Carbide behavior during high temperature creep in DZ40M Co-base superalloy

The carbide behavior of a directionally solidified Co-base superalloy DZ40M has been investigated after creep at 700°C and 900°C. During the high temperature creep, a great amount of secondary carbide, M₂₃C₆ precipitated in alloy matrix. At 700°C, the M₂₃C₆ precipitation occurred in slip bands, indicating that it was stress-induced, while at 900°C, M₂₃C₆ formed surrounding the primary carbides, which is a characteristic of thermally aged alloy. The M₂₃C₆ particles pinned up dislocations and subgrain boundaries, strengthening the alloy matrix effectively. The creep crack initiation of DZ40M alloy was exclusively related to the primary carbides. The fracture of the primary carbide and the crack at the interface between them and matrix resulted in the crack formation. Furthermore, it was found that at 900°C, the surface of specimens was oxidized severely and the preferentially oxidized primary carbides acted as crack initiation sites.