ATMOSPHERIC STRESS-CORROSION CRACKING OF A SUPERPLASTIC 7475-ALUMINUM-ALLOY

The influence of different heat treatments upon the atmospheric stress corrosion cracking (SCC) of fine-grained 7475 Al-alloy plates has bee n investigated. The small size of the matrix precipitates and grain-bo undary precipitates (GBPs) was found to be the main cause of atmospher ic SCC susceptibility. Increasing the size of the matrix precipitates and GBPs by increasing the degree of aging could improve the atmospher ic SCC resistance. The size of the matrix precipitates was the major f actor affecting the atmospheric SCC resistance when GBPs were larger t han a critical size that could nucleate hydrogen bubbles. However, if the size of the GBPs was smaller than this critical size, the improvem ent of atmospheric SCC resistance due to grain refinement, resulting f rom a more homogeneous slip mode, could not be obtained because hydrog en embrittlement became serious. fly measuring the electrical conducti vity, the influence of matrix precipitates, but not that of GBPs, on S CC susceptibility could be obtained. Retrogression and reaging (RRA) t reatment could effectively improve the atmospheric SCC resistance of T 6 temper because RRA temper could produce larger sizes of both the mat rix precipitates and GBPs than could T6 tempered condition.