Mechanism of oxide particle evolution in the MA957 ODS ferritic alloy
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Mechanism of oxide particle evolution in the MA957 ODS ferritic alloy H. Sakasegawa1, F. Legendre1, L. Boulanger1 L. Chaffron2, T. Cozzika2, J. Malaplate2, J. Henry2, Y. de Carlan2, and M. Brocq1 1 CEA Saclay DEN/DMN/SRMP, 91191 Gif sur Yvette cedex, France. 2 CEA Saclay DEN/DMN/SRMA, 91191 Gif sur Yvette cedex, France. ABSTRACT In our past work, two types of nanometer-sized oxide particle were found in the commercial ODS (Oxide Dispersion Strengthened) ferritic alloy MA957. They were nonstoichiometric Y-, Ti-, O-enriched clusters (2 ~ 15 nm) and stoichiometric Y2Ti2O7 (15 ~ 35 nm). The particles which have an important role for the strengthening were confirmed to be nonstoichiometric Y-, Ti-, O-enriched clusters (denoted by YxTiyOz in this work). Based on that result, the stability and evolution mechanism of oxide particle at high temperatures were studied in this work. After annealing (1473 K x 1 h), most of the non-stoichiometric YxTiyOz particles were stable and their size increased very little. However, some these particles apparently became Y2Ti2O7 particles with increasing size and yttrium content. They possibly contributed to the growth of Y2Ti2O7. INTRODUCTION ODS alloys have superior creep properties. As is well known, these excellent creep properties result from very fine oxide particles dispersed in the matrix. However, there has been no common understanding about the nature of the very small oxide particles. In the literature, two hypotheses about the identification were seen, 1: non-stoichiometric Y-, Ti-, O enriched clusters and 2: stoichiometric Y2Ti2O7. In our past work [1], the chemically extracted residue method and extraction replica method were applied to the commercial ODS ferrtic alloy MA957. Then, observations using XRD (X-Ray Diffractometer) and FEG-STEM (Field Emission Gun - Scanning Transmission Electron Microscope) with EDS (Energy Dispersive X-Ray Spectrometer) system were performed. From these results, it was concluded that the composition of nano-metric particle is related to the particle size. The nanometer-sized particles consist of at least two types, 1: nonstoichiometric Y-, Ti-, O-enriched clusters (denoted by YxTiyOz, Y/Ti < 1) from ≈ 2 to ≈ 15 nm and 2: stoichiometric Y2Ti2O7 from ≈ 15 nm to ≈ 35 nm. The particles which have an important role for the strengthening were confirmed to YxTiyOz particles. In the present work, based on the past results, the stability and evolution mechanism of these oxide particles were studied. EXPERIMENTAL DETAILS The specimen used in this study was the commercial ODS ferritic alloy MA957 with nominal compositions of Fe-14Cr-0.3Mo-1.0Ti-0.25Y2O3 in wt%. Annealing was performed at 1473 K for 1 h to study stability and evolution mechanism of oxide particles at high
temperatures. As for microstructural observations, extraction replicas were prepared to precisely observe nanometer-sized precipitates, in addition to electro-polished thin films. A specimen was mechanically polished up to a mirror finish and then electrochemically etched in
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