Dislocation boundary width changes due to cyclic hardening
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THIS is a continuation of previous work by Bassim et al.’s research group. The present investigation is a fundamental study of deformation mechanics during saturation fatigue of polycrystalline aluminum. Li et al.[1] conducted fully reversed strain-controlled push-pull tests on polycrystalline commercial pure aluminum and noted the absence of the plateau region in the cyclic stress-strain curve. Tsou[2] studied the cyclic deformation behavior of pure polycrystalline aluminum over a range of strain amplitudes using conventional fatigue testing methods. Cyclic hardening response revealed a rapid increase in stress in the early stage of deformation and a gradual decreasing rate toward a steady state of stress amplitude response. In addition, the cyclic stress strain curve exhibited an increase in the saturated stress amplitude with increasing plastic strain amplitude. The increase in the stress was found to be faster at lower strain amplitude and became more gradual at higher strain amplitude. This trend has been observed for most fcc polycrystals. The dominant surface slip character is the persistent slip band (PSB) structure, which could not be found within the interior of the grains during transmission electron microscopy (TEM) investigation. The explanation of the existence of PSBs on the surface and not in the interior could be due to less grain boundary restriction on the slip activity near the surface, where the dislocation activity is closer to that of single-slip condition. The TEM observations revealed dislocation cell structure as the dominant structure of deformed aluminum. It was observed that cell boundary width at lower plastic strain amplitude consists of a large number of dislocation meshes near and within the cell walls. The dislocation boundary width decreases with increasing strain amplitude and cells are more equiaxed at large strain amplitude. Giese et al.[3] conducted a fatigue study on coarse-grained polycrystalline aluminum tested at room temperature in tension-compression mode under total strain control conditions. They concluded that the dependence of the stress amplitude on the cumulative strain is determined by the plastic strain amplitude. At small amplitudes, a softening stage has been found, while at large cumulative strains, the cyclic hardening curves exhibit a tenA.O. MOHAMED and Y. EL-MADHOUN, Graduate Students, and M.N. BASSIM, Professor, are with the Department of Mechanical and Industrial Engineering, University of Manitoba, Winnipeg, MB, Canada R3T 2N2. Contact e-mail: [email protected] This article is based on a presentation made in the symposium entitled ‘‘Fourth International Alloy Conference,’’ which occurred in Kos, Greece, from June 26 to July 1, 2005, and was sponsored by Engineering Conferences International (ECI) and co-sponsored by Lawrence Livermore National Laboratory and Naval Research Laboratory, United Kingdom. METALLURGICAL AND MATERIALS TRANSACTIONS A
dency to saturation. The dependence of the saturation stress on plastic strain amplitude has been shown to
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