Mechanisms of brittle intergranular fracture in Al-Li alloys and comparison with other alloys

Abstract Evidence that brittle intergranular fracture in Al-Li alloys is associated with segregation of lithium, and maybe the formation of two-dimensional grain-boundary phases, is briefly reviewed. Particular attention is drawn to similarities between embrittlement of Al-Li alloys and segregation-induced embrittlement in other materials such as steels. High-resolution fractographic studies of Al-Li alloys to elucidate the atomic mechanisms of brittle intergranular fracture are then described. Brittle intergranular fracture surfaces were often featureless when examined by scanning electron microscopy, but transmission electron microscopy of replicas often revealed very shallow dimples, indicating that crack growth occurred by a very localized microvoid-coalescence process. The observations are explained in terms of decreased cohesion across grain boundaries due to lithium enrichment and grain-boundary structural changes. Low cohesion may facilitate void nucleation by decohesion (in the absence of grain-boundary precipitates in very underaged alloys) and may promote void growth by facilitating the emission of dislocations from crack tips. The applicability of the proposed mechanism to brittle intergranular fracture in other materials is also briefly discussed.