Unraveling the temperature dependence of the yield strength in single-crystal tungsten using atomistically-informed crystal plasticity calculations

We use a physically-based crystal plasticity model to predict the yield strength of body-centered cubic (bcc) tungsten single crystals subjected to uniaxial loading. Our model captures the thermally-activated character of screw dislocation motion and full non-Schmid effects, both of which are known to play a critical role in bcc plasticity. The model uses atomistic calculations as the sole source of constitutive information, with no parameter fitting of any kind to experimental data. Our results are in excellent agreement with experimental measurements of the yield stress as a function of temperature for a number of loading orientations. The validated methodology is then employed to calculate the temperature and strain-rate dependence of the yield strength for 231 crystallographic orientations within the standard stereographic triangle. We extract the strain-rate sensitivity of W crystals at different temperatures, and finish with the calculation of yield surfaces under biaxial loading conditions that can be used to define effective yield criteria for engineering design models

MOBILITÉ DES DISLOCATIONS DANS LES CRISTAUX MOLECULAIRES CAS DE L'HEXAMÉTHYLÈNE-TÉTRAMINE

L'étude de la mobilité des dislocations dans l'hexaméthylène-tétramine, cristal moléculaire de structure cubique centrée, a été abordée à la fois par l'aspect macroscopique de la plasticité (essais de compression à température ambiante) et par des expériences de fluage dans le domaine microplastique, effectuées directement sur chambre de topographies aux rayons X (méthode de Lang). Les résultats obtenus suggèrent fortement des mécanismes fondamentaux analogues à ceux observés dans les métaux, en particulier par le caractère vis 1/2 prépondérant qui apparaît bien avant la limite élastique macroscopique.A study of the mobility of dislocations in the cubic centered molecular crystal hexamine has been undertaken, both macroscopically through room temperature compression experiments and in the microplastic range by creep experiments performed directly on a X-ray topographic Lang camera. The results strongly suggest fundamental mechanisms analogous to the ones observed in metals, in particular by the very marked 1/2 screw character which appears well before the macroscopic elastic limit

X-ray dislocation substructure observations and strengthening mechanisms in α-iron single crystal between room temperature and 123 K

α-Iron single crystals have been investigated in the (101) [111] slip system between 123 and 296 K up to 20 % deformation. Stress relaxation and creep tests have been used to measure activation parameters during tensile straining. Berg-Barrett X-ray reflection topography was used to examine the overall dislocation substructure. Below 200 K the observed dislocation substructure is homogeneous ; the activation volume is independent of strain and the stress-strain curves show an almost zero work-hardening rate : the whole plasticity is controlled solely by the Peierls friction. Above 200 K the dislocation substructure becomes more and more heterogeneous as temperature increases ; the activation volume decreases with strain and the work-hardening rate is around 10 -3 μ : thermal activation overcomes the lattice friction.La consolidation du fer α monocristallin a été étudiée sur le système (101) [111] entre 296 K et 123 K jusqu'à 20 % de déformation. Les paramètres d'activation thermique ont été mesurés tout le long de la courbe de déformation par relaxation de la contrainte appliquée et par des essais différentiels en fluage, parallèlement on a examiné la sous-structure de dislocations par topographie X de Berg-Barrett. En-dessous de 200 K, la sous-structure de dislocations observée est homogène, le volume d'activation est indépendant de la déformation et les courbes contrainte-cisaillement réduits présentent un taux de durcissement quasiment nul : toute la plasticité est régie par le seul frottement de réseau. Au-dessus de 200 K, la sous-structure de dislocations devient de plus en plus hétérogène au fur et à mesure que la température croît, le volume d'activation décroît avec la déformation et le taux de durcissement n'est plus nul et vaut 10-3 μ : l'activation thermique l'emporte sur le frottement de réseau

Compressive creep of ultrafine grained MgO polycrystals

Steady state compressive creep of ultrafine-grained MgO has been studied in the temperature range 0.3 to 0.38 Tm. The specimens were fabricated by reactive hot pressing i.e. by hot-pressing of Mg(OH)2 during the decomposition reaction, which produced dense specimens (90 % of the theoretical density) with 1 000 Å average grain size. The value of the stress exponent was found to be unity and the activation enthalpy was of the order of 36 kcal/mole.Le fluage stationnaire de l'oxyde de Magnésium polycristallin à grains ultra-fins a été étudié entre 0,3 et 0,38 Tm. Les échantillons ont été fabriqués par frittage réactif durant la réaction de décomposition de Mg(OH)2 . Ce type de frittage produit un matériau dense (90 % de la densité théorique) possédant une taille de grains très fine (1 000 Å). La sensibilité de la vitesse de fluage stationnaire avec la contrainte s'exprime par un exposant de contrainte de l'ordre de l'unité et l'enthalpie d'activation est faible de l'ordre de 36 kcal/mole

Chromatin freeze fracture electron microscopy: a comparative study of core particles, chromatin, metaphase chromosomes, and nuclei.

Chromatin gels, metaphase chromosomes, and intact nuclei were studied by freeze fracturing followed by electron microscopy. The results complement and extend those obtained by classical electron microscopy techniques as they are obtained without fixation or dehydration. The freeze fracturing technique permits a determination of the hydrated diameters of nucleosomes in chromatin and in nuclei to be 13 nm by comparing to simultaneously studied test objects. Nucleosomes in chromatin fibers are closely spaced but are discrete particles in all conditions studied. In the presence of divalent ions, most chromatin in solution, chromosomes, and nuclei is organized into fibers whose thickness is larger than 40 nm. The images are not at all compatible with a super bead organization of the nucleofilament. Freeze fractures of intact nuclei provides information on the distribution of chromatin in a hydrated unfixed state. The images suggest that most of the chromatin is localized in large domains in contact with the inner nuclear membrane