Thermodynamic analysis of precipitation behavior of M23C6 carbide in Nimonic 105 superalloy

The Time-Temperature-Precipitation diagram of M23C6 carbide formation was established for a Nimonic 105 Ni-based superalloy by means of microstructural observation and mathematical-thermodynamic analysis. The results showed that during heat treatment at 750 degrees C, M23C6 carbide usually forms at grain boundaries due to the decomposition of MC carbides, indicating that the grain boundaries promote M23C6 carbide precipitation. Also, by increasing temperature or time, diffusion rate increases and new diffusional paths (such as y matrix and twinning) are provided leading to the nucleation of these phases inside the matrix by means of gamma -> M23C6 + gamma' reaction. According to thermodynamic analysis, the interactions between M23C6 formation Gibbs free energy and diffusion activation energy of elements, especially carbon, defines M23C6 precipitation behavior. The peak of the Time-Temperature-Precipitation diagram was calculated to be about 900 degrees C based on experimental results, and about 927 degrees C based on thermodynamic results. These two temperatures can be considered as approximately equal.