Microstructure, thermal stability and corrosion behavior of ultrasonic shot peened AA7034

Microstructure, thermal stability at room temperature and corrosion behavior of ultrasonic shot peened AA7034 were investigated in this work. Microstructure modification induced by ultrasonic shot peening was obtained by using OM, SEM, EBSD and XRD, and it was found that a ∼200 μm thick gradient surface layer with nano-scale grains on the topmost layer was induced on the peened sample. XRD peaks of the pre-existing aging-induced precipitates, i.e., η′ and η phases, disappeared after peening and did not show up during 52 months-long natural aging treatment, showing good thermal stability at room temperature. After peening, a significant improvement in intergranular corrosion resistance is evident. Electrochemical characterizations like open circuit potential and electrochemical impedance spectroscopy were conducted on various depths of the peened sample and the results indicated that uniform corrosion rate was largely accelerated by the presence of foreign Fe-rich contamination, implying that modifications should be made to reduce/avoid the foreign contamination. Additionally, mechanisms of corrosion improvement and good thermal stability of the nanocrystalline surface layer of AA7304 are also discussed.