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-0-1 5mass%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 bcc 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.