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I.

INTRODUCTION

HIGH-STRENGTH aluminum alloys of the A1-Zn-Mg type (7xxx series) are widely used in airframe construction. These alloys are usually chosen because of their high strength and stiffness, which are derived from precipitation hardening. However, 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 conditionY ~1 Their resistance to SCC can be increased by overaging to T73 temper but with a concomitant loss of about 10 to 15 pct in strength. Cinar51 has reported a heat treatment known as retrogression and reaging (RRA), which was claimed to give SCC resistance equivalent to that of T73 temper together with T6 strength levels, t6-91 This treatment has been applied to materials in the T6 condition and consists of reheating the material for a short time in the temperature range of 200 ~ to 280 ~ (retrogression treatment), followed by reaging using the same conditions as in the original T6 age. An optimum condition chosen for SCC testing was retrogression for 5 minutes at 220 ~ followed by reaging, which produced the best SCC results. ~7,1~ Burleight4] summarized three main mechanisms for SCC in aluminum alloys. They are anodic dissolution, hydrogeninduced cracking, and passive film rupture, f2,3~He also indicated that anodic dissolution is generally favored in the 2xxx series aluminum alloys, whereas hydrogen-induced cracking is favored in the 7xxx series. The SCC susceptibility of 7xxx series aluminum alloys is well known to be strongly affected by their microstructural characteristics. Three principal microstructural features have been discussed concerning the influence of SCC. They are the precipitate free zone (PFZ), matrix precipitate structure, and

T.C. TSAI, Doctoral Candidate, and T.H. CHUANG, Professor, are with the Institute of Materials Science and Engineering, National Taiwan University, Taipei, Taiwan, 106, Republic of China. Manuscript submitted April 24, 1995. METALLURGICALAND MATERIALSTRANSACTIONS A

grain-boundary precipitate (GBP) structure,tlH4] Generally, overaging can improve the SCC resistance of high-strength aluminum alloys by means of the more homogeneous slip mode and the reduction of slip planarity, which are due to the increase of matrix precipitate size and the associated change from GP zones to semicoherent 7' and incoherent ~7 precipitates, v4,15,~61 The homogeneous slip mode can effectively reduce either stress-assisted anodic dissolution along grain boundaries t~71 or hydrogen transported to the grain boundaries for inducing cracking by means of mobile dislocationsY 5,~7j On the other hand, the increasing size of GBPs during overaging has been proposed to explain the higher SCC resistance in the 7xxx series aluminum alloys.[8.~31The larger r/precipitates in the grain boundary can act as sacrificial anodest~31 or as trapping sites for atomic hydrogen to retard intergranular SCC. t8,~~ By trapping atomic hydrogen, the bubbles of molecular hydrogen nuc