Deformation, strength and corrosion resistance evolutions of AA7475 under creep-ageing with various pre-treatments

Abstract

Multi-step ageing has been developed as a novel treatment for 7xxx aluminium alloys to achieve both high strength and superior corrosion resistance. When creep-ageing these multi-step aged Al-Zn-Mg-Cu alloys to fabricate large scale aerospace components, however, effective strategies to enhance their creep deformation while maintaining a satisfactory balance between strength and corrosion resistance remain lacking. This study systemically investigated the evolution of deformation, strength and corrosion resistance during creep-ageing of a multi-aged AA7475 with various pre-treatments including stress-free and stressed pre-ageing. The effects of the pre-treatments on the material behaviour were examined through uniaxial creep-ageing and subsequent tensile tests, intergranular corrosion (IGC) tests, and corresponding microstructural investigations. It was found that during the creep-ageing process, the creep deformation was promoted, while the strength and corrosion resistance were reduced in the pre-treated specimens when compared to the as-received material. Specifically, the tensile creep aged (TCA) pre-treated specimens exhibited the largest reduction in material strength, while those with pre-aged initial state have the most decrease in corrosion resistance. Creep-ageing of materials in various initial states led to the increase in grain boundary precipitate (GBP) size and precipitate free zone (PFZ) width to varying extents, as well as diverse distributions of precipitates along grain boundaries, which exerted substantial effects on the deformation, strength evolution, and corrosion resistance of the material. This study provides valuable insights into developing feasible treatment strategies to improve creep deformation and enhance creep-ageing efficiency in high strength Al-Zn-Mg-Cu alloys, while maintaining an optimal balance between strength and corrosion resistance of the material.