Precipitates and boundaries interaction in ferritic ODS steels

• Published in: Journal of nuclear materials 2016-04, Vol.472, p.118-126
• Main Authors: Sallez, Nicolas, Hatzoglou, Constantinos, Delabrouille, Fredéric, Sornin, Denis, Chaffron, Laurent, Blat-Yrieix, Martine, Radiguet, Bertrand, Pareige, Philippe, Donnadieu, Patricia, Bréchet, Yves
• Format: Article
• Language: English
• Subjects: Chemical Sciences; Dispersion hardening steels; Dissolution; Extrusion; Ferritic stainless steels; Grain boundaries; Material chemistry; Precipitates; Precipitation; Transmission electron microscopy
• ISSN: 0022-3115; 1873-4820
• DOI: 10.1016/j.jnucmat.2016.01.021

In the course of a recrystallization study of Oxide Dispersion Strengthened (ODS) ferritic steels during extrusion, particular interest was paid to the (GB) Grain Boundaries interaction with precipitates. Complementary and corresponding characterization experiments using Transmission Electron Microscopy (TEM), Energy Dispersive X-ray spectroscopy (EDX) and Atom Probe Tomography (APT) have been carried out on a voluntarily interrupted extrusion or extruded samples. Microscopic observations of Precipitate Free Zones (PFZ) and precipitates alignments suggest precipitate interaction with migrating GB involving dissolution and Oswald ripening of the precipitates. This is consistent with the local chemical information gathered by EDX and APT. This original mechanism for ODS steels is similar to what had been proposed in the late 80s for similar observation made on Ti alloys reinforced by nanosized yttrium oxides: An interaction mechanism between grain boundaries and precipitates involving a diffusion controlled process of precipitates dissolution at grain boundaries. It is believed that this mechanism can be of primary importance to explain the mechanical behaviour of such steels. •To study the microstructural evolution of a ferritic ODS steel during its extrusion, observations have been carried on samples resulting from a voluntarily interrupted extrusion and extruded materials.•A highly heterogeneous precipitate population have been observed. Nanosized coherent precipitates (2–5 nm) on both sides of the grain boundaries despite grain boundary migration after precipitation due to further thermo-mechanical processing as well as coarse precipitates (10–40 nm) alignments are observed on the grain boundaries and within the grains, parallel to the grain boundaries.•Asymmetrical PFZs can be observed around precipitates alignments and grain boundaries. Using TEM with EDX and APT we have been able to ensure that the PFZs are chemically depleted.•Based on the previous observations, a dissolution mechanism of the metastable yttrium precipitates at the grain boundaries is proposed.