SEGREGATION AT SPECIAL GRAIN BOUNDARIES IN Fe-Si ALLOY BICRYSTALS

La ségrégation du silicium était étudiée avec la spectrométrie Auger aux joints de grains symmétriques Σ=3, {112} et Σ=5, {013} dans des échantillons bicrystallins d'un alliage Fe-12.9 at % Si qu'étaient équilibrés dans la région 773 - 1173 K. L'enthalpie de ségrégation, ƊH = -8.1 ± 0.5 kJ/mol, et l'entropie, ཀྵS = -6.9 ± 0.4 J/(mol K) étaient déterminées pour le joint de grain {013} aux températures 973 - 1173 K. Pour le joint de grand {112}, les valeurs correspondantes ཀྵH = -4 ± 3 kJ/mol et ཀྵS = -4 ± 3 J/(mol K) étaient obtenues. Aux deux joints, une diminution de la ségrégation du Si était observé à 773 K. De plus une distribution à profondeur élongée de l'enrichissement du Si était observée malgré les hautes températures et malgré les longs temps du chauffage. Ces deux phénomènes sont probablement causés par des effets de l'ordre dans cet alliage. L'addition du phosphore à cet alliage est la cause d'une diminution du content du Si au joint de grain {112}.The segregation of silicon to the Σ=3, {112} and Σ=5, {013} symmetrical grain boudaries was studied in well-defined bicrystals of an Fe-12.9 at % Si alloy annealed in the range 773 - 1173 K using Auger electron spectroscopy. The segregation enthalpy, ƊH = -8.1 ± 0.5 kJ/mol, and entropy, ƊS = -6.9 ± 0.4 J/(mol K), were determined for the {013} grain boundary in the temperature range 973 - 1173 K. For the {112} boundary the corresponding values ཀྵH = -4 ± 3 kJ/mol and ཀྵS = -4 ± 3 J/(mol K) were obtained. At both these boundaries, a decrease of the segregation of Si was observed at 773 K. In addition, a wide in-depth distribution of silicon enrichment was observed away from the boundaries in spite of relatively high annealing temperatures and times. Both effects are probably due to the ordering effects in this alloy. Addition of phosphorus to this alloy caused a depletion of silicon at the {112} grain boundary

Drag of a Cottrell atmosphere by an edge dislocation in a smectic-A liquid crystal

In a recent letter (P. Oswald et al., EPL 103, 46004 (2013)), we have shown that a smectic-A phase hardens in compression normal to the layers when the liquid crystal is doped with gold nanoparticles. This is due to the formation of Cottrell clouds nearby the core of the edge dislocations and the appearance of an additional drag force that reduces their mobility. We theoretically calculate the shape of the Cottrell cloud and the associated drag force as a function of the climb velocity of the dislocations. The main result is that the drag force depends on velocity and vanishes when the temperature tends to the smectic-A-to-nematic transition temperature. The role of the diffusion anisotropy is also evaluated

Faceting and stability of smectic A droplets on a solid substrate

It is shown that a smectic A droplet deposited on a solid substrate treated for strong homeotropic anchoring is faceted at the top in spite of the fact that there are no steps at the free surface, but instead edge dislocations in the bulk. The radius of the facet and the full profile of the curved part of the droplet are determined as a function of the temperature in the vicinity of a nematic-smectic A phase transition. It is shown that the observed profiles do not correspond to the actual equilibrium shape, but to metastable configurations close to their point of marginal stability. In addition, we predict that the profiles must be different for a given temperature depending on whether the droplet has been heated or cooled down to reach this temperature. Finally, we discuss the problem of the formation of giant dislocations in big droplets (Grandjean terraces)

Effect of Temperature on Grain Refinement of Mg-3Al-1Zn Alloy Processed by Equal Channel Angular Pressing

In this work the effect of temperature on grain refinement of Mg-3Al-1Zn alloy (AZ31B) processed by equal channel angular pressing using route A is described. The deformation sequences consisted of equal channel angular pressing passes at 200C followed by passes at 150°C. Nonhomogeneous grain size distribution promotes shear band formation at 150°C. Shear bands with microcracks inside were analyzed by electron backscatter diffraction technique