Influence of water vapour on the creep and fatigue properties of TiAl

In two publications the influence of water vapour on the oxidation behaviour and the mechanical properties of a technical γ-TiAl based alloy at temperatures of 500, 700 and 750 °C is discussed. The present paper deals with the mechanical properties while the oxidation behaviour was discussed in the preceding paper in this journal [30]. Results are presented from creep and fatigue investigations in dry synthetic air and air with 10% water vapour. Under primary creep conditions the presence of water vapour led to an increase of the creep rates. A similar effect was found in a certain stress range under secondary creep conditions. In fatigue the effect of water vapour depended on strain rate and surface finish. In particular, at low strain rates a decrease in the number of cycles to failure becomes evident in the water vapour-containing environment. Further effects are due to surface roughness and preoxidation. The results are discussed on the basis of a potential effect of hydrogen on dislocation movement, potential hydrogen embrittlement and the formation of brittle subsurface zones resulting from the oxidation process.