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6/30/04

7:23 AM

Page 2725

The Effect of Grain Size on Low-Cycle Fatigue Behavior of Al-2024 Polycrystalline Alloy A. MOHAMED, Y. EL-MADHOUN, and M.N. BASSIM 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  104 s1 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.

I. INTRODUCTION

THE effect of grain size on the plastic deformation of polycrystalline material can be explained in terms of strain compatibility, which constitutes the foundation of grain boundary effects. The need to maintain continuity of plastic strain across grain boundaries between randomly oriented neighbors demands that multiple slip systems or shear modes be brought into operation when polycrystalline material as a whole is deformed. As the strain level increases, the number of different slips systems necessary to maintain continuity of strain increases. While strain compatibility induces the operation of many slip systems, the latter causes the rate of strain hardening to increase by interaction between individual systems generating dislocation barriers debris, which affect the mobility and mean free path of dislocation in the interior of the grain size.[1,2] However, Morrison[3] found at large difference in saturation stress, and at high plastic strain amplitude, fine-grain nickel saturates at a higher stress amplitude than the coarse-grain nickel. Mughrabi et al.[4] reported that grain size also influences the slip character of the material, and it was shown that the onset of multiple slips occurs at lower plastic strain amplitudes in fine-grain than in coarsegrain copper. Llanes[5] concluded that grain size effect was more pronounced at low plastic strain amplitude and diminished at high plastic strain amplitude. Mughrabi et al.[6] studied the cyclic stress-strain response of polycrystalline copper with grain sizes ranging from 25 to 400 m and found that grain size had very little effect on cyclic stressstrain response. Rasmussen and Pedersen[7] found an insigniA. MOHAME