Grain-boundary segregation in an N18 nickel-based superalloy.

3D reconstruction of a small volume (15x15x29 nm3) that was analyzed in a polycrystalline nickel base superalloy N18 developed for jet turbines. Only boron (red dots) and Al (green dots) are represented. Other elements (e.g Ni) are omitted for clarity. This image reveals the grain-boundary (GB) segregation of boron atoms. The presence of (001) planes (spacing close to 0.35 nm) only at the left side of the GB exhibits the misorientation between grains.

The thickness of the boron-enriched layer at the GB is close to 2 nm. Such superalloys derive their excellent creep properties in turbine applications from the presence of a high volume fraction (~50%) of ordered Ni3Al-type precipitates. This reconstruction exhibits the presence of a small Al-enriched-Ni3Al γ’ precipitate on the left side of the GB. Al-enriched (001) planes alternate with Al-depleted planes (Ni-enriched, Ni is omitted for clarity).

This stacking sequence of (001) planes is typical of the L12 ordered structure of Ni3Al-type precipitates. A small part of a much larger Al-enriched γ’ particle appears on the right side of the GB. Grey envelopes are Al concentration iso-surfaces (CAl = constant)) that show interfaces between precipitates and the surrounding Al-depleted parent phase.

Download Larger File (JPEG, 432KB).
Download the whole document (MSWord, 460KB).

E. Cadel, D. Lemarchand, S. Chambreland, D. Blavette, Acta Mater. 50 (2002) 957-966
D. Lemarchand, E. Cadel, S. Chambreland, D. Blavette, Phil. Mag. A vol. 82-9 (2002) pp1651-1669