Evolution of Oxide Particles during Fabrication Processes of 12Cr ODS Steel
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Evolution of Oxide Particles during Fabrication Processes of 12Cr ODS Steel Xiaodong Mao1,2, Tae Kyu Kim1, Sung-Soo Kim1, Chang Hee Han1 Kyu Hwan Oh2, Jinsung Jang1,* 1 Nuclear Materials Division, Korea Atomic Energy Research Institute, 989-111 Daedeok-daero Yuseong-gu, Daejeon 305-353, Republic of Korea 2 Department of Materials Science and Engineering, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 151-742, Republic of Korea ABSTRACT 12Cr ODS steel samples were prepared by mechanical alloying of the metal powders with 20-30 nm Y2O3 particles followed by isostatic pressing, hot rolling and final heat treatment. Evolutions of oxide particles such as YTaO4 and YCrO3 after each fabrication step were investigated by using TEM with EDS. Crystallographic correlation between oxide particles and the matrix was investigated in a HIPped sample, and interactions between dislocations and oxide particles were observed in hot rolled or heat treated sample. Size distributions of oxide particles were measured by carbon replica samples and it was found that coarsening of oxide particles from 9 to 12 nm occurred during hot rolling process. Additional isothermal annealing at 1250 ÛC revealed that phase transformation of oxide particles from monoclinic YTaO4 to face centered cubic Y3TaO7 was observed. INTRODUCTION Uniformly dispersed nano-sized oxide particles are considered to contribute significantly to the high temperature strength and the irradiation resistance of ODS alloys [1-2]. Such oxide particles act as pinning points to dislocation movement and stable sinks for irradiation induced defects [3-5]. The contribution of oxide particles to strength and irradiation resistance would be mainly determined by the particle size distribution and the inter-particle spacing. Finer oxide particles with higher number density would be beneficial. Processing parameters would play an important role in controlling the dispersion of the oxide particles, since the particles were found to have reprecipitated during the consolidation process [6] and coarsening of oxide particles may occur during the subsequent fabrication processes. Coarsening of Y-Al-O particles and transformation of YAM (Y4Al2O9), YAH (hexagonal, YAlO3) and YAP (perovskite, YAlO3) particles to YAG (garnet, Y3Al5O12) were reported in ODS alloys such as PM2000TM or MA60001TM after heat treatment [7,8]. This study focuses on the evolution of the oxide particles in 12Cr ODS steel at different fabrication processes: HIPing, hot rolling and heat treatment. Annealing at 1250 ÛC was also applied to investigate the stability of the fine oxide particles. EXPERIMENT 12Cr ferritic ODS steel was fabricated by mechanical alloying (MA) and hot isostatic pressing (HIP) processes. The chemical composition of the ODS steel is given in Table 1. 1
PM2000TM and MA6000TM are trademarks of Plansee and SMC, respectively
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Elemental metal powders with yttria powder of about 30 nm in size were mechanically alloyed by using a planetary ball mill (Fritsch, Pulverisette-5) at 200 rpm in an Ar
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