The mechanical properties of a Ti-49Al alloy were investigated by tensile tests at different strain rates in vacuum, air, or a flowing hydrogen gas, before or after the specimens were exposed to a high temperature hydrogen gas. The results indicate that the strain rate hardly affect the tensile properties in a strain rate range of 5.6 × 10-6to 2 × 10-4s-1. A large amount of the internal hydrogen decreases the ductility more than the external hydrogen. The internal hydrogen induces embrittlement by decreasing the cohesive strength of the lattice, whereas the external hydrogen embrittlement may be attributed to hydrogen-enhanced localized plastic deformation. The presence of a large amount of internal hydrogen increases the width of the stacking fault; that is, it decreases the stacking fault energy. © 2002 Elsevier Science Ltd. All rights reserved.
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