Y. de Carlan

A SANS investigation of the irradiation-enhanced α–α′ phases separation in 7–12 Cr martensitic steels

Abstract Five reduced activation (RA) and four conventional martensitic steels, with chromium contents ranging from 7 to 12 wt%, were investigated by small angle neutron scattering (SANS) under magnetic field after neutron irradiation (0.7–2.9 dpa between 250 and 400 °C). It was shown that when the Cr content of the b.c.c. ferritic matrix is larger than a critical threshold value (∼7.2 at.% at 325 °C), the ferrite separates under neutron irradiation into two isomorphous phases, Fe-rich (α) and Cr-rich (α′). The kinetics of phase separation are much faster than under thermal aging. The quantity of precipitated α′ phase increases with the Cr content, the irradiation dose, and as the irradiation temperature is reduced. The influence of Ta and W added to the RA steels seems negligible. Cold-work pre-treatment increases slightly the coarsening of irradiation-induced precipitates in the 9Cr–1Mo (EM10) steel. In the case of the low Cr content F82H steel irradiated 2.9 dpa at 325 °C, where α′ phase does not form, a small irradiation-induced SANS intensity is detected, which is probably due to point defect clusters. The α′ precipitates contribute significantly to the irradiation-induced hardening of 9–12 wt% Cr content steels.

Assessment of consolidation of oxide dispersion strengthened ferritic steels by spark plasma sintering: from laboratory scale to industrial products

Abstract Oxide dispersion strengthened steels are new generation alloys that are usually processed by hot isostatic pressing (HIP). In this study, spark plasma sintering (SPS) was studied as an alternative consolidation technique. The influence of the processing parameters on the microstructure was quantified. The homogeneity of the SPSed materials was characterised by electron microprobe and microhardness. A combination of limited grain growth and minimised porosity can be achieved on semi-industrial compact. Excellent tensile properties were obtained compared to the literature.

Small Angle Neutron Scattering Study of Irradiated Martensitic Steels: Relation Between Microstructural Evolution and Hardening

Martensitic/ferritic steels (containing 7–13 % Cr) are candidate materials for internal structures in pressurized water, fast breeder, and fusion reactors. Approval for use requires verification of structural stability under neutron irradiation in relation to the evolution of mechanical properties. In this context, several conventional and Reduced Activation (RA) martensitic materials were neutron irradiated at 325°C up to 6 dpa. They were investigated by Small Angle Neutron Scattering (SANS) under a magnetic field after various doses. It was shown that when the Cr content of the b.c.c. ferritic matrix was larger than a critical threshold value (∼ 7.2 at.% at 325°C), the ferrite separated under neutron irradiation into two isomorphous phases, Fe-rich (α) and Cr-rich (α′). The kinetics of phase separation is much faster than under thermal aging. The quantity of precipitated α′ phase increases with the Cr content and the irradiation dose. In the case of steel with the lowest Cr content (F82H) irradiated at 5.6 dpa at 325°C, the α′ phase does not form, and the SANS signal suggests a small contribution due to vacancy clusters. It was believed that these could contribute to the “black dots” observed by TEM. Furthermore, we studied the microstructural features responsible for the secondary hardening phenomenon detected in the as-quenched F82H martensitic steel during irradiation or annealing. In addition, the microstructural evolution of the Oxide Dispersion Strengthened (ODS) steel MA957, which presents an excellent hardening/ductility compromise after irradiation, has been also characterized. The stability of the oxides has been elucidated, and an important α′ volume fraction has been detected. The contribution of α′ to the irradiation-induced hardening was assessed. This, although not negligible, is not the critical factor in normalized and tempered or cold-worked steels. However, it may be the main contribution to hardening in MA957.

Oxide dispersion-strengthened/ferrite-martensite steels as core materials for Generation IV nuclear reactors

Abstract Oxide dispersion-strengthened (ODS) steels are the most promising candidate materials for fuel cladding of Generation IV nuclear reactors. The progress and current status for development of ODS/F-M (ferrite-martensite) steels conducted mainly in Japan and France are overviewed. Fabrication routes of cladding tube are mentioned for ferrite-type ODS steels using a recrystallized process and a martensite-type one using α-γ phase transformation. The optimized process is identical for both countries. The joining process between cladding and end-plug has also been developed by using the pressurized resistance upset welding method. The improvements brought by ODS/FM steels in high-temperature strength and irradiation resistance are verified. Finally, the different environmental effects on the ODS behavior, including irradiation, are reviewed.