LOW CYCLE FATIGUE AND FATIGUE CRACK GROWTH IN Al-Li, Al-Li-Zr AND 8090 ALLOYS

Abstract
The evolution of the properties in low cycle fatigue and fatigue crack growth of binary Al-Li, ternary Al-Li-Zr and 8090 alloys were compared. The respective influences of the main microscopic features of Al-Li alloys were studied. The shearing of δ' precipitates induces the formation of intense and heterogeneous slip bands (PSB) and of intergranular cracking. The Al₃Zr precipitates prevent the strong PSB formation in the recristallized Al-Li-Zr alloy, but the texture enhances brittleness in the unrecristallized one. Owing to δ', S' and T₁ precipitates, the 8090 alloy exhibited a flow stress almost twice that of other Al-Li alloys, but its fatigue life was reduced by the brittleness of the PFZ at grain boundaries (GB). All the alloys exhibited a very good crack growth resistance, with ƊKs threshold higher than 9MPa√m. Very efficient closure effects were due to the crack propagation into PSB or GB. Expressed in terms of ƊKeff, the resistance of the alloys were comparable to that of the other Al-alloys. The correlation with low cycle fatigue could be achieved through the use of the HRR elastoplastic field and the cyclic stress-strain curve. When related to the cyclic plastic strain ahead of the crack tip, the crack propagation rates could be correlated to the fatigue life in low cycle fatigue.