Microstructures and mechanical properties in ultra fine-grained oxide-dispersion ferritic stainless steels

Microstructures and mechanical properties were investigated in ultra fine-grained oxide-dispersion steels with 24 mass% Cr ferrite matrix fabricated from mechanical milled powders. Powder mixtures of Fe, Cr and Y₂O₃ were mechanically milled in an argon gas atmosphere and then consolidated at around 1100 K. After 720 ks mechanical milling (MM), nanocrystalline ferrite grain structures were formed within powder particles. As for Y₂O₃ particles, it was observed that they were not only crushed to several nanometers but also decomposed during MM, and then oxides like Y₂O₃ and YCrO₃ finely precipitated in ultra fine grained structure on annealing at around 1200 K. Such ultra fine oxide particles effectively retarded the growth of matrix grains. The sample with 1 vol.% Y₂O₃ addition maintained ultra fine ferrite grains of 0.4 μm even after the annealing of 1573 K-3.6 ks by the Zener pinning effect of the oxide particles of about 10 nm in diameter. Tensile strength of the bulk samples markedly rose as the ferrite grain size became smaller in the grain size range below 1 μm and reached 1.93 GPa for the bulk sample with 1 vol.% Y₂O₃ addition and with the smallest mean grain size of 0.16 μm.