Evidence for Dislocation Transport of Hydrogen in Aluminum
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I. INTRODUCTION CONSIDERABLE evidence has now been accumulated which indicates that the presence of cathodically-charged internal hydrogen can degrade the ductility of high-strength aluminum alloys, particularly those based on the A1-Zn-Mg system.l-7 A number of investigators have shown that not only can the reduction of area in a tensile test be reduced by the presence of hydrogen, but that such a loss is often associated with the occurrence of an intergranular fracture mode, particularly in alloys with an equiaxed grain shape.~'8-1z The occurrence of hydrogen embrittlement in high strength aluminum alloys under internal hydrogen conditions suggests further that the more general form of environmental embrittlement, namely, stress corrosion cracking, may itself be cortrolled by the direct action of hydrogen in these alloys, rather than by anodic dissolution. A number of authors have discussed this possibility at length. 1.6,8,10-21 A complicating factor in such an argument and, in fact, in completely understanding the direct role of hydrogen, is that lattice transport of hydrogen is extremely slow in aluminum, making it difficult to rationalize how hydrogen can keep up with a moving crack, or be transported to a crack region solely by volume diffusion. One solution to this apparent dilemma would be the existence of a more rapid transport mode for hydrogen, namely, as solute atmospheres associated with mobile dislocations. 22'23 In this way transport distances per unit time can be increased by orders of magnitude, and if this is a rate controlling step in the embrittlement process, a direct correlation should be found between the degree of embrittlement and the amount of dislocation-associated hydrogen transport. Transport of hydrogen with mobile dislocations would also have to be compared to possible transport by pipe diffusion or grain boundary diffusion. To test these ideas, an aluminum alloy, similar to 7075 and of equiaxed grain structure, was chosen for study. Previous work on high-purity A1-Zn-Mg ternary alloys has provided interesting data, but many microstructural and compositional details of commercial alloys are necessarily absent. There is evidence 19'2~that the often-complex interactions of microstructure and hydrogen-assisted fracture which occur in commercially-processed alloys cannot be J. ALBRECHT, formerly Postdoctoral Associate at Carnegie-Mellon University, is now with Brown Boveri Research Center, Baden CH-5405, Switzerland. I.M. BERNSTEIN and ANTHONY W. THOMPSON are Professors, Department of Metallurgical Engineering and Materials Science, Camegie-Mellon University, Pittsburgh, PA 15213. Manuscript submitted August 11, 1981. METALLURGICAL TRANSACTIONS A
duplicated in model alloys. Specimens of the equiaxed 7075-type alloy, if embrittled, should show a greater percentage of intergranular embrittlement than would longitudinal specimens of the "pancake" or disk-shaped grain structure typical of wrought product, if this is indeed the preferred fracture mode. 6 Further, a test technique was devised
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