Fracture toughness of 2090 Al-Li-Cu extrusions with high and low hydrogen contents

The effects of different hydrogen contents, produced by variations in melting and casting procedures, on the short-transverse fracture toughness of 2090 Al-Li-Cu alloy extrusions with different alkali-metal impurity (AMI) concentrations were studied. The fracture toughness data and fractographic observations suggest that hydrogen does not have a substantial effect on intergranular fracture resistance for hydrogen contents between 0.2 and 0.8 wt. ppm. A small detrimental effect of hydrogen may occur for material with low AMI content, but small differences in lithium content made it difficult to reach a definite conclusion. Little or no effect of hydrogen on fracture could be explained on the basis that hydrogen is present as small, discrete lithium hydride phases at grain boundaries (rather than segregated as solute along grain boundaries). Since there are numerous grain-boundary precipitates present in aged Al-Li alloys, these lithium hydride particles at boundaries do not significantly increase the area Fraction of particles at the boundaries and, hence, do not significantly influence microvoid nucleation and growth. For material with high AMI content, the fracture behaviour is dominated by liquid metal embrittlement due to the presence of liquid Na-K phases, and hydrogen content does not appear to be important. (C) 1998 Elsevier Science S.A. All rights reserved.