Ultra Grain Refining and Decomposition of Oxide during Super-heavy Deformation in Oxide Dispersion Ferritic Stainless Steel Powder

Mechanical milling using a high energy planetary ball mill was applied to the powder mixtures of iron, chromium and yttria (Y₂O₃) (Fe-24mass%Cr-0-15mass%Y₂O₃) to introduce a very large strain into the iron-base matrix, and microstructural changes during mechanical milling were investigated in relation to decomposition behavior of Y₂O₃ particles. Mechanical milling of more than 36 ks was long enough to allow the mechanical alloying of iron and chromium powders. After the milling of 36 ks, ultrafine bec crystalline grains of 10 to 20 nm were formed within Fe-24mass%Cr-15mass%Y₂O₃ powder mixture and 15 mass% of Y₂O₃ particles were almost decomposed. The resultant powder mixture markedly hardened to about 1 000 Hv. The decomposition of Y₂O₃ particles can be explained as being due to the formation of an amorphous grain boundary layer where yttrium and oxygen atoms are enriched. As a result, it is proposed that, for the dissolution of Y₂O₃, bcc crystalline grains should be refined to a nanometric size to provide a sufficient volume fraction of the grain boundary layer, and that Y₂O₃ particles should be crushed to several nanometers to produce the driving force for the decomposition of Y₂O₃ particles.