Water Vapour Embrittlement and Hydrogen Bubble Formation in Al–Zn–Mg Alloys

Abstract

It has been found by transmission electron microscope observations that lens-shaped bubbles form on grain boundaries of Al–6% Zn–3% Mg alloy specimens pre-exposed to moist air but not in specimens pre-exposed to dry air. The bubbles are surrounded by strong elastic strain fields and their density depends on the time and temperature of pre-exposure, the grain boundary plane and mis-orientation, the specimen thickness, composition and presence of stress. In aged alloys the bubbles form preferentially on grain boundary precipitates. The bubbles are believed to result from the precipitation of molecular hydrogen formed by reaction between the specimen and the water vapour. The pressure of hydrogen in the bubbles is sufficient to propagate intergranular cracks even in the absence of an externally applied stress. However, the experimental observations suggest that it is the hydrogen in solution on the grain boundaries between the bubbles that is particularly damaging and not the hydrogen precipitated in the bubbles themselves.