Selective evolution of secondary γ′ precipitation in a Ni-based single crystal superalloy both in the γ matrix and at the dislocation nodes

Two distinct evolutionary processes of secondary γ′ precipitates in a Ni-based single crystal superalloy were revealed by means of Cs-corrected high-resolution transmission electron microscopy (HRTEM) and corresponding X-ray energy dispersive spectroscopy analyses. Upon creep testing at elevated temperatures, it was determined that large quantities of secondary γ′ precipitates were formed. The precipitates were found to select two distinctive forms and routes: one randomly dispersed in the γ matrix (2nd γ′γ), and the other formed at nodes of dislocation networks (2nd γ′d) in regular arrays. These two types of secondary γ′ precipitates exhibited different evolutionary morphologies and mechanisms. The 2nd γ′γ precipitates progressively evolved in shape from spherical to cuboidal and then to butterfly-like shapes, and the mean size also increased gradually from 11 nm to 34 nm. The 2nd γ′d precipitates exhibited stable cuboidal morphology and a mean size of ∼30 nm. These differences are attributable to the different element transport conditions for the two types of precipitates, i.e., an uphill diffusion for the 2nd γ′γ precipitates and a dislocation network pipe transportation for the 2nd γ′d precipitates. The 2nd γ′γ precipitates show severe segregation of γ′-rich elements of Ni and Al and deficiency of γ′-poor elements of Cr compared with the 2nd γ′d precipitates. [Display omitted]