Evolución microestructural durante el envejecimiento a alta temperatura de una aleación 35Ni-25Cr-Nb

AUTOR(ES)
FONTE

Matéria (Rio J.)

DATA DE PUBLICAÇÃO

19/07/2018

RESUMO

ABSTRACT The dendritic-type microstructure of the 35Ni-25Cr-Nb alloy, in the as-cast condition, consists of an austenitic matrix and two types of eutectic primary carbides; Nb-rich MC type and Cr-rich M23C6 type both presents in interdendritic edges and grain boundaries. During aging at 800ºC for different times, M23C6 type secondary carbides precipitate in the matrix while MC carbides would transform to a Ni-Nb silicide, known as G phase (Ni16Nb6Si7). The microstructure was characterized by optical microscopy, scanning electron microscopy (SEM) with mapping, electron backscatter diffraction (EBSD) and dilatometry, in order to detect the presence of this undesirable phase and analyze the conditions under which it is presented. Silicon was found on edges of primary Nb-rich carbides. This could be indicating that the transformation of this carbide towards silicide rich in Ni and Nb, is in an incipient stage. Apparently, the transformation could be taking place from the outside to the inside of the Nb carbide. In addition, the contraction observed in the dilatometry curve would be associated to the difference on the thermal expansion coefficients between the matrix and the primary eutectic carbides and not to the transformation of the M7C3-type carbide to the M23C6-type.

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