Microstructure Evolution in Nano-reinforced Ferritic Steel Processed By Mechanical Alloying and Spark Plasma Sintering
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HIGH-CHROMIUM ferritic oxide-dispersion strengthened (ODS) steels displayed considerable interest for high-temperature applications.[1–3] These materials exhibit excellent creep properties due to a specific microstructure reinforced by a very fine and homogeneous dispersion of nano-oxides. This requires the use of powder metallurgy where yttria particles and prealloyed ferritic powder are mixed together. Then, hot isostatic pressing (HIP) and/or hot extrusion (HE) are conventional processes to consolidate the materials.[4] Recently, spark plasma sintering (SPS) has emerged as a novel tool to process nanostructured materials.[5–8] This technique has been assessed for mechanically alloyed
XAVIER BOULNAT, Ph.D. Student, and YANN DE CARLAN, Researcher, are with the Service de Recherches Me´tallurgiques Applique´es, CEA, DEN, 91191 Gif-sur-Yvette, France. Contact e-mail: [email protected] MICHEL PEREZ, Professor, DAMIEN FABREGUE, Associate Professor, and THIERRY DOUILLARD, Researcher, are with the Universite´ de Lyon, INSA-Lyon, MATEIS UMR CNRS 5510, 69621 Villeurbanne, France. MARIE-HE´LE`NE MATHON, Researcher, is with the Laboratoire Le´on Brillouin, CEA-CNRS, CEA/Saclay, 91191 Gif-sur-Yvette, France. Manuscript submitted July 10, 2013. Article published online November 9, 2013 METALLURGICAL AND MATERIALS TRANSACTIONS A
ODS ferritic steels, providing dense materials with a fine and heterogeneous microstructure.[9] The appearance of bimodal grain structure composed of ultrafined grains with coarser grains has been referred to enhance ductility when comparing to monomodal nanostructured materials.[9–13] Srinivasarao et al.[14] tailored the nanosized to coarse grains ratio to obtain excellent compromise between tensile strength and elongation with a milled iron powder consolidated by SPS. Austenitic and martensitic steels also show heterogeneous microstructures after SPS consolidation, depending upon the use of dry or wet milling process.[15] Other metallic materials such as Fe-Mo,[16] Mg-Mn-Zr[17] showed similar properties. As these materials are produced by powder metallurgy, the link between the initial nanostructure and the resulting microstructural features and mechanical properties of the final compacts needs to be better understood. More specifically, ODS steels involve secondphase particles that are known to drastically influence either the subgrain or the grain growth.[18–21] Heterogeneous grain structure is often reported in ODS steels but such structure is tough to tailor and make reproducible. One of the main reasons is that the whole precipitation state in ODS steels depends upon numerous factors among which the consolidation parameters and the content of yttrium and titanium. Many studies were focused on nanosized particles,[22,23] rarely with an VOLUME 45A, MARCH 2014—1485
overall view of the precipitation state in the alloy. This is why the influence of precipitation on grain growth has been scarcely studied. Therefore, one needs to understand both the role of the initial structure due to powder
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