The option of extending the fatigue life of precipitation hardenable aluminium AA2024 alloys under high cycle fatigue loading conditions by dynamic precipitation in under-aged material has been explored. The fatigue life enhancement should be due to formation of clusters or precipitates at deformation induced damage sites which is to retard crack initiation. To this aim the fatigue lifetime and microstructural changes due to fatigue loading of peak aged and under-aged aluminium AA 2024 samples, containing low and high solute levels respectively, have been studied.
For fatigue testing at an R-value of −0.4 a substantial increase in fatigue life time for the under-aged material was observed at stress ranges at 265 MPa. Detailed microstructural investigations at (sub-)nanometer level using Positron Annihilation (PA), Thermo-Electric-Power (TEP) TEM revealed changes in the solute level and precipitate state during cyclic static loading in the plastic regime but no changes in solute level during fatigue loading were detected. Hence we did not succeed in correlating the improvement in high cycle fatigue life times for under-aged material to dynamic precipitation effects. More work is required to determine the effect of higher solute levels on the fatigue lifetime for a wider range of testing conditions.
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