Challenges in Developing High Performance Al-Li Alloys

Abstract

The goal of replacing ‘conventional’ Al alloys with Al-Li alloys to reduce weight of aerospace structures has had only limited success to date, despite one of the largest alloy-development programs ever undertaken. One of the reasons being, for some product forms and crack-plane orientations, fracture toughness is lower compared with conventional Al alloys due to a greater propensity for low-energy intergranular fracture. Proposed explanations for brittle intergranular fracture include: (i) planar slip resulting in high stresses where slip bands impinge on grain-boundaries, (ii) embrittlement due to alkali-metal-impurity phases, and (iii) grain-boundary structural changes associated with segregation of lithium. The present paper reviews evidence for and against these proposed mechanisms based on (i) studies of 8090 and 2090 alloys, and the more recently developed 2297 alloy, and (ii) comparisons of fracture of these alloys with other materials that exhibit grain-boundary weakness. It appears that lithium segregation to grain boundaries is the prime cause of low-energy intergranular fracture in Al-Li alloys. Embrittlement by alkali-metal impurities is not important providing that impurity levels are less than about 5 ppm (as is the case for most commercially produced alloys). Planar slip and other factors probably play only a minor role.