Microstructural stability of secondary phases in an ODS ferritic steel after thermal aging at 873 K

Abstract

An oxide dispersion strengthened (ODS) steel with nominal composition Fe–14Cr–2W–0.4Ti–0.3Y2O3 (wt%) has been manufactured by mechanical alloying of pre-alloyed powders with nanosized Y2O3, compacted by hot isostatic pressing and hot cross rolled. In order to evaluate the long-term thermal resistance of the alloy, it has been subjected to 2000 h of thermal aging at 873 K, which is a relevant temperature for nuclear reactor applications. A thermodynamic equilibrium simulation indicates the precipitation of Laves phase under the aging parameters used. This prediction is confirmed from the detailed multi-technique characterization performed. Before aging, Ti-rich oxides, Cr-W-rich precipitates (M23C6 type) and a homogeneous Y-rich nanoprecipitate dispersion are observed. After aging, some W-rich precipitates are identified as Laves phase, while M23C6 carbides, Ti-rich oxides and Y-rich nanoprecipitates remain stable. Mechanical characterization performed in a previous research showed higher hardness, a loss of total elongation and a slight shift of the ductile-to-brittle transition temperature (DBTT) towards a higher value after aging, with similar strength values before and after the long-term thermal treatment. These changes can be due to the redistribution of precipitates together with the Laves phase formation. The mechanical properties not being dramatically affected seem to be due to the observed stability of Y-rich nanoprecipitates distribution.