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

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 400degreesC). It was shown that when the Cr content of the b.c.c. ferritic matrix is larger than a critical threshold value (similar to7.2 at.% at 325degreesC), the ferrite separates under neutron irradiation into two isomorphous phases, Fe-rich (alpha) and Cr-rich (alpha'). The kinetics of phase separation are much faster than under thermal aging. The quantity of precipitated alpha' 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 325degreesC, where alpha phase does not form, a small irradiation-induced SANS intensity is detected, which is probably due to point defect clusters. The alpha' precipitates contribute significantly to the irradiation-induced hardening of 9-12 wt% Cr content steels. (C) 2003 Elsevier Science B.V. All rights reserved.