Secondary carbide precipitation in a directionally solified cobalt-base superalloy

Abstract

A directionally solidified cobalt-base alloy, DZ40M, was solidified with a columnar grained austenitic matrix with a 〈001〉 preferential orientation and primary carbides of chromium-rich M7C3 and MC at grain boundaries and interdendritically. Secondary carbides in DZ40M alloy are chromium-rich M23C6 and tungsten-rich M6C. The M23C6 carbide has a cube-cube orientation relationship with the austenitic matrix. Initial precipitation of secondary carbide, M23C6, occurred on dislocations in the austenitic matrix of the as-cast DZ40M alloy during cooling. Aging treatment (100 to 1000 hours at 850°C) produced a profusive precipitation of the M23C6 carbide mainly around the primary carbides. In the interior of grains, M23C6 precipitated preferentially on dislocations and stacking faults. Subsequently, M23C6 grew into laths near the primary carbides and coalesced into chains. The precipitation behavior of M23C6 can be explained by the following reaction: 23M+6C→M23C6. The primary carbides are a carbon reservoir for the precipitation of M23C6. The M6C carbide was found only on the surface of the primary M7C3 carbide adjacent to tungsten-rich MC in the aged condition. The precipitation of the tungsten-rich M6C is atributed to the tungsten segregation, which resulted from decomposition of the tungsten-rich MC and good lattice match between M6C and M7C3. The inhomogeneity of secondary precipitation is due to the uneven distribution of alloying elements.