversity
(sexual and racial) are not intrinsically good. On the other hand, diversity as
instrumental good can be evaluated empirically and can give rise to policies,
but these may be very weak. Finally, symptoms and side effects are not actually
about diversity. We consider the example of female enrollment in science and
engineering, interpreting the various arguments found in the literature in
light of this polysemy.
Keywords: ethics, policy, higher education, female students, minority
students, affirmative actionComment: 7 page
Phase-field model for grain boundary grooving in multi-component thin films
Polycrystalline thin films can be unstable with respect to island formation
(agglomeration) through grooving where grain boundaries intersect the free
surface and/or thin film-substrate interface. We develop a phase-field model to
study the evolution of the phases, composition, microstructure and morphology
of such thin films. The phase-field model is quite general, describing
compounds and solid solution alloys with sufficient freedom to choose
solubilities, grain boundary and interface energies, and heats of segregation
to all interfaces. We present analytical results which describe the interface
profiles, with and without segregation, and confirm them using numerical
simulations. We demonstrate that the present model accurately reproduces the
theoretical grain boundary groove angles both at and far from equilibrium. As
an example, we apply the phase-field model to the special case of a Ni(Pt)Si
(Ni/Pt silicide) thin film on an initially flat silicon substrate.Comment: 12 pages, 5 figures, submitted to Modelling Simulation Mater. Sci.
En
Wire edge dependent magnetic domain wall creep
open13While edge pinning is known to play an important role in sub-μm wires, we demonstrate that strong deviations
from the universal creep law can occur in 1 to 20 μm wide wires. Magnetic imaging shows that edge pinning
translates into a marked bending of domain walls at low drive and is found to depend on the wire fabrication
process and aging. Edge pinning introduces a reduction of domain wall velocity with respect to full films which
increasingly dominates the creep dynamics as the wire width decreases. We show that the deviations from the
creep law can be described by a simple model including a counter magnetic field which links the width of the
wire to the edge dependent pinning strength. This counter field defines a key nonuniversal contribution to creep
motion in patterned structures.openHerrera Diez, L.; Jeudy, V.; Durin, G.; Casiraghi, A.; Liu, Y. T.; Voto, M.; Agnus, G.; Bouville, D.; Vila, L.; Langer, J.; Ocker, B.; Lopez-Diaz, L.; Ravelosona, D.Herrera Diez, L.; Jeudy, V.; Durin, G.; Casiraghi, A.; Liu, Y. T.; Voto, M.; Agnus, G.; Bouville, D.; Vila, L.; Langer, J.; Ocker, B.; Lopez-Diaz, L.; Ravelosona, D
Magnetic domain wall curvature induced by wire edge pinning
open14In this study, we report on the analysis of the magnetic domain wall (DW) curvature due to magnetic field induced motion in Ta/CoFeB/MgO and Pt/Co/Pt wires with perpendicular magnetic anisotropy. In wires of 20 mu m and 25 mu m, a large edge pinning potential produces the anchoring of the DW ends to the wire edges, which is evidenced as a significant curvature of the DW front as it propagates. As the driving magnetic field is increased, the curvature reduces as a result of the system moving away from the creep regime of DW motion, which implies a weaker dependence of the DW dynamics on the interaction between the DW and the wire edge defects. A simple model is derived to describe the dependence of the DW curvature on the driving magnetic field and allows us to extract the parameter sigma (E), which accounts for the strength of the edge pinning potential. The model describes well the systems with both weak and strong bulk pinning potentials like Ta/CoFeB/MgO and Pt/Co/Pt, respectively. This provides a means to quantify the effect of edge pinning induced DW curvature on magnetic DW dynamics.embargoed_20210815Herrera Diez, L.; Ummelen, F.; Jeudy, V.; Durin, G.; Lopez-Diaz, L.; Diaz-Pardo, R.; Casiraghi, A.; Agnus, G.; Bouville, D.; Langer, J.; Ocker, B.; Lavrijsen, R.; Swagten, H. J. M.; Ravelosona, D.Herrera Diez, L.; Ummelen, F.; Jeudy, V.; Durin, G.; Lopez-Diaz, L.; Diaz-Pardo, R.; Casiraghi, A.; Agnus, G.; Bouville, D.; Langer, J.; Ocker, B.; Lavrijsen, R.; Swagten, H. J. M.; Ravelosona, D
Long-wave infrared integrated resonators in the 7.5-9 mu m wavelength range
We present broadband on-chip resonators based on SiGe graded-index waveguides operating in the long-wave infrared spectral range from 7.5 to 9.0 mu m wavelength range. A quality factor up to 10(5) has been measured, while an intrinsic quality factor of 1.13 x 10(5) has been extracted from the measurements. Thermal tuning of the phase in the micro-ring has been used to overcome the limitation of the experimental setup in terms of spectral resolution. These results pave the way toward the development of integrated frequency comb operating in the long-wave infrared range
Methodology for evaluating the radiological consequences of radioactive effluents released in normal operations. July 1979, Joint Report by the National Radiological Protection Board and the Commissariat a L'Energie Atomique.
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