Influence of heat treatments on microstructure of single crystal Ni ₃Al-base superalloy

An observation was made on the microstructure of a single crystal Ni ₃Al-base superalloy IC21 at as-cast condition and various heat treatment conditions using optical microscope (OM) and scanning electron microscopy (SEM). The microstructure of the alloy after heat treatment under different solid-solution temperatures and various aging temperatures was investigated. The microstructure observations and analysis indicated that the as-cast alloy consisted of γ', γ and the white phase which was block like in the interdendritic regions. The size of γ' phase in the dendrite region was larger than that in the interdendritic region. The incipient melting temperature of IC21 alloy was about 1345°C and the heat treatment window was 65°C. The solid-solution temperature of the γ' phase in the interdendritic regions was lower than that in the dendrite regions. The higher the solid-solution treatment temperature, the homogeneous the size of γ' precipitate. The size, morphology and the volume fraction of γ' precipitate after complete heat treatment were all connected to the primary aging temperature. With the increase of primary aging temperature, the size and the volume fraction of γ' precipitate increase, the morphology of the γ' precipitate gradually changed from near-spherical to cuboidal and then to elongate shape. Appropriate microstructural feature was achieved through the 1320°C/10 h/aac+1060°C/2 h/ac+870°C/32 h/ac treatment.