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 gamma' 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 gamma' precipitates were formed. The precipitates were found to select two distinctive forms and routes: one randomly dispersed in the gamma matrix (2nd gamma'(gamma)), and the other formed at nodes of dislocation networks (2nd gamma'(d)) in regular arrays. These two types of secondary gamma' precipitates exhibited different evolutionary morphologies and mechanisms. The 2nd gamma'(gamma) 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 gamma'(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 gamma'(gamma) precipitates and a dislocation network pipe transportation for the 2nd gamma'(d) precipitates. The 2nd gamma'(gamma) precipitates show severe segregation of gamma'-rich elements of Ni and Al and deficiency of gamma'-poor elements of Cr compared with the 2nd gamma'(d) precipitates. (C) 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.