Decomposition of primary MC carbide and its effects on the fracture behaviors of a cast Ni-base superalloy

Primary MC degeneration in long-term thermally exposed cast Ni-base superalloy K452 was investigated in detail. It is shown that the primary MC degeneration is a diffusion-controlled process where various transformation products are sequentially present within the reaction region during exposure. The primary MC degeneration produces a DM or SM microstructure on the MC/γ interface at the initiation of exposure, which hinders the diffusion of alloying elements via its ordered γ′ layers and leads to the presence of the η and α-(W, Mo) phases, respectively during the intermediate and late exposure. In addition, the primary MC deterioration is examined to be detrimental to the long-term thermally exposed alloy due to its contribution to the cracking incidence especially when the MC is present at the grain boundaries.