Abstract
The relationship between degeneration of the primary M7 C3 carbide and precipitation of the secondary M23 C6 carbide has been investigated in a cobalt base superalloy after 100 h aging at 850°C. The results indicated that the primary M7 C3 carbide could not transform in situ into the secondary M23 C6 carbide. The M23 C6 formed by a direct reaction between chromium and carbon atoms. The dissolution of M7 C3 provided the precipitation of M23 C6 with essential carbon, which acted as a carbon reservoir. Near M7 C3, the chromium content is a controlling factor of M23 C6 precipitation, while away from it, the carbon content dominated the reaction. The precipitate free zone around M7 C3 is attributed to chromium depletion which is inherited from the as cast condition.