Combination of back stress strengthening and Orowan strengthening in bimodal structured Fe-9Cr-Al ODS steel with high Al addition

Oxide dispersion strengthened (ODS) steels of Fe-9Cr-xAl (x = 0.4, 2.5, 4.5 and 8.0 wt%) have been fabricated by mechanical alloying, hot isostatic pressing and subsequent forging. Both the ferrite matrix and yttria particles were coarsened by the addition of Al. However, a bimodal structure consisting of coarse grains closely surrounded by fine grains is formed, and develops with increasing Al addition. Moreover, at the same time, nano-sized alumina particles precipitate in the interior of the bimodal ferrite grains. The tensile strength increases gradually with Al addition due to back stress strengthening combined with Orowan strengthening. The formation mechanism of the bimodal structure and intragranular precipitation of the Al added ODS steels were also discussed.