Effect of hydrogen on the fracture toughness of the titanium alloys Ti6Al4V and Ti5Al2.5Sn before and after neutron irradiation

Titanium alloys are considered as potential candidate structural materials for the first wall, the blanket and the magnetic coil structures of fusion reactors. Titanium alloys are interesting materials because of their high specific strength and low elastic modulus, their good resistance to radiation damage and their fast induced radioactivity decay. The attachment of the first wall modules of the ITER FEAT fusion reactor is designed using flexible connectors made from titanium alloys. Two classical industrially available alloys, the monophase alpha Ti5Al2.4Sn alloy and the duplex alpha + beta Ti6Al4V alloy are candidate materials for the application. Titanium alloys with an alpha structure are believed to have a good resistance against radiation embrittlement and alpha + beta alloys should possess the best tolerance to hydrogen embrittlement. Hydrogen has been loaded in the material using a Sievert's apparatus operated at 750degreesC. The study concentrates on two hydrogen levels, 150 wppm at which no hydrides are expected in both alloys and 400 wppm were hydrides could form in the alloys. The fracture toughness is measured using mini charpy specimens tested in a three point bend fixture. The 3 x 4 x 27 mm mini charpys have been irradiated in the DR3 reactor of RISO in Denmark, at 60degreesC and 350degreesC to a dose of 0.1 dpa. The specimen have been cracked to a/w = 0.5 after irradiation. The fracture toughness properties of both alloys is reported as a function of the hydrogen level, before and after irradiation.