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°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 oxidized 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.