Microstructural evolution of oxide-dispersion-strengthened Fe–Cr model steels during mechanical milling and subsequent hot pressing

Oxide-dispersion-strengthened (ODS) ferritic steels of Fe–9Cr–0.3Y2O3 and Fe–9Cr–0.2Ti–0.3Y2O3 (in mass) incorporating nanoscale oxide particles, were produced by mechanical milling (MM) followed by hot pressing (HP). Microstructural evolution of these two types of ODS steels were structurally characterized at each step of the elaboration processes by means of scanning electron microscope (SEM), transmission electron microscope (TEM), X-ray diffraction (XRD), and optical microscope. The observations of structure of the mixed powders and the nanoscale oxide particles in both ODS steels after MM indicate that the initial powders, coupled with the original yttria powders, get fractured by severe plastic deformation and ultrafine bcc grains (~20 nm) of the matrix and Y2O3 nanocrystals with irregular edges are formed during MM. The addition of titanium (Ti) promotes the refinement of bcc grains, Y2O3 nanocrystals and the formation of amorphous phase of Y2O3 during MM. TEM observations of these two Oxide-dispersion-strengthened (ODS) steels exhibit a very fine structure of micrometer-scale grains in which large number of nanoscale oxide particles are distributed after HP process. The observation of some unreinforced domains without the nanoscale oxide particles indicates that there still exist inhomogeneous areas, although the size of those oxide particles reaches nanoscale. Threshold stress of the HPped Fe–9Cr–0.2Ti–0.3Y2O3 steel with the relatively homogeneous dispersion was carefully evaluated on the basis of higher magnified images of the nanoscale oxide particles. Different values of threshold stress were obtained due to the various dispersions of the nanoscale oxide particles within different areas. That may be the reason why the threshold stress cannot be clearly obtained by the results of creep tests.