Hydrogen diffusion in Al-Li alloys
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INTRODUCTION
A L U M I N U M alloys, in general, absorb hydrogen during casting, fabrication, and/or heat treatment as a result of their interaction with moisture in the surrounding atmosphere. The severity of this interaction, however, is directly related to the chemical composition of the given alloy; e.g., magnesium and lithium-containing alloys interact more severely with moisture than other alloys. In molten A1-Li alloys, the solubility of hydrogen is more than three times that in pure liquid aluminum, E1,2,3] and consequently, they are difficult to degas during casting. This also makes it difficult to prevent subsequent contamination of the melt after degassing during casting. In addition, the solubility of hydrogen in the solid state of the alloys is more than an order of magnitude higher than that in aluminum and other of its alloys. ~2'31 Consequently, A1-Li alloy products typically contain a comparatively higher hydrogen concentration ( > 0 . 3 0 cm3/ 100 g) than other aluminum alloys. The effects of high levels of hydrogen on the mechanical properties (e.g., short transverse fracture toughness and yield tensile strength) of these alloys have not yet been established. Hydrogen could, however, lower strength or toughness properties via: (a) precipitation of the hydrogen atoms at internal defects, (b) interaction of the dissolved hydrogen to reduce the cohesive strength of the lattice, (c) association of hydrogen with dislocations either to restrict dislocation mobility or to provide localized hydrogen accumulations and thereby embrittle the lattice, and (d) formation of a hydrogen-rich phase (e.g:, LiH, LiA1H4, and Nail) whose mechanical properties differ from those of the matrix. This is often referred to in the literature E4-91despite the lack of conclusive evidence that lithium hydride (or sodium hydride) could form in the alloys under production conditions. Attempts at direct correlation of the hydrogen content of the alloys to their mechanical properties t1~ and fracture behavior have been cursory and inadequate. Pre-
P.N. A N Y A L E B E C H I , Staff Scientist, is with Alcoa Laboratories, A l u m i n u m Company of America, Alcoa Center, P A 15069. Manuscript submitted October 4, t989. METALLURGICAL TRANSACTIONS B
vious studies have also been complicated by the difficulty in dissociating the effect of hydrogen from the role of other undesirables, such as inclusions. More importantly, the inability to control the hydrogen levels of the samples being examined has led to conflicting inferences about the role of hydrogen in the relatively poor short transverse fracture toughness properties of the alloys. The study of the impact of hydrogen in the alloys requires control of the hydrogen to known values. This can be accomplished by controlled solid-state degassing of the samples at appropriate temperature and time conditions. Calculation of these temperatures and times, in the form of degassing curves or nomograms, requires the knowledge of the diffusion coefficient of hydrogen in the alloys.
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