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

Abstract

The Time–Temperature–Precipitation diagram ofM₂₃C₆carbide 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 °C,M₂₃C₆carbide usually forms at grain boundaries due to the decomposition ofMC carbides, indicating that the grain boundaries promoteM₂₃C₆carbide precipitation. Also, by increasing temperature or time, diffusion rate increases and new diffusional paths (such as γ matrix and twinning) are provided leading to the nucleation of these phases inside the matrix by means of γ→M₂₃C₆+ γ′ reaction. According to thermodynamic analysis, the interactions betweenM₂₃C₆formation Gibbs free energy and diffusion activation energy of elements, especially carbon, definesM₂₃C₆precipitation behavior. The peak of the Time–Temperature–Precipitation diagram was calculated to be about 900 °C based on experimental results, and about 927 °C based on thermodynamic results. These two temperatures can be considered as approximately equal.