Precipitation of Y-rich nano-oxides in FeCr alloy: An ion beam synthesis study

Y-rich nano-oxide precipitates were successfully produced by ion beam synthesis in high purity Fe–10%Cr. After annealing at 1100 °C of Y and O ion-implanted FeCr, a very high density (>1023 m−3) of small nano-oxides that are fairly uniform in size is achieved. A careful analysis of the structure and composition of these precipitates using HRTEM, STEM-HAADF (scanning transmission electron microscopy coupled with high-angle annular dark-field), and STEM-EDX (scanning transmission electron microscopy coupled with energy dispersive x-ray spectroscopy) demonstrates characteristics that are similar to those of the nano-oxides in non-Ti-doped oxide dispersion strengthened (ODS) steels. Both cubic and monoclinic Y2O3 precipitates are identified, and a Cr-rich shell has begun to form around them. In contrast to precipitates with a cubic structure, those with a monoclinic structure are observed to exhibit large distortions. Comparison of the precipitate characteristics with those from a previous study involving sequential Y, Ti, and O ion implantation into the same material indicates that the additional presence of Ti substantially reduces the size of the precipitates, consistent with observations in conventionally produced ODS steels, and stabilizes the cubic structure of yttria at the expense of the monoclinic structure. The present ion beam synthesis investigation demonstrates that the mechanism of Ti-stimulated precipitate refinement is not related to the specific precipitating structure because Ti addition promotes smaller Y–Ti precipitates as compared to pure Y2O3 ones even when these Y–Ti oxide phases have crystallographic structure and lattice parameters matching those of pure cubic yttria.