Bending tests on T91 samples implanted with 0.25 at.% helium: Experiments and mechanical analysis

In order to investigate helium effects on the fracture properties of martensitic mod 9Cr–1Mo (T91) steel, miniature Charpy specimens were implanted at 250 °C in the notch region to 0.25 at.% helium using a degraded 34 MeV 3He ion beam and subsequently submitted to static bending tests at room temperature. For the six implanted specimens, a ‘pop-in’ phenomenon, which is an arrested unstable crack extension, was systematically recorded during testing. In the implanted zones of the samples, the fracture mode was fully brittle with both intergranular and cleavage fracture, whereas for unimplanted samples tested at −170 °C, the fracture mode was found to be 100% cleavage. Finite element simulations of the tests performed on unimplanted and implanted specimens were also carried out to determine stress and strain fields at the onset of crack propagation. Based on these computations, the fracture toughness of implanted T91 was tentatively evaluated using the Beremin model of the local approach to brittle fracture.