Key aspects of carbide precipitation during solidification in the Ni-base superalloy, MAR M002

Key differences have been shown to exist between the predicted solidification path using thermodynamic calculations compared with that determined experimentally using thermal analysis coupled with microscopy in the Ni-base superalloy, MAR M002. When nucleation undercooling is negligible, the measured liquidus temperature is significantly greater than that obtained from thermodynamic calculations. Primary solidification corresponds to freezing of γ-phase, and a near-constant freezing rate occurs during primary growth prior to nucleation and initial growth of MC carbides. This nucleation and growth occurs via a eutectic reaction, L → γ + MC during the latter stages of solidification, and freezing terminates through the four-phase eutectic reaction, L → γ + MC + γ′. There is a two-fold decrease in freezing rate during the terminal stages of solidification. The freezing sequence of the carbides is markedly different to that predicted by thermodynamic calculations, which instead predict growth of MC directly from the liquid before nucleation of γ-phase and also to the formation of borides, which were not experimentally observed.