Abstract
The influence of different heat treatments upon the atmospheric stress corrosion cracking (SCC) of fine-grained 7475 Al-alloy plates has been investigated. The small size of the matrix precipitates and grain-boundary precipitates (GBPs) was found to be the main cause of atmospheric SCC suscepti-bility. Increasing the size of the matrix precipitates and GBPs by increasing the degree of aging could improve the atmospheric SCC resistance. The size of the matrix precipitates was the major factor affecting the atmospheric SCC resistance when GBPs were larger than a critical size that could nucleate hydrogen bubbles. However, if the size of the GBPs was smaller than this critical size, the improvement of atmospheric SCC resistance due to grain refinement, resulting from a more homo-geneous slip mode, could not be obtained because hydrogen embrittlement became serious. By meas-uring the electrical conductivity, the influence of matrix precipitates, but not that of GBPs, on SCC susceptibility could be obtained. Retrogression and reaging (RRA) treatment could effectively im-prove the atmospheric SCC resistance of T6 temper because RRA temper could produce larger sizes of both the matrix precipitates and GBPs than could T6 tempered condition.
Highlights
HIGH-STRENGTH aluminum alloys of the A1-Zn-Mg type (7xxx series) are widely used in airframe construction
7xxx series aluminum alloys are susceptible to stress corrosion cracking (SCC) failures observed in service, 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
It is more appropriate to consider that electrical conductivity increases with increasing degree of aging~7,37]because the main factor leading to an increase in the value of electrical conductivity, obviously derived from the Transmission electron microscopy (TEM) observations and Table VIII, is that the matrix precipitates change from GP zones to semicoherent r/' and incoherent ~7precipitates and increase the size and volume fraction of r/' and r/precipitates
Summary
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. Burleight4] summarized three main mechanisms for SCC in aluminum alloys. They are anodic dissolution, hydrogeninduced cracking, and passive film rupture,f2,3~He indicated that anodic dissolution is generally favored in the 2xxx series aluminum alloys, whereas hydrogen-induced cracking is favored in the 7xxx series. Three principal microstructural features have been discussed concerning the influence of SCC. They are the precipitate free zone (PFZ), matrix precipitate structure, and
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