La microflore de diatomées au passage mio-pliocène en Corse

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Growth and microstructure evolution of the Nb(2)A-Al3Nb eutectic in situ composite

In situ composite materials obtained by directional growth of eutectic alloys usually show improved properties, that make them potential candidates for high temperature applications. The eutectic alloy found in the Al-Nb system is composed of the two intermetallic phases Al3Nb (D0(22)) and Nb2Al (D(8)b). This paper describes the directional solidification of an Al-Nb eutectic alloy using a Bridgman type facility at growth rates varying from 1.0 to 2.9 cm/h. Longitudinal and transverse sections of grown samples were characterized regarding the solidification microstructure by using optical and scanning electron microscopy, energy dispersive spectroscopy (EDS) and X-ray diffraction. Despite both phases being intermetallic compounds, the eutectic microstructure obtained was very regular. The results obtained were discussed regarding the effect of the growth rate on the microstructure, lamellar-rod transition and variation of phase volume fraction. (c) 2004 Elsevier Inc. All rights reserved.54318719

Directional solidification, microstructure and properties of the Al3Nb-Nb2Al eutectic

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)The Al-Nb system exhibits a eutectic transformation at 1595.2 degrees C, which results in Al3Nb (D0(22)) and Nb2Al (D8(b)) phases. This paper is concerned with the processing of this eutectic by directional solidification. Alloys were prepared by arc melting and directionally solidified in Bridgman-type equipment. The resulting samples were utilized to evaluate the solidification microstructure and morphology regarding the growth conditions. Eutectic microstructures obtained were regular with lamellar morphology. Variations of the growth rate showed that an increase in this parameter causes a decrease in the lamellar spacing. With further increase in the growth rate, eutectic cells were observed as a result of constitutional undercooling. Oxidation tests of eutectic microstructures showed that this alloy suffers severe microstructure instability, with growth kinetics of the oxide scale of linear type. This indicates that the Al in the Nb-Al eutectic alloy is insufficient to form protective oxide films, such as, alpha-Al2O3. Finally, the heat treatment in argon atmosphere showed that the eutectic alloy presents a high degree of microstructure stability at 1200 degrees C. (C) 2004 Elsevier B. V. All rights reserved.27541671E153E158Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq

The late Messinian erosional surface and the subsequent reflooding in the Mediterranean: New insights from the Melilla–Nador basin (Morocco)

International audienceNew field investigations in the Cape des Trois Fourches peninsula (northern Morocco) allow us to bring new insights on the late Messinian erosional surface and the overlying marine deposits. Our results show that the late Messinian erosional surface, characterized by an erosional plateau, is interrupted by an up to 70 m deep paleovalley, thought to result from a landslide. The minimum relative sea-level drop is 70 m, probably much more with regard to the indication that a large-scale landsliding has occurred. The post-erosional deposits (Cycle 3) comprise one major depositional sequence composed of two high-order sedimentary subcycles. Deposits of the first subcycle 3A filled in the erosional basin and the paleovalleys; deposits of the second subcycle 3B overflowed the paleovalleys and the erosional plateau. We found evidence of the transgressive systems tract of the deposits corresponding to the post-Messinian crisis reflooding, for which a latest Messinian onset can neither be proved nor entirely ruled out because the rare biostratigraphic data are not sufficiently conclusive. The uppermost part of the reflooding deposits (subcycle 3A) is shown to be definitely Pliocene in age based on the occurrence of Globorotalia puncticulata and the composition of the assemblage of pectinids

Modèle 3D Terre-Mer et nouvelles données chronostratigraphiques sur le bassin Oligo-Miocène de Castelsardo, Sardaigne

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