High resolution energy dispersive spectroscopy mapping of planar defects in L12-containing Co-base superalloys

Abstract

Local chemical fluctuations in the vicinity of superlattice intrinsic stacking faults (SISFs) have been observed via high resolution energy dispersive X-ray spectroscopy (EDS) mapping in new single crystal Co- and CoNi-base superalloys containing γ′-(L12) precipitates. The SISFs were formed during high temperature tensile creep at 900°C. Chemical fluctuations were found to greatly influence the SISF energy, which was calculated from density functional theory in Co3(Al, Ta, W) compounds at 0K. The local SISF structure was found to be comprised of four D019 (0001) planes that were enriched in W and Ta, as revealed by high resolution scanning transmission electron microscopy (HRSTEM) imaging and EDS mapping. The precipitates were determined to accommodate up to 22% of the plastic deformation accrued during an interrupted creep test to 0.6% creep strain. The driving forces for segregation are discussed, and new models for shearing of the ordered precipitates are proposed.