Internal Hydrogen Embrittlement in the Aluminum - Zinc - Magnesium System
Ciaraldi, Stephen William
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https://hdl.handle.net/2142/68645
Description
Title
Internal Hydrogen Embrittlement in the Aluminum - Zinc - Magnesium System
Author(s)
Ciaraldi, Stephen William
Issue Date
1980
Department of Study
Metallurgy and Mining Engineering
Discipline
Metallurgical Engineering
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Engineering, Metallurgy
Language
eng
Abstract
An investigation has been made of the internal hydrogen embrittlement of high strength aluminum alloys. The majority of experiments were performed using a high purity Al-5.5 wt pct Zn-2.5 wt pct Mg alloy hydrogenated to levels of up to 1 at pct either by mechanical polishing in aqueous solutions or by exposure to moist air. These specimens were observed to have reduced ductilities and fracture stresses during subsequent tensile tests in argon. Embrittlement was found to be reversible and could be suppressed by thermal treatments prior to testing. Strain rate was also found to influence the occurrence of embrittlement.
Severely embrittled specimens fractured intergranularly and a fragmented surface layer was often observed on the brittle grain boundary facets. Electron diffraction studies of the layer yielded polycrystalline patterns consistent with a hexagonal aluminum hydride, AlH(,3), with a = 2.90 (ANGSTROM) and c = 4.55 (ANGSTROM). This phase was observed to be unstable and patterns obtained from the fragmented layer after exposure to air were found to correspond to FCC aluminum. Transmission electron microscope and other studies revealed that the phase was not pre-existing prior to stressing. These results suggest that the mechanism of internal hydrogen embrittlement in the Al-Zn-Mg system involves the formation of stress induced hydrides.
Experiments were also performed using alloy 7075 and two high purity quaternary alloys of compositions, Al-6.16 wt pct Zn-2.23 wt pct Mg-1.55 wt pct Cu, and Al-6.29 wt pct Zn-2.40 wt pct Mg-2.53 wt pct Cu. Embrittlement was not obtained in the commercial 7075 alloy but both high purity quaternary alloys were susceptible to internal hydrogen embrittlement. A similar intergranular fragmented layer was also observed on the fracture surfaces of the high purity alloy having 2.53 wt pct Cu.
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