Phase transformation of η and σ phases in an experimental nickel-based superalloy

Both nodular σ phase and platelet η phase were found in the as-cast microstructure of an experimental nickel-based superalloy. The nodular σ phase decomposed into γ phase leaving serrated transformation interfaces between σ and γ during the heat treatment process. The formation mechanism of nodular σ phase and platelet η phase was investigated by analyzing the orientation relationship of σ/γ and η/γ. It was found that σ phase nucleated and grew from the liquid during solidification in the interdendritic area. The η phase had regular platelet morphology and definite orientation relationship with γ phase, which indicated that it was formed by means of solid-state precipitation.