MICROCHEMICAL ANALYSIS OF PRECIPITATE FREE ZONES IN 7075-AL IN THE T6, T7 AND RRA TEMPERS

Abstract

The microchemistry within precipitate free zones (PFZ) in the commercial alloy 7075-Al subjected to various tempers was investigated using analytical electron microscopy. Segregation or depletion of Zn and Cu atoms near grain boundaries in as-quenched samples, which has been reported by other investigators, was not found in this study. Considerable depletion of Zn was observed in the samples in all three tempers investigated (T651, T73 and RRA). The magnitude of the depletion of Zn was relatively insensitive to the temper condition, being slightly larger in the RRA temper than in the other two. The depletion of Cu was far more sensitive to the aging condition, being larger in both the T73 and RRA tempers than in the T651 temper. The increased resistance to stress-corrosion cracking of 7075-Al in the T73 or RRA tempers is attributed primarily to the reduction of the Cu concentration within the PFZs.