Structure of sticky-hard-sphere random aggregates: The viewpoint of contact coordination and tetrahedra

International audienceWe study more than 10 4 random aggregates of 10 6 monodisperse sticky hard spheres each, generated by various static algorithms. Their packing fraction varies from 0.370 up to 0.593. These aggregates are shown to be based on two types of disordered structures: random regular polytetrahedra and random aggregates, the former giving rise to δ peaks on pair distribution functions. Distortion of structural (Delaunay) tetrahedra is studied by two parameters, which show some similarities and some differences in terms of overall tendencies. Isotropy of aggregates is characterized by the nematic order parameter. The overall structure is then studied by distinguishing spheres in function of their contact coordination number (CCN). Distributions of average CCN around spheres of a given CCN value show trends that depend on packing fraction and building algorithms. The radial dependence of the average CCN turns out to be dependent upon the CCN of the central sphere and shows discontinuities that resemble those of the pair distribution function. Moreover, it is shown that structural details appear when the CCN is used as pseudochemical parameter, such as various angular distribution of bond angles, partial pair distribution functions, Ashcroft-Langreth and Bhatia-Thornton partial structure factors. These allow distinguishing aggregates with the same values of packing fraction or average tetrahedral distortion or even similar global pair distribution function, indicative of the great interest of paying attention to contact coordination numbers to study more precisely the structure of random aggregates

Structure and microstructure evolution of Al-Mg-Si alloy processed by equal-channel angular pressing

An ultrafine grained Al–Mg–Si alloy was prepared by severe plastic deformation using the equal-channel angular pressing (ECAP) method. Samples were ECAPed through a die with an inner angle of F = 90° and outer arc of curvature of ¿ = 37° from 1 to 12 ECAP passes at room temperature following route Bc. To analyze the evolution of the microstructure at increasing ECAP passes, X-ray diffraction and electron backscatter diffraction analyses were carried out. The results revealed two distinct processing regimes, namely (i) from 1 to 5 passes, the microstructure evolved from elongated grains and sub-grains to a rather equiaxed array of ultrafine grains and (ii) from 5 to 12 passes where no change in the morphology and average grain size was noticed. In the overall behavior, the boundary misorientation angle and the fraction of high-angle boundaries increase rapidly up to 5 passes and at a lower rate from 5 to 12 passes. The crystallite size decreased down to about 45 nm with the increase in deformation. The influence of deformation on precipitate evolution in the Al–Mg–Si alloy was also studied by differential scanning calorimetry. A significant decrease in the peak temperature associated to the 50% of recrystallization was observed at increasing ECAP passes.Peer ReviewedPreprin

Capas delgadas optenidas por pirólisis

Los procedimientos de pirólisis para el depósito de capas delgadas experimentan una expansión constante en la industria del vidrio plano. Una descripción rápida de dichos procedimientos nos permite precisar sus ventajas y las cualidades de los productos obtenidos. Se analizan las funciones nuevas que estas capas pirolizadas pueden conferior a los acristalamientos. Esta exposición es ilustrada finalmente con el ejemplo de capas poco emisivas destinadas a la construcción o al automóvilPyrolisis procedures for depositing thin layers are experiencing a constant growt in the plate glass industry. A quick description of those procedures enables to state their advantages and quality of the products obtained. The new functions that these pyrolized layers may confer to glasses are analyzed. This review is completed with the example of low emissive layers intented for the building and car industryPeer reviewe

Deuxième partie. — Structure des métaux et alliages amorphes

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VERRES METALLIQUES COVALENTS ET IONIQUES

Nous montrons ici les possibilités et les limites des modèles de sphères adhésives pour décrire la structure des verres métalliques, covalents ou ioniques (I).The possibilities and limitations of adhesive sphere models for the structural description of metallic, covalent and ionic glasses are discussed

ÉTUDE DE LA STRUCTURE DES ALLIAGES MÉTALLIQUES AMORPHES PAR DIFFRACTION DES NEUTRONS

On présente les premiers résultats de l'étude de la structure de Ni-P et Co-P par diffraction des neutrons. Les mesures d'intensité ont été effectuées jusqu'à Kmax = 30 pour Ni-P. On montre que la fonction de distribution radiale de cet alliage ne peut plus être notablement améliorée par une nouvelle prolongation de la fonction d'interférence. Des détails concernant la distribution des atomes de phosphore apparaissent dans le cas de l'alliage Co-P en utilisant l'inversion de contraste de ces 2 atomes vis-à-vis des R. X. et des neutrons.Preliminary results of neutron diffraction experiments on Ni-P and Co-P amorphous alloys are discussed. Scattering data for Ni-P have been recorded up to Kmax = 30. It is shown that a new extension of the interference function should not provide significant contribution to the Ni-P R. D. F. First details on the P atom distribution are given by using the reversion of the scattering amplitude ratios for Co and P atoms in X-Rays and neutrons diffraction patterns

STUDY OF LIQUID NICKEL-VANADIUM ALLOYS BY NEUTRON DIFFRACTION AND MODEL SIMULATION

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