Nature of Grain Boundary Precipitates and Stress Corrosion Cracking Behavior in Al 7075 and 7079 Alloys

Transmission electron microscopy (TEM) has been employed to investigate the nature and microchemistry of Grain boundary precipitates in Al 7079 and Al 7075 aged at the peak and over aged conditions to correlate with the stress corrosion cracking (SCC) behavior. The SCC plateau velocity in Al 7079 at T6 and T7 conditions is two to four orders of magnitude higher, respectively, as compared to Al 7075 T6 and T7. TEM studies revealed that the precipitates within grains in Al 7079 at the peak and overaged conditions is mostly T-phase (Al₁₆Zn₃₃Mg₃₂), while the precipitate at grain boundaries is Al-Mg-Zn type quasicrystalline precipitate (Al₁₅Mg₄₄Zn₄₁). The grain boundary phase in Al 7075 at peak and over aged conditions is hexagonal η phase with stoichiometry Mg(Cu_xZn_1−x)₂. We demonstrate that Al 7079 is more susceptible to SCC as compared to Al 7075 as a result of the formation of the quasicrystalline phase with relatively high Mg content at grain boundaries.