Planar defect formation in the γ' phase during high temperature creep in single crystal CoNi-base superalloys

The structure and formation mechanism of extended planar defects in the gamma/gamma' microstructure of creep deformed CoNi-base single crystal superalloys have been studied by conventional and advanced transmission electron microscopy (TEM). Planar defects in numerous isolated as well as contiguous gamma' precipitates on 011) planes reveal a characteristic configuration whereby superlattice intrinsic stacking faults (SISF) are fully embedded within antiphase boundaries (APB). Detailed analysis revealed that a leading 1/3[1 12] superpartial dislocation first creates an SISF. The SISF is then transformed into an APB by a trailing 1/6[112] partial dislocation. The partial is left inside the precipitate and remains as a dislocation loop. Thus, the entire shearing process constitutes a crystallographic slip of type 1/2[112]. A force balance analysis indicates that the initial APB energy exceeds the SISF energy. However, energy dispersive X-ray spectroscopy (EDXS) indicates pronounced local reordering and diffusion processes near both types of planar defects. The APB qualitatively adopts the composition of the gamma phase whereas the SISF locally changes its composition towards that of the Co3W D0(19) phase. We propose that these atomic diffusion processes determine the formation and shrinkage of the loops. A post mortem in situ TEM heating experiment shows that with increasing temperature the APBs exhibit complete faceting into 000) planes followed by coarsening, eventually leading to disintegration of the gamma' precipitate. This indicates a detrimental impact of APBs as potential nuclei for fragmentation of the gamma/gamma' microstructure in CoNi-base superalloys. (C) 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.