Mechanical Alloying of Lanthana-Bearing Nanostructured Ferritic Steels

A novel nanostructured ferritic steel powder with the nominal composition Fe-14Cr-1Ti-0.3Mo-0.5La₂O₃ (wt.%) was developed via high energy ball milling. La₂O₃ was added to this alloy instead of the traditionally used Y₂O₃. The effects of varying the ball milling parameters, such as milling time, steel ball size and ball to powder ratio, on the mechanical properties and microstructural characteristics of the as-milled powder were investigated. Nanocrystallites of a body-centered cubic ferritic solid solution matrix with a mean size of approximately 20 nm were observed by transmission electron microscopy. Nanoscale characterization of the as-milled powder by local electrode atom probe tomography revealed the formation of Cr-Ti-La-O-enriched nanoclusters during mechanical alloying. The Cr:Ti:La:O ratio is considered "non-stoichiometric". The average size (radius) of the nanoclusters was about 1 nm, with number density of 3.7 × 10²⁴ m^-3. The mechanism for formation of nanoclusters in the as-milled powder is discussed. La₂O₃ appears to be a promising alternative rare earth oxide for future nanostructured ferritic steels.