Recently, Al-Li alloys are of great interest in the aircraft industry due to their high modulus and low density. However, their low ductility and poor fatigue resistance remain as a problem for their future use.
Although the aluminium alloys have been widely investigated, results concerning low cycle fatigue(LCF) properties of Al-Li alloys are rather scarce, therefore the low cycle fatigue properties of an Al-Li 2090 has been studied at room temperature under the strain rate of $3\times10^{-4}/\sec$, and LCF was described in terms of the cyclic stress behaviour and fatigue life.
Fatigue softening is observed for all strain amplitudes. Detailed TEM investigations reveal that the δ' precipitates are sheared by dislocations, resulting in planar slip. This softening may be attributed to a change of the particle strengthening within these bands.
The peak aged Al-Li 2090 alloy reveal a non-ideal plastic strain fatigue life behaviour with a discontinuity or break in the CoffinManson curve. Changes in the slope of strain-life curves have been associated with changes in the homogeneity of deformation.
The sample cycled at lower strain amplitude exhibited much shorter life than would be predicted by extrapolation from the high plastic strain amplitude portion of the Coffin-Manson plot. It is believed this is associated with very localized deformation that occurs at low strain amplitudes.
All the appearance of the Al-Li 2090 LCF fracture surfaces were very similar to that of the monotonic fracture surfaces. No fatigue striations representing the crack propagation stage were found. Most of the fracture surface appeared to be due to the overload fracture, indicating that fatigue failure occurred shortly after macrocrack initiation. It is suggested that most of the fatigue life of this alloy was spent in initiating the fatal crack so the differences in strain-life behavior can be attributed to differences in fatigue crack initiation resistance. From the test results of the crack initiation modes, it is found that the crcak initiation modes change from grain boundaries at a higher strain amplitudes to slip bands at a lower one. The reason for variation of crack initiation sites as a function of strain amplitude is attributed to change in deformation processes as a function of the plastic strain amplitude.
