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in oxide dispersion-strengthened (ODS) ferritic steels is being pursued with renewed interest as these are candidate materials for application in nuclear power plants.[1–3] ODS steels are usually synthesized through mechanical alloying (MA) followed by consolidation by hot extrusion and/or hot isostatic pressing.[1–3] With these consolidation techniques, full density is achieved after processing at very high temperatures [~1273 K (~1000 C)] for longer duration (>1 hour).[2,3] Processing at high temperatures leads to significant grain growth as well as coarsening of dispersoids present in these steels resulting in reduced strength and microstructural stability. It is therefore beneficial if the MA powders could be consolidated at lower temperatures. Recent studies have shown that spark plasma sintering (SPS) is quite effective in preventing grain growth while still obtaining ‡99 pct densification at much lower temperature, time, and pressure.[4,5] SPS is widely used for sintering ceramics owing to its faster heating rate and a shorter processing KARTHIKEYAN RAJAN, Master’s Student, VADLAMANI SUBRAMANYA SARMA, Associate Professor, and B.S. MURTY, Professor, are with the Department of Metallurgical and Materials Engineering, Indian Institute of Technology Madras, Chennai 600036, India. Contact e-mail: [email protected] THANGARAJU SHANMUGASUNDARAM, Post-Doctoral Fellow, is with the Institute for Applied Materials, Karlsruhe Institute of Technology, 76131 Karlsruhe, Germany. Manuscript submitted February 28, 2013. Article published online July 9, 2013 METALLURGICAL AND MATERIALS TRANSACTIONS A

time.[6–8] As a result of spark discharge, plasma is believed to be generated between the powder particles, and this contributes to the faster heat and mass transfer.[6–10] However, no direct experimental evidence for plasma generation has been reported so far, though the effectiveness of the SPS process has been proven beyond doubt.[4,11,12] Recently, an indirect evidence for plasma formation was postulated in the synthesis of diamond from carbon nanotube materials in Ar atmosphere.[13] The role of current on mass transport as the possible reason is being investigated for understanding the effectiveness of SPS processing vis a vis conventional sintering.[4] We have recently reported that bulk ultrafine-grained (UGC) ODS ferritic steels (with >99 pct density) could be synthesized through MA and SPS at 1073 K (800 C) and 30 MPa with sintering times shorter than 10 minutes.[5] Oxide dispersoids stabilize the microstructure through Zener pinning and contribute to the enhanced creep resistance of UFG ferritic steels.[1–3,5] However, the role of oxide dispersoids on sintering kinetics during conventional sintering as well as SPS is not well understood. In the current study, we studied the sintering mechanism(s) that control the densification and compared the experimental and calculated densification rates in a Fe-9Cr-1Mo ferritic steel with and without yttria dispersoids. The steel compositions analyzed in the current study are (wt pct) Fe-9C