From microscopic to atomistic scale: Temperature effect on yttria distribution in mechanically alloyed FeCrMnNiCo powder particles

Mechanical alloying (MA), the state-of-the-art processing step to produce oxide dispersion strengthened materials, shows a deficiency regarding time and costs hindering a broader applicability. Therefore, in order to investigate the effect of cryogenic MA temperatures and to understand the mechanism behind the refinement and dissolution of yttria, face-centered cubic FeCrMnNiCo powders are mechanically alloyed with yttria at room and cryogenic temperatures using a novel cryogenic attritor. Mechanically alloyed powders are thus analyzed using a comprehensive set of experimental methods. Transmission electron microscopy reveals a stronger decrease of the oxide particle size upon cryogenic MA while at both temperatures the hereby observed particles in a size over 10 nm still show yttria crystal structure. Nevertheless, a substantial amount of yttria is refined below 10 nm forming nanoclusters without detectable crystal structure. Positron annihilation spectroscopy suggests a vacancy assisted dissolution of yttria into these nanoclusters while detailed investigation of these nanoclusters by atom probe tomography suggests smaller clusters in the cryoalloyed sample. The results imply that this vacancy assisted dissolution seems to be enhanced at cryogenic temperatures as first principle calculations and a change of the chemical composition of the nanoclusters imply higher vacancy densities at cryogenic MA temperatures stabilizing smaller nanoclusters.