Formation of Nanocale Oxide Particles and Evaluation of Threshold Stress of Fe-9Cr-0.2Ti-0.3Y_2O_3 Nano-structured Ferritic Alloy

摘要

Nanostructured ferritic alloys (NFA) of Fe-9Cr-0.2Ti-0.3Y_2O_3 (in mass) incorporating nanoscale oxide particles, was produced by mechanical milling (MM) followed by hot pressing (HP). The formation process of the nanoscale oxide particles were structurally characterized at each step of the elaboration processes by means of transmission electron microscope (TEM). The observations of structure of the mixed powders and the nanoscale oxide particles in the NFA after MM indicated 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 formation of amorphous phase of [YMO]_(amorphous) during MM. Microstructure of the NFA observed by TEM exhibited a very fine structure of nanostructured grains in which large number of nanoscale oxide particles were distributed after HP process. HRTEM observations of partially and fully crystallized oxide nanoparticles in the matrix of NFA gain an insight into the formation mechanism of nanoscale oxide particles in HP process. The formation process of nanoscale oxide particles consist of the reaction between Y2O3 fragments and titanium (Ti) and crystallization of [YMO]_(amorphous).Threshold stress which can quantitatively shows the effect of dispersion strengthening 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.