Study of Nano-cluster Formation in Fe-18Cr ODS Ferritic Steel by Atom Probe Tomography
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Study of Nano-cluster Formation in Fe-18Cr ODS Ferritic Steel by Atom Probe Tomography O. Kalokhtina1*, B. Radiguet1, Y. de Carlan2 and P. Pareige1 1 Groupe de Physique des Matériaux, UMR CNRS 6634, Université et Insa de Rouen Avenue de l’Université, 76801 Saint Etienne du Rouvray, France 2 CEA Saclay, Service de Recherches de Métallurgiques Appliquées, 91191 Gif sur Yvette CEDEX, France. ABSTRACT A high chromium ferritic Oxide Dispersion Strengthened steel was produced by mechanical alloying of Fe-18Cr-1W-0.3Ti-0.3Ni-0.15Si and 0.5% Y2O3 (wt.%) powders in industrial attritor, followed by hot extrusion at 1100°C. The material was characterized by Atom Probe Tomography on each step of manufacturing process: as-milled powder and in final hot extruded state. In addition, to get information on clustering kinetics the powder was also characterized after annealing at 850°C during 1 hour. Atom Probe Tomography revealed that the oxide dispersion strengthened steel Fe-18Cr contains nanometer scale yttrium- and oxygenenriched nanoclusters in as-milled state. Their evolution is shown after subsequent annealing and hot extrusion. More well defined nanophases also enriched in Ti are observed. A mechanism of their formation is proposed. Mechanical alloying results in supersaturated solid solution with presence of small Y- and O-enriched clusters. Subsequent annealing stimulates incorporation of Ti to the nuclei that were previously formed during mechanical alloying. INTRODUCTION Many research programs are devoted to improve existing materials and to develop new ones that will be able to meet the different challenges for new Generation IV (GEN IV) and fusion nuclear systems. Fe-based alloys with a body-centered-cubic (bcc) structure are known to be highly resistant to the influence of radiation effects, compared to austenitic alloys (Fe-based alloys with face-centered cubic structure) [1]. In addition, high chromium levels are known to provide good corrosion resistance. One of the main properties which moderates the use of ferritic–martensitic F/M steels of the Fe–9/18Cr type is their creep resistance. To extend their range of operating temperatures, Oxide-Dispersion-Strengthened (ODS) ferritic and martensitic steels were developed [2,3]. Due to the presence of a high density (~1023 to ~1024 m-3) of nanoclusters (1-10 nm in diameter) dispersed in the Fe-Cr matrix, these materials exhibit a good resistance to swelling under irradiation [4,5], and better creep properties [6,7] at high temperatures. In order to obtain such microstructures, ODS steels are usually produced by complex process including mechanical alloying (MA) and thermo-mechanical treatments. MA enables the production of metal systems in which insoluble non-metallics compounds such as oxides, carbides, nitrides, silicides can be uniformly dispersed throughout the metal matrix [2]. In the case of ODS, it is widely accepted that precipitation of Y-Ti-O rich nanophases appears during hot extrusion or hot isostatic pressing (HIP) processes while mechanical mi
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