Role of GP II Zones and Metastable -ɳ′ Precipitates on the Environmentally Assisted Cracking Behavior of AA 7050 Alloy

The electrochemical kinetics of matrix precipitates -metastable MgZn2 (ɳ′) and GP II zones towards environmentally assisted cracking (EAC) have been investigated using cathodic polarization tests and slow strain rate tests. Retrogression and re-aging treatment (RRA) and modified interrupted aging treatment (MA3) are employed to produce high fractions of ɳ′ and GP II zones respectively. The higher ɳ′ fraction resulted in a three times higher hydrogen evolution on the alloy, thus decreasing the elongation values in the cathodically charged and uncharged 3.5 wt pct NaCl solution. Furthermore, the gauge surface of the ɳ′ dominated alloy showed pitting along the grain boundaries in 3.5 wt pct NaCl solution and a high amount of sub grain formation when tested in cathodically charged solution. On the other hand, long transgranular cracks were observed in the case of the alloy dominated with GP II zones, suggesting that the higher coherency of GP II zones promoted the formation of planar slip events. To correlate the effect of hydrogen on dislocation generation, dislocation densities were calculated using XRD peak broadening methodology. It suggested that the cathodic charging resulted in a 1.5 times higher dislocation density for the alloy with a higher volume fraction of GP II zones. Such a variation caused a higher drop in tensile strength for the case of the alloy having higher GP II zones in the cathodically charged condition.