Impact of irradiation on the tensile and fatigue properties of two titanium alloys

The attachment of the first wall modules of the ITER FEAT fusion reactor is designed using flexible connectors made from titanium alloys. An assessment of the tensile and fatigue performance of two candidate alloys, a classical two phase Ti6Al4V alloy and a monophase alpha alloy Ti5Al2.5Sn, has been carried out using 590 MeV protons for the simulation of the fusion neutrons. The dose deposited was up to 0.3 dpa and the irradiation temperature was between 40 degreesC and 350 degreesC. The unirradiated tensile performances of both alloys are roughly identical. The radiation hardening is much stronger in the alpha + beta alloy compared with the alpha alloy, and the ductility is correspondingly strongly reduced. A very fine precipitation observed by TEM in the primary and secondary alpha grains of the dual phase alloy seems to be the cause of the intense radiation hardening observed. Two different regimes have been observed in the behaviour of the cyclic stresses. At a high imposed strain, the softening is small in the Ti6Al4V and larger in the Ti5Al2.5Sn. At a low imposed strain, and for both alloys, cyclic softening occurs up to about 800 cycles, but then a transition occurs, after which a regime of cyclic hardening appears. This cyclic hardening disappears after irradiation. In both materials, and for all test conditions, the compressive stress of the hysteresis loop was found to be larger than the tensile stress. The stress asymmetry seems to be triggered by the plastic deformation. The fatigue resistance of the Ti5Al2.5Sn alloy is slightly better than that of the Ti6Al4V alloy. The irradiation did not significantly affect the fatigue performance of both alloys, except for high imposed strains, where a life reduction was observed in the case of the Ti6Al4V alloy. SEM micrographs showed that the fractures were transgranular and pseudo-brittle. (C) 2001 Elsevier Science B.V. All rights reserved.