Development and comparison of processing routes for the manufacture of oxide dispersion strengthened steels

• Main Author: Hong, Zuliang
• Other Authors: Grant, Patrick S. ; Zhang, Hongtao
• Published: University of Oxford 2017
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.729238

Ferritic oxide dispersion strengthened (ODS) steels reinforced by evenly and densely distributed Y, Ti and O enriched nano-sized precipitates (NP) have emerged as one of the most promising structural materials candidates for next generation (Generation IV) nuclear fission and future nuclear fusion concepts, since they can both withstand more intensive neutron irradiation, and maintain strength and creep properties at higher operating temperatures than current standards. The conventional processing route for ODS steels is a two-step powder metallurgy process, consisting of mechanical alloying (MA) of an Fe based matrix and a normally nano-sized Y₂O₃ powder followed by consolidation into bulk. However, major disadvantages such as prolonged processing time, small batch size, tendency for contamination and high inherent cost of feedstock powders have prevented industrial application of the MA based route. This thesis has explored two alternative processing routes for ODS steels. In the first approach, Y₂O₃ was replaced with Fe₃Y during MA, followed by hot isostatic pressing (HIP) consolidation. The use of Fe₃Y did not disrupt the standard MA process for powders, while the consolidated alloy possessed a narrower grain size distribution, higher room temperature ductility and slightly better high temperature microhardness compared with a reference alloy produced using Y₂O₃. In the second approach, the whole MA process was replaced with melt spinning of an Fe-5Y or Fe-1Y-1Ti (wt%) alloy. Relatively uniform spatial distribution of Y and Ti was achieved in the melt spun ribbons. The process demonstrated consistent yield (> 60% by weight), fast processing time (< 10 s), good scalability (up to > 100 g feedstock material) and repeatability across equipment in three different institutes. Internal oxidation investigations also validated the potential of forming < 20 nm Y oxides in the ribbons. The Fe-1Y-1Ti melt spun ribbon was then pulverised and consolidated into bulk by field assisted sintering (FAST). Simultaneous internal oxidation was achieved with the addition of nano-sized Fe₃O₄ powder, and successfully generated a high number density of sub-micron Y and/or Ti enriched oxides. Further formation of oxides took place during annealing, resulting in an approximate 20% increase in hardness at temperatures below 300 °C. However, the use of Fe₃O₄ powder reduced sinterability of the ribbons and undermined density of the alloy, which lead to mechanically incoherent prior ribbon boundaries and a significant property drop above 400 °C.