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.