The fatigue properties of the Ti-alloy IMI 834 at high temperature (500°C) are compared in air as an aggressive environment and in vacuum as an inert reference environment. This comparison includes the environmental effect on the low cycle fatigue properties, where it is distinguished between the effect on crack nucleation and on the propagation of these self-initiated small surface cracks, as well as on the propagation behavior of large through-cracks. Two different microstructural conditions are considered, a pure lamellar structure and a bi-modal structure, consisting of equiaxed primary α phase in a transformed lamellar matrix. The characterization of the propagation behavior of large through-cracks includes the effects of mean stress variations and therefore considers the presence or absence of crack closure effects. The observed pronounced reduction in the resistance against crack nucleation and crack propagation of small surface cracks as well as large cracks in the aggressive air environment is discussed on the basis of hydrogen embrittlement caused by the water vapor present in the air environment.
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