Microstructural analysis of 9% Cr martensitic steels containing 0.5 at.% helium

Microstructural examinations by transmission electron microscopy and small angle neutron scattering were performed on 100 μm thick specimens of 9Cr–1Mo (EM10) and modified 9Cr–1Mo (T91) martensitic steels homogeneously implanted with 23 MeV α particles to a concentration of 5000 appm. Two implantation temperatures were selected, 250 and 550 °C, which correspond respectively to the lower and higher bounds of the operation temperature range foreseen for the window of accelerator driven systems devoted to waste transmutation. 250 °C is also the maximum operating temperature of the European spallation source window. The TEM samples were punched out from implanted tensile specimens following testing, which revealed, as detailed in a companion paper [P. Jung et al., these Proceedings], drastic hardening and complete ductility loss for the specimens implanted at 250 °C. Helium bubbles were detected in both materials implanted at 250 and 550 °C and bubble size distributions as well as number densities were determined. Furthermore, it was found that the bubbles are at thermodynamic equilibrium. Based on the microstructural results, it is shown that the high degree of hardening of specimens implanted at 250 °C is due to the high density of tiny helium bubbles they contain. It is furthermore suggested that the brittle, intergranular fracture mode displayed by these specimens results from the combined effects of pronounced intragranular hardening and weakening of prior austenite grain boundaries due to helium.