1,249 research outputs found
Approche numérique de la plasticité induite par transformation diffusionnelle : investigations dans le cas de l’interaction plasticité classique TRIP
Le phénomène mécanique appelé TRIP (Transformation Induced Plasticity), correspondant à une déformation permanente induite par une transformation de phase, peut apparaître lors d’une transformation sous chargement extérieur comme lorsque la phase parente a été pré-écrouie. Le TRIP peut apparaître en particulier pendant des opérations de soudage d’aciers, lors de la ou des étapes de refroidissement. Le modèle le plus courant qui permet de tenir compte d'un pré-écrouissage de l'austénite pour prédire le développement de TRIP (modèle de Leblond) s'est montré imparfait pour reproduire les résultats expérimentaux dans le cas d’une phase parente pré-écrouie, d’après ce qu’ont montré Taleb et Petit-Grostabussiat. Ceci a donc motivé le développement de méthodes alternatives, à savoir par éléments finis, pour tenter de mieux rendre compte par modélisation de ces effets de pré-écrouissage. Cet article présente les investigations numériques concernant la transformation bainitique d’un macrovolume cubique dans le cas idéalisé où un seul germe est présent au centre du volume d’étude (ceci constitue un cas test), ainsi que dans le cas où des germes apparaissent aléatoirement dans l’espace et dans le temps selon une loi de distribution donnée.Mots clés: Plasticité induite par transformation; modélisation micromécanique; transformation diffusionnelle; éléments finis. The mechanical phenomenon called TRIP (Transformation Induced Plasticity), corresponding to a permanent strain induced by a phase transformation, can be due to an externally applied load or to a pre-hardening of the parent phase. TRIP can appear particularly during a welding operation on steels, during the cooling step(s). The most currently used model which allows to take into account the pre-hardening of austenite for the prediction of TRIP (Leblond model) has shown to be incorrect to reproduce the experimental results in the case of a pre-hardened austenite, according to the work of Taleb and Petit-Grostabussiat. This has motivated the development of alternative methods, with finite elements analysis, to try to improve the prediction of this effect of pre-hardening. This article presents the numerical investigations concerning the bainitic transformation of a cubic macro-volume. Two cases are treated: the ideal case where a single nucleus is present at the center of the volume (test case); the case where nuclei appear randomly in space and in time, according to a given distribution law.Keywords: Transformation induced plasticity; micromechanical modelling; diffusive transformation; finite elements
Angle-resolved photoemission study of the role of nesting and orbital orderings in the antiferromagnetic phase of BaFe2As2
We present a detailed comparison of the electronic structure of BaFe2As2 in
its paramagnetic and antiferromagnetic (AFM) phases, through angle-resolved
photoemission studies. Using different experimental geometries, we resolve the
full elliptic shape of the electron pockets, including parts of dxy symmetry
along its major axis that are usually missing. This allows us to define
precisely how the hole and electron pockets are nested and how the different
orbitals evolve at the transition. We conclude that the imperfect nesting
between hole and electron pockets explains rather well the formation of gaps
and residual metallic droplets in the AFM phase, provided the relative parity
of the different bands is taken into account. Beyond this nesting picture, we
observe shifts and splittings of numerous bands at the transition. We show that
the splittings are surface sensitive and probably not a reliable signature of
the magnetic order. On the other hand, the shifts indicate a significant
redistribution of the orbital occupations at the transition, especially within
the dxz/dyz system, which we discuss
Symmetry breaking in commensurate graphene rotational stacking; a comparison of theory and experiment
Graphene stacked in a Bernal configuration (60 degrees relative rotations
between sheets) differs electronically from isolated graphene due to the broken
symmetry introduced by interlayer bonds forming between only one of the two
graphene unit cell atoms. A variety of experiments have shown that non-Bernal
rotations restore this broken symmetry; consequently, these stacking varieties
have been the subject of intensive theoretical interest. Most theories predict
substantial changes in the band structure ranging from the development of a Van
Hove singularity and an angle dependent electron localization that causes the
Fermi velocity to go to zero as the relative rotation angle between sheets goes
to zero. In this work we show by direct measurement that non-Bernal rotations
preserve the graphene symmetry with only a small perturbation due to weak
effective interlayer coupling. We detect neither a Van Hove singularity nor any
significant change in the Fermi velocity. These results suggest significant
problems in our current theoretical understanding of the origins of the band
structure of this material.Comment: 7 pages, 6 figures, submitted to PR
Significant reduction of electronic correlations upon isovalent Ru substitution of BaFe2As2
We present a detailed investigation of Ba(Fe0.65Ru0.35)2As2 by transport
measurements and Angle Resolved photoemission spectroscopy. We observe that Fe
and Ru orbitals hybridize to form a coherent electronic structure and that Ru
does not induce doping. The number of holes and electrons, deduced from the
area of the Fermi Surface pockets, are both about twice larger than in
BaFe2As2. The contribution of both carriers to the transport is evidenced by a
change of sign of the Hall coefficient with decreasing temperature. Fermi
velocities increase significantly with respect to BaFe2As2, suggesting a
significant reduction of correlation effects. This may be a key to understand
the appearance of superconductivity at the expense of magnetism in undoped iron
pnictides
Giant Anisotropy of Spin-Orbit Splitting at the Bismuth Surface
We investigate the bismuth (111) surface by means of time and angle resolved
photoelectron spectroscopy. The parallel detection of the surface states below
and above the Fermi level reveals a giant anisotropy of the Spin-Orbit (SO)
spitting. These strong deviations from the Rashba-like coupling cannot be
treated in perturbation theory. Instead, first
principle calculations could accurately reproduce the experimental dispersion
of the electronic states. Our analysis shows that the giant anisotropy of the
SO splitting is due to a large out-of plane buckling of the spin and orbital
texture.Comment: 5 pages, 4 figure
Minimally invasive osteotomy for distal radius malunion: A preliminary series of 9 cases
AbstractThe rate of malunion after distal radius fractures is 25% after conservative treatment and 10% after surgery. Their main functional repercussion related to ulno-carpal conflict is loss of wrist motion. We report a retrospective clinical series of minimally invasive osteotomies. The series consisted of 9 cases of minimally invasive osteotomies with volar locking plate fixation. All osteotomies healed. The average pain was 5.3/10 preoperatively and 2.1/10 at last follow-up. The mean Quick DASH was 55.4/100 preoperatively and 24.24/100 at last follow-up. Compared to the opposite side, the average wrist flexion was 84.11%, the average wrist extension was 80.24%, the average pronation was 95.33% and the average supination was 93.9%. With similar results to those of the literature, our short series confirms the feasibility of minimally invasive osteotomy of the distal radius for extra-articular malunion.TypeCase-series.Level of evidenceIV
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