Deformation and damage mechanisms in an α/β 6242 Ti alloy in fatigue, dwell-fatigue and creep at room temperature. Influence of internal hydrogen

In β-forged Ti6242, dwell periods during ambient stress-controlled fatigue substantially reduce the lifetime. Cracks initiate due to the coalescence of shear-induced cavities nucleated at α/β interfaces. This damage is most often due to pyramidal slip and enhanced by dwell periods. The influence of the hydrogen content on the tensile, creep and fatigue behaviour, damage mechanisms, and crack growth was investigated. Hydrogen reduced the proportional limit and the serrations on the tensile curves but increased the conventional yield stress at high strain rate, so that it either slowed down or enhanced room temperature creep, depending on the applied stress. When a H-induced reduction in creep occurred, H had a beneficial influence on the fatigue and dwell-fatigue lives, in spite of an acceleration of fatigue crack growth. Observations of the H distribution, using the microprint technique show that even though most of it resides in the β phase, some is transported into the α phase, especially in heavily deformed α laths.