Environment-Induced Cracking of High-Strength Al-Zn-Mg-Cu Aluminum Alloys: Past, Present, and Future

Abstract

Environment-induced cracking (EIC) during commercial use of aluminum alloys started over 125 y ago (mid-1890s), some 45 y earlier than previously documented, with earliest failures for Al-Zn-Mg-Cu, 7xxx series alloys occurring a decade later. Needs for lighter, thicker, and stronger alloy products, first driven by WW1 and WW2 militaristic requirements and subsequently by relentless demands from modern aircraft industry designers, resulted in major in-service EIC in commercial high-strength Al-Zn-Mg-Cu alloys in the United States and United Kingdom during the 1950s, 1960s, and 1970s. These were avoidable had research findings from France, Germany, and Japan from the 1930s and 1940s been implemented. Unprecedented U.S. Government R&D funding during the late 1960s, 1970s, and early 1980s led to AA7050 and similar alloys, that essentially eliminated EIC issues during commercial usage for several decades. EIC assessment for the following “new-generation” high-strength alloys relied totally on standard ASTM test methods, incapable of providing data directly relatable to the service conditions. Although EIC service issues for the latest generation of 7xxx series alloys remains manageable, the premature appearance of EIC requires a quantitative understanding of EIC initiation under environmental and mechanical conditions directly relatable to intended use, to prevent unexpected failures for future alloys. Directions for future high-strength 7xxx series aluminum alloy development and EIC assessment to provide quantitative data relatable to service conditions and input for structural design and for service life prediction are discussed.