Carbide behaviour during high temperature low cycle fatigue in a cobalt-base superalloy

The carbide behaviour of a directionally solidified cobalt-base superalloy has been investigated after low cycle fatigue at 900 degrees C. During fatigue, primary carbides, M7C3 and MC, decomposed sluggishly and a great amount of secondary carbide, chromium-rich M23C6 precipitated. The inhomogeneous distribution of M23C6 brought about a different dislocation substructure. In the vicinity of the primary carbides, densely-distributed fine M23C6 pinned up dislocations effectively, resulting in a uniform distribution of dislocations, while in the interior of grains, since precipitates were coarse and scarce, dislocations were arranged in a planar array and piled up in the front of the precipitates. M23C6 also acted as an obstacle deflecting fatigue crack. Primary carbides on the surface of specimens were oxidizied preferentially, causing a precipitate depletion around them. The oxidized primary carbides were crack initiation sites. The primary carbides hindered fatigue crack propagation, causing the formation of shear steps. (C) 1999 Kluwer Academic Publishers.