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Two different sets of fatigue specimens were heat treated at different times or temperatures to investigate the effect of grain size on the low-cycle fatigue behavior of Al-2024 polycrystalline alloy. Straincontrolled low-cycle fatigue testing with a strain rate of 1 ϫ 10 Ϫ4 s Ϫ1 was conducted at room temperature. The fatigue response of the alloy was evaluated macroscopically in terms of cyclic stress strain response and microscopically in terms of appearance of cyclic slip bands. The cyclic stress strain response of Al-2024 polycrystalline alloy exhibited a definite plateau region where saturation stress remained constant with plastic strain. It was found that the smaller the grain size, the lower the saturation stress and the longer the plateau, whereas the larger the grain size, the higher the saturation stress and shorter the plateau (i.e., reverse grain size effect). Microscopic observations using scanning electron microscope revealed that persistent slip bands (PSBs) were observed at 45 deg orientations from the grain boundary. The volume fraction of PSBs was higher in small-grained Al-2024 polycrystalline alloy as compared to large-grained Al-2024 polycrystalline alloy.

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In this article, cyclic deformations of pure aluminum specimens were characterized by analyzing the cyclic hardening response. Microstructural observations were done by means of transmission electron microscopy (TEM). Microscopic analysis showed that dislocation tangles migrate to the boundary and dislocation boundaries become thinner, condensed, and well defined as plastic strain increases.