Low-energy intergranular fracture in Al–Li alloys

An age-hardened Al–Li–Cu–Mg–Zr alloy (AA8090) used for a substantial part of the structure of a helicopter (in order to save weight) exhibited low-energy intergranular fracture during accidents, resulting in extensive damage. With this experience in mind, various hypotheses for brittle intergranular fracture in Al–Li alloys are reviewed, and possible remedial measures outlined. The effects of variables such as alloy composition, ageing times and temperatures, crystallographic texture, and test temperature on brittle-intergranular-fracture resistance are described, and it is concluded that lithium segregation to grain boundaries is primarily responsible for brittle intergranular fracture. Lithium segregation occurs mainly during the primary ageing treatment; but secondary ageing during service, where temperatures can reach 50–80 °C, could increase lithium segregation and further increase susceptibility to brittle intergranular fracture. The evidence also suggests that the prevalence of a planar-slip-mode in Al–Li alloys, resulting in dislocation pile-ups at grain boundaries, is not responsible for brittle intergranular fracture, contrary to widespread opinion.

► Al–Li alloys have a propensity for brittle intergranular fracture.
► The use of Al–Li alloys in aerospace structures is therefore problematical.
► Lithium segregation to grain boundaries is mainly responsible for brittle fracture.
► Planar slip in Al–Li alloys is not responsible for brittle intergranular fracture.
► There are no obvious solutions to the problem besides reducing the Li contents.