Cryogenic Toughness of Commercial Aluminum-Lithium Alloys: Role of Delamination Toughening

Mechanisms influencing the plane-strain fracture toughness behavior of commercial aluminum-lithium alloys at cryogenic temperatures are investigated as a function of microstructure and plate orientation. It is confirmed that certain alloys show a marked increase in tensile ductility and toughness with decrease in temperature, although such behavior is not found in the short-transverse orientations, nor for all alloys and aging conditions. Specifically at lower temperatures, the majority of A1-Li alloys, namely 2090-T8E41, 8091-T8X, 8090-T8X, and 2091-T351, show a significant increase in fracture toughness in the in-plane orientations (L-T,T-L), without any apparent change in fracture mode. Such behavior is attributed primarily to loss of through-thickness constraint resulting from enhanced short-transverse delamination (termed crack-divider delamination toughening), consistent with observed reductions in plane-strain ductility and short-transverse (S-L,S-T) toughness. Conversely, in underaged microstructures of 8091, 8090, and peak-aged 2091, a decrease in toughness with decreasing temperature is found for both L-T and S-L orientations, behavior which is associated conversely with a fracture-mode change from ductile void coalescence to brittle transgranular shear and intergranular delamination at lower temperatures.