EFFECTS OF AGING TREATMENTS ON RESISTANCE TO INTERGRANULAR FRACTURE OF 8090 AL-LI ALLOY PLATE

The relationships between ageing treatments, microstructure, and the intergranular fracture resistance of Al-Li-Cu-Mg-Zr 8090 alloy plate were studied. The main aim of the work was to obtain a better understanding of how previously reported double-ageing treatments increase the short-transverse fracture toughness. Transmission electron microscopy (TEM) of thin foils from re-aged bulk material and of thin foils re-aged in situ indicate that re-ageing treatments which almost double the fracture toughness produce only a small extent of dissolution and coarsening of delta' precipitates, with no change in S' precipitates or other microstructural features. These effects on matrix precipitation do not appear to influence the slip characteristics. Analytical TEM (using plasmon-peak energies obtained from parallel electron energy loss spectra to measure lithium concentrations) indicates that lithium segregation develops at grain boundaries during natural ageing of re-solution treated and quenched material. The ageing time to develop lithium segregation corresponded roughly to the time which resulted in a transition from transgranular fracture to intergranular fracture for testing at -196-degrees-C. The activation energy associated with the kinetics of this low-temperature embrittlement of very under-aged material was the same as that previously determined for (i) the re-embrittlement of material toughened by double-ageing treatments, and (ii) the diffusion of lithium to grain boundaries in 8090 plate. These (and other) observations support the view that lithium segregation at grain boundaries is largely responsible for facilitating intergranular fracture and that double-ageing treatments decrease the degree of lithium segregation.