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

The relationships between ageing treatments, microstructure, and the i ntergranular fracture resistance of Al-Li-Cu-Mg-Zr 8090 alloy plate we re studied. The main aim of the work was to obtain a better understand ing of how previously reported double-ageing treatments increase the s hort-transverse fracture toughness. Transmission electron microscopy ( TEM) of thin foils from re-aged bulk material and of thin foils re-age d in situ indicate that re-ageing treatments which almost double the f racture toughness produce only a small extent of dissolution and coars ening of delta' precipitates, with no change in S' precipitates or oth er microstructural features. These effects on matrix precipitation do not appear to influence the slip characteristics. Analytical TEM (usin g plasmon-peak energies obtained from parallel electron energy loss sp ectra to measure lithium concentrations) indicates that lithium segreg ation develops at grain boundaries during natural ageing of re-solutio n treated and quenched material. The ageing time to develop lithium se gregation corresponded roughly to the time which resulted in a transit ion 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 diffusio n of lithium to grain boundaries in 8090 plate. These (and other) obse rvations support the view that lithium segregation at grain boundaries is largely responsible for facilitating intergranular fracture and th at double-ageing treatments decrease the degree of lithium segregation .