Hydrogen bubbles in embrittled Al-Zn-Mg alloys

Abstract

Al-Zn-Mg alloys become embrittled during exposure to moist environments due to hydrogen penetration of grain boundaries. The result of this hydrogen penetration due to surface reaction with water vapour of both bulk specimens and electron-transparent “thin foils”, has been studied at high resolution in the JEM 100 C transmission electron microscope as a function of alloy composition and ageing treatment. In bulk specimens of alloys solution-heated, water-quenched, and aged in water-vapour-saturated air at 70° C, the hydrogen is in the form of a mobile atomic species which is transformed to bubbles of molecular hydrogen under the action of the electron beam. However, in electron-transparent specimens of aged alloys after exposure to water vapour the accumulated hydrogen is observed directly as bubbles. These bubbles take the form of hexagonal lenses bounded by {111} planes, and are associated with grain-boundary precipitates, particularly in over-aged microstructures, and with primary intermetallic particles in alloys containing sparingly soluble transition elements. The consequence of the observed hydrogen penetration of grain boundaries in promoting environmental debilitation of mechanical properties and stress-corrosion cracking of Al-Zn-Mg alloys is discussed.