Effect of thermo-mechanical treatments on the microstructure and mechanical properties of an ODS ferritic steel

The Fe-14Cr-2W-0.3Ti-0.3Y(2)O(3) oxide dispersion strengthened (ODS) reduced activation ferritic (RAF) steel was fabricated by mechanical alloying of a pre-alloyed, gas atomised powder with yttria nano-particles, followed by hot isostatic pressing and thermo-mechanical treatments (TMTs). Two kinds of TMT were applied: (i) hot pressing, or (ii) hot rolling, both followed by annealing in vacuum at 850 degrees C.

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Nadine Baluc, Pablo Federico Mueller, Zbigniew Oksiuta, Philippe Spätig
2011
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https://infoscience.epfl.ch/record/171378?ln=en

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Steel

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The formation and evolution of oxide particles in oxide-dispersion-strengthened ferritic steels during processing

Nadine Baluc

The fabrication of oxide-dispersion-strengthened (ODS) steels is a multi-stage process involving powder ball milling, degassing and consolidation by hot isostatic pressing. Y is introduced by mechanical alloying (MA) with either Y2O3 or Fe2Y so a high density of Y-Ti-O-based oxide nanoparticles is formed. The evolution of ∼2 nm oxide nanoparticles and larger >5 nm grain boundary oxides has been studied at each step of the processing. The nanoparticle dispersions produced in material MA with Fe2Y were comparable to those produced by alloying with Y 2O3. Hence the majority of oxygen which forms the nanoparticles must be incorporated from the atmosphere during MA, rather than being introduced via the alloying additions. During mechanical alloying, a high density of subnanometer particles are formed (2.5 ± 0.5 × 10 24 m-3). The oxygen content of the nanoparticles decreases slightly on annealing, and then the composition of the nanoparticles remains constant throughout subsequent processing stages. The nanoparticle size increases during processing up to ∼2 nm radius, while the number density decreases to 4 ± 0.5 × 1023 m-3. Grain boundary oxides (>5 nm) have a Ti-Cr-O-rich shell, and contain no Y before consolidation, but have similar core composition to the matrix nanoparticles after consolidation. The formation of the larger grain boundary oxides is shown to take place during the degassing and consolidation stages, and this occurs at the expense of the nanoparticles in the matrix. This work provides a mechanistic understanding of the importance of controlling the oxygen content in the powder during MA, and the resulting impact on the formation of the ODS microstructure. © 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Elsevier2013

Relation between microstructure and Charpy impact properties of an elemental and pre-alloyed 14Cr ODS ferritic steel powder after hot isostatic pressing

Nadine Baluc, Emmanuelle Boehm Courjault, Zbigniew Oksiuta

This article describes the microstructure and Charpy impact properties of an Fe-14Cr-2W-0.3-Ti-0.3Y(2)O(3) oxide dispersion strengthened (ODS)-reduced activation ferritic (RAF) steel, manufactured either from elemental powders or from an Fe-14Cr-2W-0.3Ti pre-alloyed powder. ODS RAF steels have been produced by mechanical alloying of powders with 0.3 wt% Y2O3 nanoparticles in either a planetary ball mill or an attritor ball mill, for 45 and 20 h, respectively, followed by hot isostatic pressing (HIPping) at 1,150 degrees C under a pressure of 200 MPa for 4 h and heat treatment at 850 degrees C for 1 h. It was found that the elemental ODS steel powder contains smaller particles with a higher specific surface area and a two times higher oxygen amount than the pre-alloyed ODS steel powder. After HIPping both materials exhibit a density higher than 99%. However, the pre-alloyed ODS steel exhibits a slightly better density than the elemental ODS steel, due to the reduced oxygen content in the former material. Charpy impact experiment revealed that the pre-alloyed ODS steel has a much larger ductile-to-brittle transition temperature (DBTT) (about 140 degrees C) than the elemental ODS steel (about 25 degrees C). However, no significant difference in the upper shelf energy (about 3.0 J) was measured. TEM and SEM-EBSD analyses revealed that the microstructure of the elemental ODS steel is composed of smaller grains with a larger fraction of high-angle grains (>15 degrees) and a lower dislocation density than the pre-alloyed ODS steel, which explains the lower DBTT value obtained for the elemental ODS steel.

2010