Effect of thermal exposure on the stability of carbides in Ni-Cr-W based superalloy

The microstructure evolutions of Ni-Cr-W based superalloy during thermal exposure have been investigated systematically. M₆C carbides in the alloy decompose into M₂₃C₆ carbides at temperatures from 650 to 1000°C due to its high content of Cr. The M₆C carbides decompose dramatically from 800 to 900°C. At temperatures up to 1000°C, a few M₂₃C₆ carbides form on the surface of M₆C carbides. The decomposition behavior of primary M₆C can be explained by the following reaction: M₆C→M₂₃C₆+Me (W, Ni, Cr, Mo). At temperatures below 900°C, coarse lamellar M₂₃C₆ carbides precipitate at the grain boundaries. The carbide lamellae line almost perpendicular to the grain boundaries. While the temperature is above 1000°C, discrete M₂₃C₆ carbides precipitate at the grain boundaries. Moreover, there are lots of small M₂₃C₆ particles precipitated around M₆C carbides from 650 to 1000°C.