The embrittlement of Al-Zn-Mg and Al-Mg alloys by water vapor
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I.
INTRODUCTION
T H R O U G H O U T the numerous investigations carried out on the stress corrosion I and hydrogen embrittlement2 of A1Zn-Mg and A1-Mg alloys attention has been centered on grain boundary regions, as the predominant fracture mode is intergranular. Anodic dissolution of material at the grain boundary was thought to be the mechanism controlling stress-corrosion cracking 3 until the relatively recent discovery that intergranular decohesion occurred4 and that specimens could be embrittled by exposing them to hydrogen-producing environments in the absence of stress, a phenomenon which could be reversed by heat treatment after exposure, 5 testing in vacuum, or natural recovery effects with time. 6 These observations suggested that hydrogen embrittlement was the operative mechanism in stresscorrosion cracking and preexposure embrittlement and led to several investigations into the way in which hydrogen affects the mechanical properties of the boundaries 7'8'9and to the possibility of utilizing grain boundary precipitates as trap sites to control the ingress of hydrogen. 10 Fairly recent work by Christodoulou u has added to the growing evidence that hydrogen embrittlement occurs when AI-Zn-Mg alloys are exposed to water vapor. This work shows that MgZn2 precipitates on the grain boundaries of these alloys act as traps for the embrittling hydrogen atoms and proposes that they act in this way by causing hydrogen molecules to form around them, producing less debilitating bubbles of hydrogen gas. Hydrogen bubbles on the grain boundaries of AI-Zn-Mg and AI-Mg" alloys which have been exposed to water vapor have also been observed by other workers using transmission electron microscopy ~2'13(TEM), but the bubbles have not always been seen to be associated with MgZn2 grain boundary precipitates, and it has been proposed that they result from the decomposition of hydrides (particularly magnesium hydride9'14) by the electron beam. Thus, the trapping of hydrogen by the grain boundary C . D . S . TUCK is Research Scientist with Alcan IntemationaI Ltd., Banbury Research Laboratories, Banbury, Oxon, United Kingdom. Manuscript submitted June 11, 1984. METALLURGICAL TRANSACTIONS A
MgZn2 precipitates in A1-Zn-Mg could be via hydride formation rather than by a discharge mechanism. It was decided to repeat the experiments of Christodoulou using thicker specimens (3 mm as opposed to 0.5 mm) and an alloy more akin to commercial material (7004-type) to try to clarify the mechanisms by which grain boundary chemistry affects the embrittlement and recovery processes. The effect of iron, nickel, and copper additions to pure AI-Zn-Mg was also studied as these are known to promote stress corrosion resistance in A1-Zn-Mg.
II.
EXPERIMENTS
A medium strength super-pure base A14.5Znl.5Mg alloy was used with and without small additions of iron, nickel, and copper. A15Mg was also included. The alloy compositions are given in Table I. They were D.C. cast, scalped, homogenized for 16 hours at 465 °C, hot-rolled to 5 ram, and cold-rolled to
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