Reactive ball-milling is a promising technique for producing nanocomposite materials. Indeed, when appropriate reactants and conditions are chosen, ball-milling can involve a chemical reaction which usually promotes nano-structuration. However, it is difficult to maintain the metastable nanostructure once it is formed. Indeed during ball-milling, takes place a competition between mechanically forced processes which disorder the system and thermal activation which tends to bring it back to the equilibrium state. Up to now it is not possible to predict how to control the balance between both. So we study this issue on a specific nanocomposite material.

The Fe-Y₂O₃ system was chosen because of its interest for ODS (Oxide Dispersion strengthened) steels studied for future nuclear power plant. We synthesized the Fe-38%atY₂O₃ metal-oxide nanocomposite by ball-milling YFe₃ and Fe₂O₃ in the stoichiometric proportions of the following chemical reaction: 2YFe₃ + Fe₂O₃ à 8Fe + Y₂O₃. Then the material was briefly annealed. The material was characterized at different steps by X-Ray Diffraction, Mössbauer Spectroscopy and Atom Probe Tomography.

We observe that, after few hours of ball-milling which mixes the reactants, the desired chemical reaction propagates but unexpectedly quickly and violently, leading to an unwanted coarse microstructure composed of two phases, α-iron and Y₂O₃. Afterwards, ball-milling gradually refines the microstructure while changing the phase composition and leads to the formation of two nanometric interconnected phases: α-iron super-saturated in yttrium and oxygen, and an yttrium and oxygen rich phase, with a ratio of oxygen over yttrium close to 1. After annealing, phase composition changes while phases coarsen but slightly enough to stay smaller than 100 nm.

Finally, we will explain the mechanism of formation with the concept of competition between thermal and forced processes. We will also identify the processing parameters which control it.

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