Sub-nanometre elemental analysis of Cu cluster in Fe-Cu-Ni alloy using aberration corrected STEM-EDS.

In order to understand the mechanism of Cu clustering and Ni effects on Cu clusters, a sub-nano scale structural and elemental analysis of Cu-bearing steels was carried out through an aberration corrected scanning transmission electron microscope. Based on systematic observation condition adjustment by changing the electron scattered angle, we adopted an inner scattered semi angle of between 50 and 70 mrad for the scanning transmission electron microscope energy dispersive X-ray spectroscopy analysis. Under this condition, Cu precipitates such as 9R are clearly recognized through atomic scale scanning transmission electron microscope annular dark field images. Cu clusters, before transforming to 9R, are detected on the dislocation and have a periodical strain field. Ni enrichment is also observed in the vicinity of Cu clusters. Considering these enrichment phenomena of Cu and Ni, it is found that Cu diffuses rapidly through dislocation, while at the same time, Ni enrichment occurs by ejection from Cu clusters and forms a shell structure.