Microstructure evolution during high temperature aging of a 35Ni-25Cr-Nb alloy Evolución microestructural durante el envejecimiento a alta temperatura de una aleación 35Ni-25Cr-Nb

LISSARRAGUE, MATÍAS SOSA; PRADO, FERNANDO; PICASSO, ALBERTO; LIMANDRI, SILVINA

REVISTA MATéRIA

UNIV FED RIO DE JANEIRO

Año: 2018 vol. 23

1517-7076

The dendritic-type microstructure of the 35Ni-25Cr-Nb alloy, in the as-cast condition, consists of an austenit-ic 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 pres-ence 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 pri-mary eutectic carbides and not to the transformation of the M7C3-type carbide to the M23C6-type.