Primary MC decomposition and its effects on the rupture behaviors in hot-corrosion resistant Ni-based superalloy K444

The mechanism of primary MC decomposition and its influence on the rupture behaviors in the hot-corrosion resistant superalloy K444 are investigated by the combination of scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results reveal that the blocky primary MC mainly distributes at interdendritic regions and grain boundaries after the standard heat treatment. During long-term thermal exposure, primary MC decomposes gradually, which can be summarized into three stages. Firstly, the primary MC reacts with the γ matrix and produces the M23C6 particles and γ′ film, which can be described as MC + γ → M23C6 + γ′. In the second stage, the un-decomposed primary MC arrests the Ni element and forms the M6C particles and η phase, which can be expressed as MC + γ → M6C + η. In the third stage, the remaining primary MC reacts with the diffused Ni element and forms the η phase with M6C and M23C6 particles inside, which can be described as MC + γ → M6C + M23C6 + η. The reaction of the second and third stages should be ascribed to the high (Ti + Nb + Hf)/Al ratio and segregated W, Mo and Cr element in the decomposed region. In addition, the platelet-like σ phase is found in the third stage. The primary MC decomposition has different effects on the rupture behaviors between tensile and stress-rupture test.

► Primary MC decomposed during long-term thermal exposure.
► The reaction of MC + γ → M23C6 + γ′ occurred in the first decomposition stage.
► The reaction of MC + γ → M6C + η happened in the second stage.
► Lastly, the MC + γ → M6C + M23C6 + η occurred and promote the formation of σ phase.
► The primary MC decomposition has different effects on tensile and stress rupture behavior.