Low cycle fatigue behaviour of TZM molybdenum alloy in divertor water coolant

Load controlled low cycle fatigue (LCF) tests on TZM Mo-alloy have been conducted up to specimen rupture, using two triangular load waveforms of 17 and 20 kN amplitudes, and in high temperature water (250 degrees C), either deaerated or containing separate additions of oxygen (10(-5) M/l), hydrogen (10(-3) M/l) and hydrogen peroxide (10(-4)-10(-5) M/l), in order to simulate the coolant chemistry inside the divertor tubings during fusion reactor operation. Extensions of TZM fatigue lives, compared to those obtained in the 20 kN experiments. were found to occur in all aqueous environments under the 17 kN cyclic load amplitude test, which induced a lower total strain range. It was also observed that, independently of the stressing conditions, the environments in terms of their deleterious effects on the LCF performances of TZM alloys, could be ranked in the following order: oxygenated water < deaerated water < hydrogenated water. Conflicting results were conversely obtained in hydrogen peroxide solutions, where the fatigue resistance was the lowest under 20 kN loading, but became the highest at a reduced amplitude of 17 kN. On the basis of visual assessments, and TZM open circuit potential values, continuously recorded during testing against an Ag/AgCl reference electrode, an idea about the corrosion behaviour of TZM alloy in high temperature water environments was proposed. Qualitative fractographic analysis was performed by scanning electron microscopy (SEM), to identify the predominant failure modes and suggest, in connection with the electrochemical evaluations, candidate cracking mechanisms, which accounted, reasonably, for the fatigue life data.