Ductile-to-brittle fracture transitions in 8090 Al-Li alloys

Variables influencing transitions from ductile transgranular fracture to brittle intergranular fracture in a very underaged 8090 Al-Li-Cu-Mg-Zr alloy plate, in which there were no grain-boundary precipitates or precipitate-free zones, have been studied. For the short-transverse crack-plane orientation, ductile-to-brittle transition temperatures increased with increasing ageing times, and were lower for more strongly textured material. Slip was more uniformly distributed at lower temperatures, suggesting that localised planar slip was not a factor contributing to embrittlement. The activation energy for embrittlement was 84 +/-5 kJ/mol for most conditions, which is consistent with embrittlement due to lithium diffusion to grain boundaries and subsequent segregation. There were also notable similarities with segregation-induced fracture in other alloys. The ductile-to-brittle fracture transitions are possibly associated wi th changes in atomic structure at grain boundaries below a critical temperature that is dependent on the degree of lithium enrichment at grain boundaries and the grain-boundary misorientation. (C) 2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.