Development of a High-Strength Ultrafine-Grained Ferritic Steel Nanocomposite
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THE superior void swelling resistance and high temperature strength of oxide-dispersion-strengthened (ODS) ferritic steels as compared with the matrix alloy have made them prospective structural materials for the elevated temperature applications in future nuclear power plants.[1,2] The commonly used route to produce the ODS materials is mechanical alloying followed by the hot deformation process, which can result in finely dispersed complex oxide with 2-nm to 4-nm diameter size enriched with Ti, Y, and O. Thermal studies of the ODS steels have shown that these nanoclusters are stable during long-term creep tests even up to 1173 K (900 C) and mainly precipitate on the grain boundaries during heat treating.[3,4] Hence, it appears that these precipitates can act as barriers to grain growth according to Zener pinning, leading to highly stable ultrafine-grained microstructure.[5,6] At elevated temperatures, the ultrafine grains usually degrade in strength because of the shortened path for the diffusion of atoms that encourages the diffusional creep, but the ROOHOLLAH RAHMANIFARD, Assistant Professor, is with the School of Emerging Sciences and Technologies, Iran University of Science and Technology, Tehran, Iran. Contact e-mail: rahmanifard@ iust.ac.ir HASAN FARHANGI, Associate Professor, is with the School of Metallurgy and Materials Engineering, College of Engineering, University of Tehran, Tehran, Iran. ABDUL JAVAD NOVINROOZ, Associate Professor, is with the Physics Group, Science Department, Takestan Branch, Islamic Azad University, Takestan, Iran. SAMIRA MONIRI, Researcher, is with the Department of Physics, Karaj Branch, Islamic Azad University, Karaj, Iran. Manuscript submitted February 25, 2012. Article published online October 17, 2012 990—VOLUME 44A, FEBRUARY 2013
ultrafine-grained ODS steels have exhibited excellent creep resistance. The inhibition of Fe self-diffusion and the nanoclusters formation together with Cr and W segregation on the grain boundaries are the reasons that can slow down creep rate dramatically.[6,7] It has been found that 12YWT and 14YWT are the best interests of the ultrafine-grained ferritic ODS steels.[5–11] Beside the very good creep resistance, these materials show higher tensile properties when compared with other developed ODS steels throughout the tensile testing temperatures. However, 12YWT has an improper ductile-to-brittle transition temperature (DBTT) of about 373 K (100 C) while 14YWT has the DBTT of about 123 K (150 C). 14YWT chemically contains a higher yittria and Cr content with respect to 12YWT. In spite of reports given by Olier et al.[12] and Hadraba et al.[13] on the deterioration of impact properties due to the increase of yittria content from 0.3 to 0.5 pct, the oxide particles can promote the ultrafine-grained structure as mentioned above, which may result in the improved impact properties. However, since the difference of yittria content between 14YWT and 12YWT is small (0.05 pct), it cannot significantly affect the impact behavior. Additionally, an increase in t
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