Stress corrosion cracking of superplastically formed 7475 aluminum alloy
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I.
INTRODUCTION
THERMOMECHANICAL treatment has been employed to produce a fine grain size in high-strength 7475Al alloy,[1] which possessed superplasticity.[2] The substantial advantages for fabrication of aerospace components by superplastic forming in comparison with conventional fabrication methods are lower weight, lower production cost, less wastage, and more efficient design. However, an undesirable phenomenon for the superplastic forming of a fine-grained 7475Al alloy is the formation of cavitation,[3,4] which will reduce the subsequent mechanical properties.[5] Furthermore, it is well known that 7xxx series aluminum alloys are susceptible to stress corrosion cracking (SCC) failures observed in service, particularly when they are aged to the near-peak-strength T6 tempered condition.[6] Recent studies have also shown that in a T6 tempered 7475Al alloy, prior superplastic deformation has been found to suffer atmospheric SCC.[7] For the same reason, a possible degradation of SCC resistance, when the superplastically formed 7475Al alloy is used for structural aerospace components, could be particularly serious. Burleigh[8] summarized that the main mechanism for SCC in 7xxx series aluminum is hydrogen-induced cracking. Generally, the increase of matrix precipitate size and the associated change from GP zones to semicoherent h' and incoherent h precipitates will result in a more homogeneous slip mode and the reduction of slip planarity.[9] The homogeneous slip mode can effectively reduce hydrogen transported to the grain boundaries for inducing cracking by means of mobile dislocations.[9,10] On the other hand, the larger grain boundary precipitates (GBPs) can act as trapping sites for atomic hydrogen to retard intergranular T.C. TSAI, Postdoctor, J.C. CHANG, Doctoral Candidate, and T.H. CHUANG, Professor, are with the Institute of Materials Science and Engineering, National Taiwan University, Taiwan, 106, Republic of China. Manuscript submitted October 23, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS A
SCC.[11,12] By trapping atomic hydrogen, the bubbles of molecular hydrogen nucleate at GBPs, thus lowering the concentration of atomic hydrogen in the grain boundary and preventing hydrogen embrittlement, which is due to atomic hydrogen reducing grain boundary cohesion.[12,13] An RRA[14] heat treatment was claimed to give SCC resistance equivalent to that of T73 temper together with T6 strength level.[11,15,16] The RRA tempered condition can provide a larger size of GBPs to reduce SCC susceptibility[8,11] and a greater volume fraction of coherent matrix precipitates to retard the loss of strength[16] than can the T6 tempered condition. Previous investigations have paid much attention to the effects of superplastic deformation on the subsequent service mechanical properties.[5,17,18] However, it is very rare to report on the SCC testing. The purpose of this investigation was to evaluate the influences of different amounts of superplastic deformation and various postforming heat treatments on SCC susceptibility o
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