Evaluation of Ductile-Brittle Transition Behavior of Helium-implanted Reduced Activation 9Cr-2W Martensitic Steel by Small Punch Tests

摘要

Helium implantation was performed by cyclotron with a beam of 36MeV-alpha particles at temperature below 429K using a beam energy degrader to obtain homogeneous distribution of helium in a reduced activation 9Cr-2W martensitic steel up to a concentration of 120 at. ppm. The ductile-brittle transition temperature (DBTT) was evaluated by small punch (SP) tests in which the disk specimens of 3 mm diameter with 0.22 mm thickness were deformed by bulge mode. The shift in the DBTT (SP-DELTADBTT) and the increase in Vickers hardness (DELTAH_v) caused by helium implantation were measured to be 26K and 67kg/mm~2, respectively, which were estimated to be 65 K of standard CVN-DELTADBTT and 104 MPa of increase in the yield stress (DELTAsigma_y) from empirical relationships between them. The enhancement of irradiation hardening by helium was never recognized and no change in the fracture mode at low temperatures was observed by the helium implantation. It is considered that the shift in the DBTT is mainly associated with irradiation hardening by displacement damage.