SECONDARY HARDENING AND DISLOCATION EVOLUTION IN LOW-CYCLE FATIGUE OF POLYCRYSTALLINE ALUMINUM

Cyclic deformation of well annealed 99.999% pure Al and 99.85% commercially pure Al polycrystals is performed under constant total strain and plastic strain control, respectively. The stress response to cyclic loading is measured and secondary hardening occurs in a certain range of strain amplitudes. The evolution of dislocation microstructures and surface slip appearances during cyclic deformation are examined by TEM and SEM, respectively. It is noted that the secondary hardening is dependent on microstructural changes taking place in cyclic loading, such as the activation of secondary slip systems and the formation of a dislocation cell structure with increasing misorientations and decreasing cell sizes. Grain boundaries promote an early occurrence of secondary hardening and enhance its strength. Copyright © 1990 WILEY‐VCH Verlag GmbH & Co. KGaA