Formation of disclination lines near a free nematic interface

We have studied the nucleation and the physical properties of a -1/2 wedge disclination line near the free surface of a confined nematic liquid crystal. The position of the disclination line has been related to the material parameters (elastic constants, anchoring energy and favored anchoring angle of the molecules at the free surface). The use of a planar model for the structure of the director field (whose predictions have been contrasted to those of a fully three-dimensional model) has allowed us to relate the experimentally observed position of the disclination line to the relevant properties of the liquid crystals. In particular, we have been able to observe the collapse of the disclination line due to a temperature-induced anchoring angle transition, which has allowed us to rule out the presence of a real disclination line near the nematic/isotropic front in directional growth experiments. 61.30.Jf,61.30.G

Generalized stacking fault energetics and dislocation properties: compact vs. spread unit dislocation structures in TiAl and CuAu

We present a general scheme for analyzing the structure and mobility of dislocations based on solutions of the Peierls-Nabarro model with a two component displacement field and restoring forces determined from the ab-initio generalized stacking fault energetics (ie., the so-called $\gamma$-surface). The approach is used to investigate dislocations in L1$_{0}$ TiAl and CuAu; predicted differences in the unit dislocation properties are explicitly related with features of the $\gamma$-surface geometry. A unified description of compact, spread and split dislocation cores is provided with an important characteristic "dissociation path" revealed by this highly tractable scheme.Comment: 7 two columns pages, 2 eps figures. Phys. Rev. B. accepted November 199

Thermodynamic aspects of anisotropy of grain boundary segregation

La dépendance de l'entropie envers l'enthalpie en fonction des donnés thermodynamiques de ségrégation de Si, P et C sur les joints de grains différents dans α-Fe a été présentée. La linéarité de cette dépendance a été démontré pour chaque segrégant. L'analyse de cette relation principale ainsi que de la capacité différente de Si, et de P et C former des alliages ordonnés permet d'expliquer : (i) les valuers positives de l'entropie de ségrégation de P et C et les valeurs négatives de Si, (ii) les valeurs élevées de l'entropie de ségrégation dans le cas de joints de grains spéciaux en comparison des joints de grains généraux, (iii) l'anisotropie inverse de ségrégation des joints de grains pour les températures élevées.Previous thermodynamic data on Si, P and C segregation at different grain boundaries in α-Fe were plotted as a dependence of entropy vs. enthalpy. The linear dependence between these two quantities was proved for each segregant. Based on the analysis of this principal relationship as well as of the different ability of silicon on one hand and phosphorus and carbon on the other hand to form ordered alloys, it was possible to explain (i) positive values of segregation entropy for P and C but negative ones for Si, (ii) higher values of segregation entropy for special grain boundaries as compared to general ones, and (iii) inverse anisotropy of grain boundary segregation at higher temperatures

Segregation in intermetallics : on the possible origins of a low-level intergranular segregation.

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