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I. INTRODUCTION

THE 5083 Al alloy has been widely used as a structural material in the marine environment. Grain refinement of this alloy can be obtained by thermomechanical treatment, which improves its mechanical properties and results in superplasticity.[1] Using the superplastivity of material, a superplastic forming technology has been developed, which has become important in industries that manufacture more complex workpieces. This process might replace the conventional stamping processes, especially for those materials with lower formability.[2] A superplastic 5083 Al alloy (Al 5083SP) has been commercialized and several superplastically formed components have been produced for the automotive industry to reduce vehicle weight and improve fuel efficiency.[3] The alloy also has potential application in architectural structures. However, it has been reported that Al-Mg alloys containing more than 3 wt pct Mg, including the 5083 Al alloy, will be sensitized at temperatures above 100 8C, which can be attributed to the formation of continuous precipitation of the highly anodic b phase (Mg2Al3) at grain boundaries.[4,5,6] During exposure to corrosive environments, galvanic effect occurs between the grain boundary precipitates and the grain interiors, which leads to severe anodic attack at grain boundaries. This phenomenon causes an increase of stress corrosion cracking (SCC) susceptibility in this alloy. Since the superplastic forming process of an Al 5083SP alloy is performed at a temperature between 450 8C and 500 8C,[7] SCC susceptibility should be noted. Furthermore, J.C. CHANG, Associate Researcher, formerly with the Institute of Materials Science and Engineering, National Taiwan University, is with the Chemical Systems Research Division, Chung Shan Institute of Science and Technology, Lung-Tan, 325, Taiwan, Republic of China. T.H. CHUANG, Professor, is with the Institute of Materials Science and Engineering, National Taiwan University, Taipei, 106, Taiwan, Republic of China. Manuscript submitted April 7, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS A

another undesirable phenomenon, i.e., the formation of cavities, could occur during the superplastic forming of certain alloys, especially for aluminum alloys.[7–10] The cavitation not only deteriorates the mechanical properties of this alloy but also increases its susceptibility to corrosion. Although the superplastic formability of an Al 5083SP alloy has been evaluated by many researchers,[8–12] studies of the corrosion behavior of this alloy are limited.[13,14] In our previous study,[13] it was found that a superplastically formed Al 5083SP alloy revealed an increased tendency for intergranular corrosion, which may cause the increase of the SCC susceptibility of this alloy. However, to the authors’ knowledge, no work concerning SCC for the superplastically formed Al 5083 alloy has been reported, even though it would be very important from a practical point of view. In the present work, therefore, the SCC behavior of this alloy before and after superplastic