Volume 2, Issue 9 (October 2005)
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.