Abstract
An approach to improve the performance of steels for fusion reactors is to reinforce them with oxide nanoparticles. These can hinder dislocation and grain boundary movement and trap radiation-induced defects, thus increasing creep and radiation damage resistance. The present work investigates the thermal stability of the microstructure and the evolution of defects in a 0.3% Y2O3 dispersed Eurofer steel. Samples were annealed for 1 h under vacuum, from 600 K to 1600 K, followed by cooling inside the furnace. Electron Microscopy techniques and Vickers Hardness were used to characterize the microstructure and evaluate its thermal stability. Positron Annihilation Spectroscopy Doppler Broadening (PASDB) was used to monitor the evolution of defects, such as dislocations and vacancies, and their interaction with Y-O based nanoparticles. Several types of events take place simultaneously in the material, due to its initial deformation caused by mechanical alloying, the presence of oxide particles and austenitic phase transformation. Annealing up to 1000 K shows that the Y-O based nanoparticles keep the microstructure refined. Upon cooling from 1200 K (above Ac3), martensite forms with an equiaxed morphology, instead of the conventional lath form, due to the pinning of prior-austenite grain boundaries by the oxide nanoparticles. Annealing at 1400 K and 1600 K results in the progressive coarsening of Y-O based nanoparticles and their loss of ability to pin grain boundaries. PASDB shows that annealing up to 1200 K leads to an overall decrease in defect concentration, mainly due to recovery of dislocations. After annealing at 1400 K and 1600 K, PASDB indicates the formation of a different type of positron trap. The hypothesis is that, at these temperatures, clusters of thermal vacancies are trapped by the oxide nanoparticles, accumulating at their interfaces with the matrix and being retained in the material upon cooling to room temperature.
Highlights
Steels for nuclear applications must resist severe serviceV.S.M
Reduced-activation steels have already been developed, like the Eurofer97, they present poor resistance to radiation-induced creep and the service temperature is limited to 900 K, due to thermal instability of the microstructure
The characterization of the microstructure and evolution of defects in the 0.3% Y2O3 ODS Eurofer steel, after annealing at different temperatures, lead to the following conclusions: i) The Y-O based nanoparticles and the deformation introduced in the material, during its fabrication by mechanical alloying, strongly influence the microstructure and the kinetics of phase transformations
Summary
Steels for nuclear applications must resist severe serviceV.S.M. Pereira et al / Journal of Nuclear Materials 540 (2020) 152398 level and be recycled [8]. Steels for nuclear applications must resist severe service. Reduced-activation steels have already been developed, like the Eurofer, they present poor resistance to radiation-induced creep and the service temperature is limited to 900 K, due to thermal instability of the microstructure. The fabrication method of ODS steels is based on mechanical alloying, which enables the direct mixing of metallic and oxide powders, allowing a homogeneous distribution of oxide particles in the matrix [3,9e11]. The standard manufacturing powdermetallurgy route consists of the following steps: (1) gas atomization of a pre-alloy, (2) mechanical alloying of the metallic and oxide powders, (3) consolidation of the ODS alloy by extrusion, hot isostatic pressing or plasma-assisted sintering and (4) thermomechanical treatments like hot rolling, forging and cold rolling [3,7,9e11]
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