Precipitation of the gamma prime phase in a Ni-co-based superalloy during different stages of cooling

Abstract

The cooling precipitation of the gamma primary (γ') phase in a precipitation strengthened Ni-Co-base disk superalloy was investigated by various experimental and analytical methods. The alloy was super-solvus heat treated and then slow cooled to different interrupted temperatures (IT) followed by instant water quenching. It shows that with the IT change, the γ' phase precipitated in different characteristics, such as morphologies, sizes and distributions. Through using three-dimensional atomic probe (3DAP) technique to study the γ and γ' compositions and their interface composition profiles, three types of γ' precipitates, entitled as quenching γ', primary cooling γ' and secondary cooling γ', were identified and their respective precipitation conditions and mechanisms were also discussed. According to the composition gradient across the γ/γ' interfaces and the γ' composition features, the separation mode of these γ' was determined to via the classical nucleation and growth mechanism. In addition, the effects of the γ' forming elements on the nucleation and growth processes of the different generations of γ' were evaluated. It indicates that the diffusion of Ti dominated the primary nucleation while that of Al reigned the following growth and the secondary nucleation.